The

haptic

perception

of

spatial

orientations

This

review

examines

the

isotropy

of

the

perception

of

spatial

orientations

in

the

haptic

system

It

shows

the

existence

of

an

oblique

effect

-LRB-

i.e.

a

better

perception

of

vertical

and

horizontal

orientations

than

oblique

orientations

-RRB-

in

a

spatial

plane

intrinsic

to

the

haptic

system

determined

by

the

gravitational

cues

and

the

cognitive

resources

and

defined

in

a

subjective

frame

of

reference

Similar

results

are

observed

from

infancy

to

adulthood

In

space

the

haptic

processing

of

orientations

is

also

anisotropic

and

seems

to

use

both

egocentric

and

allocentric

cues

Taken

together

these

results

revealed

that

the

haptic

oblique

effect

occurs

when

the

sensory

motor

traces

associated

with

exploratory

movement

are

represented

more

abstractly

at

a

cognitive

level

Introduction

Haptic

perception

-LRB-

or

active

touch

-RRB-

results

from

the

stimulation

of

the

mechanoreceptors

in

skin

muscles

tendons

and

joints

generated

by

the

manual

exploration

of

an

object

in

space

-LRB-

Gibson

Revesz

-RRB-

Haptic

perception

allows

us

for

example

to

identify

an

object

or

one

of

its

features

like

its

size

shape

or

weight

the

position

of

its

handle

or

the

material

of

which

it

is

made

A

fundamental

characteristic

of

the

haptic

system

is

that

it

depends

on

contact

The

tactile

perceptual

field

-LRB-

i.e.

the

portion

of

the

skin

that

is

in

contact

with

the

external

stimulus

-RRB-

has

a

limited

size

-LRB-

the

surface

of

both

hands

at

maximum

-RRB-

and

a

limited

reach

-LRB-

the

length

of

the

arm

-RRB-

It

results

from

these

characteristics

of

the

tactile

perceptual

field

that

the

perception

of

the

spatial

properties

of

the

objects

almost

always

involves

some

displacements

of

the

arm

and

the

hand

to

explore

the

stimulus

In

fact

it

is

also

known

that

the

nature

of

these

exploratory

movements

often

depends

on

the

specific

property

of

the

touched

object

-LRB-

Lederman

and

Klatzky

-RRB-

For

example

one

might

follow

the

contour

of

an

object

with

one

finger

to

perceive

its

shape

or

squeeze

it

with

the

whole

hand

to

perceive

its

compliance

Moreover

the

haptic

system

must

integrate

information

about

the

parts

of

the

body

touching

the

object

with

information

about

the

position

of

the

body

parts

in

space

It

results

from

these

properties

of

the

haptic

system

that

the

haptic

perception

of

space

is

far

removed

from

the

proximal

stimulation

that

occurs

during

the

manual

exploration

and

depends

on

spatio-temporal

integration

of

the

kinesthetics

and

tactile

inputs

to

build

a

representation

of

the

stimulus

Studies

on

the

haptic

perception

of

space

have

generally

focused

on

the

perception

of

the

elementary

spatial

properties

of

objects

such

as

their

length

curvature

or

orientation

in

space

-LRB-

cf.

Hatwell

et

al.

Henriques

and

Soechting

Millar

-RRB-

The

hallmark

of

these

studies

is

that

the

dimension

of

interest

is

systematically

manipulated

and

that

exploratory

movements

if

present

are

very

precisely

monitored

In

general

the

aim

of

these

studies

is

to

measure

the

sensitivity

of

the

corresponding

perceptual

channel

In

addition

evidence

such

as

a

systematic

bias

or

some

variation

in

the

sensitivity

of

parameters

is

used

to

understand

the

underlying

processes

Altogether

these

studies

have

shown

that

the

haptic

perception

of

simple

spatial

properties

such

as

orientation

and

length

are

systematically

distorted

with

respect

to

the

physical

reality

For

example

it

is

well

known

that

the

haptic

perception

of

the

length

between

two

points

is

influenced

by

numerous

factors

such

as

movement

direction

-LRB-

radial

versus

tangential

-RRB-

or

velocity

-LRB-

cf.

Walsh

et

al.

Imanaka

and

Abernethy

Gentaz

and

Hatwell

-RRB-

Others

have

found

that

straight

lines

are

often

perceived

as

curved

and

vice

versa

-LRB-

Sanders

and

Kappers

see

also

Henriques

and

Soechting

-RRB-

In

the

same

vein

Kappers

-LRB-

-RRB-

and

Kappers

and

Koenderink

-LRB-

-RRB-

have

observed

that

two

parallel

-LRB-

in

physical

space

-RRB-

bars

in

different

spatial

locations

can

be

felt

as

being

almost

perpendicular

This

remarkable

effect

remains

even

after

various

types

of

feedback

Finally

it

is

noteworthy

that

judgments

of

different

spatial

properties

can

be

inconsistent

among

themselves

For

example

Fasse

et

al.

-LRB-

-RRB-

found

that

the

biases

in

the

perception

of

the

acute

angles

of

a

right

triangle

were

inconsistent

with

the

observed

biases

in

the

judgments

of

the

lengths

of

the

adjacent

and

opposed

edges

This

study

suggests

that

diverse

spatial

properties

of

an

object

might

be

processed

independently

from

each

other

and

do

not

necessarily

refer

to

a

unique

underlying

representation

of

space

The

present

review

is

concerned

by

one

very

specific

aspect

of

spatial

perception

the

haptic

perception

of

spatial

orientations

The

perception

of

orientations

is

isotropic

when

the

performances

in

orientation-adjustment

tasks

are

similar

regardless

of

the

orientation

values

and

anisotropic

when

performances

differ

according

to

these

values

Historically

the

orientation

effect

of

a

stimulus

on

the

performance

level

was

first

studied

in

various

detection

discrimination

and

reproduction

tasks

in

the

visual

modality

A

widespread

observation

termed

oblique

effect

by

Appelle

-LRB-

-RRB-

is

that

the

performance

level

is

in

general

better

when

the

stimulus

is

aligned

with

the

vertical

or

horizontal

orientation

The

original

motivation

for

studying

the

oblique

effect

in

the

haptic

modality

was

to

test

possible

explanations

of

the

visual

oblique

effect

The

existence

of

an

oblique

effect

in

the

haptic

modality

would

suggest

that

orientations

are

handled

by

the

same

processes

in

both

modalities

and

thus

would

reinforce

the

view

that

high-level

possibly

amodal

-LRB-

i.e.

independent

of

sensorial

modalities

-RRB-

processes

could

be

responsible

for

the

oblique

effect

In

contrast

its

absence

would

indicate

the

existence

of

distinct

orientation-processing

streams

in

the

haptic

and

visual

modalities

-LRB-

cf.

Gentaz

Gentaz

and

Hatwell

Heller

Millar

-RRB-

In

this

respect

we

will

show

that

orientation

processing

is

generally

anisotropic

in

the

haptic

modality

for

adults

children

and

even

in

infants

However

unlike

the

visual

oblique

effect

where

such

anisotropy

is

almost

always

present

we

will

show

that

this

effect

depends

on

the

gravitational

cues

and

memory

constraints

that

are

very

specific

to

haptics

The

second

objective

of

this

review

is

to

identify

the

frame

of

reference

associated

with

the

haptic

oblique

effect

This

question

comes

from

the

observation

that

the

concept

of

orientation

is

by

definition

relative

to

a

set

of

fixed

axes

which

define

a

coordinate

system

or

frame

of

reference

For

example

in

the

polar

coordinate

system

an

orientation

is

defined

by

the

angle

between

some

features

of

the

stimulus

such

as

its

main

axis

and

an

axis

of

the

coordinate

system

While

the

choice

of

this

set

of

axes

can

be

arbitrary

it

is

common

to

choose

the

vertical

and

its

perpendicular

-LRB-

the

horizontal

-RRB-

because

of

the

permanent

influence

of

the

vertical

gravity

force

on

Earth

In

fact

the

concept

of

verticality

is

so

dominant

that

even

in

a

drawing

put

on

a

table

that

is

on

a

horizontal

plane

the

verticality

is

projected

as

the

line

belonging

to

the

sagittal

plane

and

perpendicular

to

the

axis

of

the

body

However

the

status

of

verticality

must

be

questioned

In

the

visual

modality

various

studies

have

examined

whether

the

oblique

effect

was

tied

to

this

geocentric

reference

frame

or

to

a

retinocentric

frame

of

reference

In

this

respect

we

will

show

that

the

haptic

oblique

effect

observed

in

the

frontal

plane

is

defined

in

a

subjective

reference

frame

that

combines

ego

and

geocentric

cues

We

will

also

raise

the

question

of

which

pair

of

angles

is

used

to

code

the

orientation

of

a

stimulus

in

space

when

the

stimuli

do

not

necessarily

belong

to

a

plane

where

a

single

angle

would

suffice

to

code

the

orientation

As

a

matter

of

fact

it

is

important

to

note

that

different

pairs

of

angles

can

be

used

to

code

an

orientation

in

space

in

the

same

way

as

different

coordinate

systems

can

be

used

to

code

the

position

of

a

point

in

space

In

conclusion

we

will

argue

that

the

haptic

oblique

effect

occurs

at

a

relatively

late

stage

of

orientation

processing

when

sensory

motor

traces

associated

with

exploratory

movements

are

transformed

into

a

more

abstract

representation

of

the

orientation

because

the

experimental

factors

that

modify

the

haptic

oblique

effect

's

strength

either

contribute

to

establishing

such

a

representation

-LRB-

e.g.

gravitational

cues

-RRB-

or

favor

its

use

-LRB-

e.g.

memory

constraints

-RRB-

Finally

we

will

discuss

various

hypotheses

about

how

the

processes

involved

in

the

recoding

of

the

sensory-motor

traces

might

yield

an

anisotropy

in

the

perception

or

recollection

of

orientations

Hypotheses

on

the

origins

of

the

oblique

effects

Numerous

studies

have

shown

that

vertically

or

horizontally

oriented

visual

stimuli

are

generally

perceived

with

greater

precision

than

those

oriented

obliquely

While

the

existence

of

the

visual

oblique

effect

is

well

established

the

explanations

of

this

effect

in

vision

are

still

debated

today

-LRB-

e.g.

Baowang

et

al.

Essock

Gentaz

and

Ballaz

Gentaz

and

Tschopp

Meng

and

Qian

Westheimer

-RRB-

Although

orientation

processing

mainly

involves

the

lower

visual

areas

such

as

the

Lateral

Geniculate

Nucleus

-LRB-

LGN

-RRB-

and

the

visual

primary

cortex

-LRB-

-RRB-

it

is

completed

at

higher

levels

in

the

extra-striate

and

nonvisual

areas

-LRB-

e.g.

Goodale

et

al.

-RRB-

The

multitude

of

subcortical

and

cortical

areas

involved

in

orientation

processing

raises

the

question

of

where

the

processes

responsible

for

the

visual

oblique

effect

specifically

act

in

the

anatomo-functional

steps

of

visual

processing

Two

general

hypotheses

have

been

advanced

to

explain

two

types

of

oblique

effect

The

first

hypothesis

has

been

in

general

advanced

to

explain

the

so-called

Class

oblique

effects

a

term

coined

by

Essock

-LRB-

-RRB-

which

are

observed

in

tasks

measuring

the

basic

functional

properties

of

the

visual

system

-LRB-

e.g.

acuity

and

contrast

-RRB-

The

hypothesis

is

that

the

visual

oblique

effect

is

generated

at

the

lowest

levels

of

the

visual

system

such

as

LGN

and/or

-LRB-

for

a

discussion

about

the

major

role

of

see

Li

et

al.

-RRB-

This

hypothesis

is

supported

by

the

very

specific

neurophysiological

characteristics

of

orientation-selective

neurons

in

these

areas

In

particular

it

has

been

shown

that

neurons

sensitive

to

the

vertical

and

horizontal

orientations

are

more

numerous

sensitive

and/or

narrowly

tuned

than

neurons

sensitive

to

the

oblique

orientations

-LRB-

e.g.

Furmanski

and

Engel

Li

et

al.

Saarinen

and

Levi

-RRB-

According

to

this

hypothesis

the

processes

generating

the

visual

oblique

effect

would

therefore

be

specific

to

this

modality

In

contrast

Class

oblique

effects

are

observed

in

more

cognitive

tasks

involving

identifying

memorizing

and

categorizing

orientations

Moreover

Class

oblique

effects

are

influenced

by

factors

such

as

internal

models

-LRB-

Morgan

-RRB-

recognition

-LRB-

Heeley

and

Buchanan-Smith

-RRB-

or

attention

-LRB-

Shiu

and

Pashler

-RRB-

that

are

not

amenable

to

the

characteristics

of

the

early

stages

of

visual

processing

For

example

the

mere

fact

that

an

oblique

effect

is

present

in

the

reproduction

of

the

orientation

of

two

distant

large

dots

is

difficult

to

explain

in

terms

of

the

properties

of

direction-sensitive

neurons

since

this

stimulus

in

principle

stimulates

all

orientations

equally

-LRB-

Cecala

and

Garner

Westheimer

-RRB-

The

second

hypothesis

is

therefore

that

some

oblique

effects

might

occur

not

only

at

the

low-level

areas

of

the

visual

system

such

as

LGN

or

but

also

at

higher

level

areas

-LRB-

from

to

associative

areas

-RRB-

This

hypothesis

suggests

that

the

processes

generating

the

visual

oblique

effect

might

not

be

specific

to

this

sensory

modality

-LRB-

Heeley

et

al.

-RRB-

While

some

models

of

the

Class

oblique

effects

make

reference

to

neurophysiological

mechanisms

-LRB-

e.g.

McMahon

and

MacLeod

Westheimer

-RRB-

other

explanations

are

formulated

only

in

functional

terms

For

example

a

widely

shared

idea

is

that

orientations

are

represented

in

a

reference

frame

having

the

vertical

or

horizontal

orientations

as

principal

axes

at

some

later

stage

of

visual

processing

In

this

frame

of

reference

identification

of

these

two

orientations

as

principal

axes

can

be

carried

out

simply

by

identifying

the

stimulus

with

one

of

the

two

references

axes

whereas

the

coding

of

oblique

orientations

would

require

the

integration

of

information

derived

from

both

axes

-LRB-

Foster

and

Westland

Gentaz

and

Hatwell

Marendaz

Regan

and

Price

-RRB-

For

example

the

coding

of

oblique

orientations

might

involve

the

computation

of

the

ratio

of

the

two

values

which

corresponds

to

the

projections

of

the

oblique

on

the

references

axes

According

to

this

view

the

oblique

effect

is

caused

by

some

additional

processing

needed

to

encode

oblique

orientations

relatively

to

the

reference

axes

Alternatively

it

has

been

suggested

that

all

orientations

are

encoded

with

the

same

precision

but

that

participants

have

a

remarkably

accurate

internal

representation

of

the

vertical

and

horizontal

orientation

which

can

be

used

to

produce

the

response

when

perceived

orientation

corresponds

to

these

norms

-LRB-

Heeley

et

al.

-RRB-

In

other

words

instead

of

assuming

that

some

computation

increases

the

error

for

oblique

orientation

the

higher

level

processes

would

bring

additional

information

about

the

vertical

and

horizontal

that

decreases

the

variable

error

at

the

reference

axes

Another

possibility

adopted

by

Huttenlocher

et

al.

is

that

orientations

are

recorded

in

categorical

terms

-LRB-

Huttenlocher

et

al.

see

also

the

work

on

Categorical

Perception

Harnard

-RRB-

According

to

the

Category-Adjustment

-LRB-

CA

-RRB-

model

each

orientation

is

categorized

in

one

of

the

categories

that

partition

the

orientation

space

This

model

was

developed

to

explain

the

biases

toward

the

diagonals

observed

in

various

position

and

orientation-recall

tasks

-LRB-

see

also

Spencer

and

Hund

Haun

et

al.

-RRB-

According

to

the

CA

model

the

recall

of

an

oblique

orientation

would

be

biased

toward

the

closest

diagonal

because

it

would

combine

a

low-level

-LRB-

presumably

unbiased

-RRB-

representation

of

the

orientation

with

the

center

of

the

category

including

the

orientation

typically

one

of

the

four

quadrants

In

other

words

the

gist

of

the

model

is

that

the

mental

partitioning

or

labeling

of

the

orientations

into

a

limited

number

of

categories

is

at

the

origin

of

systematic

bias

in

the

encoding

and

recall

of

an

orientation

Finally

Spencer

et

al.

have

recently

developed

an

alternative

theoretical

framework

the

Dynamic

Field

Theory

-LRB-

DFT

-RRB-

of

spatial

cognition

to

explain

the

presence

of

systematic

biases

in

orientation-recall

tasks

-LRB-

Spencer

et

al.

-RRB-

The

DFT

is

based

on

the

dynamic

properties

of

several

topologically

organized

and

interconnected

layers

of

units

and

can

provide

insights

about

developmental

processes

such

as

the

change

in

direction

of

the

bias

in

orientation

recall

tasks

between

childhood

and

adulthood

and

about

real-time

processes

such

as

the

increase

of

the

recall

bias

with

time

More

specifically

the

DFT

includes

a

perceptual

map

that

interacts

with

the

environment

a

working

memory

map

and

a

long-term

memory

map

According

to

the

DFT

repeated

exposure

to

a

reference

axis

results

in

the

storage

of

this

piece

of

information

in

the

long-term

memory

which

can

be

at

the

origin

of

a

shift

of

the

stimulus

position

in

the

working

memory

even

in

absence

of

explicit

contextual

information

in

the

environment

It

is

important

to

note

that

both

the

CA

and

DFT

models

were

originally

conceived

to

explain

the

results

of

experiments

dealing

with

visual

stimuli

showing

the

presence

of

systematic

biases

toward

the

diagonals

in

various

orientation

or

position

recall

tasks

a

type

of

spatial

anisotropy

that

as

we

shall

see

has

not

yet

been

studied

in

detail

in

the

haptic

modality

In

the

following

section

we

start

to

review

experimental

studies

of

the

haptic

oblique

effect

Isotropic

or

anisotropic

haptic

perception

of

spatial

orientations

Existence

of

an

haptic

oblique

effect

Lechelt

et

al.

-LRB-

-RRB-

were

the

first

to

show

interest

in

the

haptic

oblique

effect

The

goal

was

to

show

the

existence

of

an

oblique

effect

in

the

haptic

system

analogous

to

the

one

observed

in

the

visual

system

for

the

abovementioned

reasons

To

that

end

they

compared

orientation

perception

in

the

visual

and

haptic

systems

by

applying

the

same

experimental

paradigm

to

both

systems

The

visual

and

haptic

stimuli

were

rods

-LRB-

cm

cm

-RRB-

which

could

take

one

of

six

orientations

Adults

were

presented

with

two

identical

rods

-LRB-

a

stimulus

rod

and

a

response

rod

-RRB-

placed

in

the

frontal

plane

on

either

side

and

equidistant

from

their

medial

plane

In

the

visual

condition

participants

worked

in

a

dark

room

with

electroluminescent

rods

and

gave

verbal

indications

to

adjust

the

response

rod

immediately

after

the

stimulus

rod

had

disappeared

In

the

haptic

condition

participants

were

blindfolded

and

the

response

rod

was

adjusted

with

the

contralateral

hand

immediately

after

the

stimulus

rod

became

unavailable

Globally

the

analysis

of

constant

errors

did

not

show

any

systematic

deviation

in

either

modality

for

all

orientations

The

analysis

of

absolute

errors

revealed

a

classical

effect

of

modality

errors

were

greater

in

the

haptic

condition

-LRB-

from

M

to

M

-RRB-

than

those

in

the

visual

condition

-LRB-

from

M

to

M

-RRB-

The

effect

of

spatial

orientations

was

significant

and

revealed

an

oblique

effect

in

both

modalities

In

vision

the

errors

were

lower

for

vertical

and

horizontal

-LRB-

M

-RRB-

than

for

the

oblique

orientations

-LRB-

M

-RRB-

In

haptics

the

errors

were

lower

for

vertical

and

horizontal

-LRB-

M

-RRB-

than

for

the

oblique

orientations

-LRB-

M

-RRB-

Lechelt

and

Verenka

-LRB-

-RRB-

replicated

these

results

with

a

slightly

modified

procedure

where

the

rod

exploration

time

was

fixed

at

s

for

both

perceptual

conditions

where

the

delay

between

exploration

and

reproduction

was

set

at

s

and

where

the

orientations

were

no

longer

indicated

by

the

experimenters

before

the

test

The

authors

concluded

that

there

was

an

oblique

effect

in

the

haptic

system

analogous

to

that

observed

in

the

visual

system

Initial

hypotheses

on

the

origin

of

the

haptic

oblique

effect

the

role

of

prior

knowledge

and

the

mode

of

reproduction

of

the

orientation

Appelle

and

Countryman

-LRB-

-RRB-

questioned

the

abovementioned

conclusions

of

Lechelt

et

al.

-LRB-

-RRB-

and

Lechelt

and

Verenka

-LRB-

-RRB-

They

argued

that

the

haptic

oblique

effect

might

be

induced

by

the

prior

visual

perceptual

experience

and/or

the

conditions

of

exploration

reproduction

movements

To

show

the

possible

effect

of

the

first

factor

the

authors

proposed

two

prior

experience

conditions

an

informed

condition

in

which

standard

orientations

were

verbally

and

visually

presented

to

participants

for

as

long

as

they

wished

and

an

uninformed

condition

in

which

participants

did

not

know

which

standard

orientations

were

being

tested

On

the

basis

of

Appelle

and

Gravetter

's

-LRB-

-RRB-

results

they

predicted

that

prior

verbal

or

visual

knowledge

of

standard

orientations

would

lead

participants

to

produce

responses

using

internal

orientation

models

as

a

reference

-LRB-

mainly

the

vertical

and

horizontal

orientations

-RRB-

instead

of

the

immediate

haptic

percept

resulting

from

the

exploration

of

the

stimulus

and

induce

the

haptic

oblique

effect

With

respect

to

the

second

factor

-LRB-

conditions

of

exploration

reproduction

movements

-RRB-

these

authors

also

argued

that

the

haptic

oblique

effect

observed

in

Lechelt

et

al.

's

experiments

could

be

due

to

the

use

of

the

contralateral

hand

to

reproduce

the

orientation

Indeed

when

one

hand

explores

the

orientation

of

a

standard

rod

and

the

other

hand

reproduces

the

same

orientation

on

a

test

rod

the

symmetrical

disposition

of

the

shoulder

hand

systems

imposes

nonhomologous

movement

patterns

to

explore

and

reproduce

oblique

orientations

-LRB-

agonist

and

antagonist

muscles

are

mobilized

differently

-RRB-

In

contrast

exploration

and

reproduction

movement

patterns

are

homologous

for

vertical

and

horizontal

orientations

Then

the

authors

proposed

two

exploration

reproduction

conditions

contralateral

condition

-LRB-

one

hand

explores

and

the

other

reproduces

-RRB-

and

an

ipsilateral

condition

-LRB-

the

same

hand

explores

and

reproduces

-RRB-

In

the

latter

case

the

exploration

reproduction

movement

patterns

are

homologous

for

all

orientations

and

the

oblique

effect

should

not

be

present

However

Appelle

and

Countryman

-LRB-

-RRB-

modified

the

experimental

paradigm

of

Lechelt

et

al.

-LRB-

-RRB-

and

Lechelt

and

Verenka

-LRB-

-RRB-

First

they

used

only

four

orientations

-LRB-

and

-RRB-

Earlier

results

justified

this

choice

as

they

showed

that

performances

did

not

differ

between

and

or

between

and

for

example

They

introduced

decoy

orientations

so

that

participants

would

not

be

aware

that

only

four

orientations

were

being

tested

They

presented

the

orientations

in

the

horizontal

plane

-LRB-

parallel

to

a

table

surface

-RRB-

instead

of

the

frontal

one

Finally

they

fixed

the

delay

between

exploration

and

reproduction

at

s.

Absolute

error

analysis

revealed

that

the

performance

on

the

oblique

orientations

and

therefore

the

amplitude

of

the

haptic

oblique

effect

varied

according

to

prior

experience

and

exploration

reproduction

conditions

Vertical

and

horizontal

orientations

were

stable

The

extent

of

the

haptic

oblique

effect

was

maximal

in

the

informed

contralateral

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

and

diminished

in

the

uninformed

contralateral

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

It

was

even

smaller

in

the

informed

ipsilateral

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

Finally

it

was

and

the

oblique

effect

thus

absent

in

the

uninformed

ipsilateral

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

In

their

study

Appelle

and

Countryman

-LRB-

-RRB-

concluded

that

prior

visual

perceptual

experience

and

exploration

reproduction

movements

were

responsible

for

the

haptic

oblique

effect

contrary

to

the

propositions

of

Lechelt

et

al.

-LRB-

-RRB-

and

Lechelt

and

Verenka

-LRB-

-RRB-

Role

of

gravitational

cues

in

the

haptic

oblique

effect

Gentaz

and

Hatwell

's

-LRB-

-RRB-

study

provides

a

test

of

Appelle

and

Countryman

's

-LRB-

-RRB-

hypothesis

-LRB-

second

factor

-RRB-

regarding

the

existence

of

a

difference

between

the

one-handed

exploratory

movement

patterns

of

an

oblique

orientation

and

the

movement

patterns

needed

to

reproduce

the

oblique

with

the

contralateral

hand

As

in

Appelle

and

Countryman

's

-LRB-

-RRB-

study

the

participants

worked

in

a

nonvisual

and

uninformed

condition

with

a

rod

oriented

at

or

and

they

reproduced

the

orientation

of

the

standard

rod

after

a

delay

The

standard

rod

was

presented

in

the

horizontal

-LRB-

as

in

Appelle

et

al.

's

experiment

-RRB-

frontal

-LRB-

parallel

to

the

surface

of

a

painting

as

in

Lechelt

et

al.

's

experiments

-RRB-

or

sagittal

-LRB-

the

median

plane

perpendicular

to

the

horizontal

and

frontal

planes

-RRB-

planes

The

task

was

performed

ipsilaterally

-LRB-

with

the

same

hand

-RRB-

or

contralaterally

-LRB-

with

the

other

hand

-RRB-

Given

that

in

the

sagittal

plane

the

oblique

orientation

exploratory

movements

of

one

hand

become

homologous

with

those

needed

to

reproduce

the

oblique

orientations

contralaterally

the

oblique

effect

should

also

be

absent

in

this

plane

if

Appelle

and

Countryman

's

-LRB-

-RRB-

hypothesis

-LRB-

difference

between

exploration

and

reproduction

movements

-RRB-

is

determinant

Absolute

error

analysis

revealed

that

results

did

not

concord

with

these

predictions

because

similar

oblique

effects

were

obtained

in

the

frontal

and

sagittal

planes

under

both

ipsilateral

and

contralateral

conditions

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

However

results

in

the

horizontal

plane

replicated

those

of

Appelle

and

Countryman

-LRB-

-RRB-

i.e.

an

oblique

effect

was

present

in

the

contralateral

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

and

absent

in

the

ipsilateral

one

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

Therefore

the

presence

or

absence

of

an

oblique

effect

does

not

appear

to

be

due

to

the

difference

of

movements

between

the

two

hands

but

seems

to

be

strongly

linked

to

the

type

of

spatial

planes

To

understand

the

results

in

the

horizontal

plane

and

in

particular

the

lack

of

an

oblique

effect

in

the

ipsilateral

condition

only

Gentaz

and

Hatwell

-LRB-

-RRB-

asked

blindfolded

adults

to

symmetrically

reproduce

a

previously

presented

orientation

-LRB-

a

right

standard

oblique

was

to

be

reproduced

at

left

and

similarly

a

left

standard

oblique

was

to

be

reproduced

at

right

-RRB-

in

the

horizontal

plane

either

ipsilaterally

or

contralaterally

Given

that

the

movements

necessary

for

the

exploration

and

the

symmetrical

reproduction

of

an

oblique

orientation

become

homologous

in

the

contralateral

condition

and

nonhomologous

in

the

ipsilateral

condition

the

oblique

effect

should

be

present

in

the

ipsilateral

condition

and

absent

in

the

contralateral

condition

if

the

Appelle

and

Contryman

's

hypothesis

was

valid

Results

did

not

concord

with

these

predictions

since

an

oblique

effect

was

present

in

both

reproduction

conditions

in

the

horizontal

plane

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

Taken

together

these

results

partially

-LRB-

see

below

-RRB-

invalidate

Appelle

and

Countryman

's

-LRB-

-RRB-

hypothesis

about

the

role

of

exploration

reproduction

movements

Gentaz

and

Hatwell

-LRB-

-RRB-

account

for

these

results

by

suggesting

that

the

haptic

oblique

effect

was

linked

to

the

gravitational

cues

produced

by

the

shoulder

hand

system

Indeed

the

need

to

relatively

move

massive

parts

of

the

body

such

as

the

arm

and

forearm

during

the

manual

exploration

of

a

stimulus

brings

antigravity

forces

into

play

The

role

that

gravity

might

play

in

the

haptic

system

is

clearly

reduced

in

the

visual

system

The

shoulder

hand

system

is

much

more

directly

submitted

to

gravitational

constraints

than

the

ocular

system

When

it

explores

a

stimulus

it

has

to

produce

antigravity

forces

and

these

in

return

provide

what

are

known

as

gravitational

cues

These

gravitational

cues

provided

by

the

proprioceptive

system

result

from

the

deformation

of

cutaneous

muscular

and

articular

tissues

and

depend

on

the

specific

muscular

forces

needed

to

maintain

or

displace

the

shoulder

hand

system

against

gravity

-LRB-

Gentaz

and

Hatwell

-RRB-

As

such

these

cues

can

provide

a

considerable

amount

of

information

on

arm

posture

or

motion

relative

to

some

fixed

geocentric

frame

of

reference

-LRB-

Paillard

-RRB-

that

is

linked

to

gravity

and

exterior

to

the

body

This

hypothesis

was

justified

by

an

analysis

of

the

task

conditions

proposed

in

the

previous

experiments

Indeed

the

size

of

the

stimulus

rod

-LRB-

cm

-RRB-

and

its

position

in

relation

to

the

participant

-LRB-

cm

-RRB-

required

significant

participation

of

the

shoulder

hand

system

during

exploration

To

explain

the

plane

effect

in

the

ipsilateral

reproduction

conditions

observed

above

-LRB-

i.e.

the

presence

of

an

oblique

effect

in

the

frontal

and

sagittal

planes

and

its

absence

in

the

horizontal

plane

-RRB-

Gentaz

and

Hatwell

-LRB-

-RRB-

hypothesized

that

participants

favored

gravitational

cues

in

orientation

processing

because

these

cues

specify

the

gravitational

vertical

which

is

used

as

a

reference

axis

The

characteristics

of

these

gravitational

cues

would

differ

according

to

exploratory

conditions

In

a

normal

situation

their

average

amplitude

is

practically

the

same

in

all

three

planes

since

the

arm

is

always

influenced

by

the

same

gravitational

forces

On

the

other

hand

the

variability

of

these

cues

is

not

the

same

depending

on

the

plane

the

experiment

is

carried

out

in

There

is

a

high

variability

in

the

frontal

and

sagittal

planes

as

the

arm

deploys

considerable

antigravitational

forces

when

moving

from

bottom

to

top

but

falls

from

top

to

bottom

with

little

effort

On

the

contrary

in

the

horizontal

plane

gravitational

cues

hardly

vary

during

exploration

as

the

arm

constantly

operates

in

a

direction

perpendicular

to

gravity

Gentaz

and

Hatwell

-LRB-

-RRB-

tested

the

hypothesis

that

the

gravitational

cues

produced

by

the

shoulder

hand

system

during

exploration

were

involved

in

the

oblique

effect

and

in

the

plane

effect

in

the

ipsilateral

reproduction

condition

The

amplitude

of

these

cues

was

modified

by

manipulating

the

level

of

gravitational

constraint

and

variability

was

modified

by

changing

the

plane

the

task

was

effectuated

in

The

oblique

effect

and

the

plane

effect

should

diminish

in

conditions

where

gravitational

cues

are

weak

and

increase

in

those

where

these

cues

are

greater

In

one

experiment

only

the

horizontal

plane

was

tested

Participants

explored

the

orientation

of

a

rod

with

one

hand

and

reproduced

it

with

the

same

hand

either

keeping

their

forearm

wrist

hand

in

the

air

-LRB-

unsupported

forearm

condition

-RRB-

or

resting

their

forearm

wrist

hand

on

the

rod

supporting

surface

-LRB-

supported

forearm

condition

-RRB-

In

the

first

condition

antigravitational

forces

were

necessarily

produced

whereas

these

forces

were

very

much

reduced

in

the

second

condition

Absolute

error

analysis

revealed

that

the

oblique

effect

was

absent

in

the

supported

forearm

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

and

was

present

in

the

unsupported

forearm

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

In

another

experiment

blindfolded

adults

were

tested

in

one

of

the

three

planes

either

in

a

natural

condition

or

in

a

lightened

condition

in

which

gravitational

cues

had

been

reduced

by

a

pulley

system

In

the

latter

case

the

participant

's

forearm

was

attached

to

a

display

which

held

it

balanced

in

the

air

when

the

weight

suspended

on

the

pulley

reached

a

certain

value

variable

from

one

participant

to

another

The

amplitude

of

the

oblique

effect

was

lower

in

the

lightened

condition

-LRB-

significant

difference

of

-RRB-

than

that

in

the

natural

condition

-LRB-

significant

difference

of

-RRB-

and

this

is

because

that

the

accuracy

of

vertical

and

horizontal

reproduction

was

deteriorated

-LRB-

thus

increasing

the

difference

with

that

of

the

oblique

orientations

which

remained

stable

-RRB-

In

addition

the

exploratory

plane

had

no

effect

Taken

together

these

results

revealed

that

gravitational

cues

are

crucial

when

the

haptic

oblique

effect

is

present

in

the

three

planes

in

blindfolded

participants

and

in

the

ipsilateral

reproduction

condition

Role

of

previous

visual

experience

and

memory

in

the

haptic

oblique

effect

The

previous

results

do

not

however

exclude

an

influence

of

high-level

factors

such

as

visual

experience

or

cognitive

resources

or

both

As

a

matter

of

fact

it

is

well

known

that

the

haptic

perception

of

the

sighted

working

in

a

nonvisual

condition

can

be

fed

by

visual

representations

-LRB-

Hatwell

Thinus-Blanc

and

Gaunet

-RRB-

Gentaz

and

Hatwell

-LRB-

-RRB-

examined

whether

the

participant

's

visual

experience

modified

the

action

of

the

gravitational

cues

observed

in

sighted

people

In

this

study

blindfolded

sighted

participants

as

well

as

early

and

late

totally

blind

people

were

asked

to

explore

a

rod

and

to

reproduce

its

orientation

ipsilaterally

The

magnitude

of

gravitational

cues

was

modified

by

manipulating

the

level

of

gravitational

constraints

-LRB-

natural

and

reduced

-RRB-

and

their

variability

was

modified

by

changing

the

task

plane

horizontal

-LRB-

weak

variability

-RRB-

and

frontal

-LRB-

strong

variability

-RRB-

By

comparing

the

performances

of

the

early

and

late

totally

blind

this

study

aimed

at

evaluating

the

role

played

by

visual

experience

and

visualization

in

the

haptic

oblique

effect

If

such

is

the

case

the

oblique

effect

should

be

present

in

the

late

blind

and

absent

in

the

early

blind

The

comparison

between

blindfolded

sighted

and

totally

blind

people

who

are

better-trained

in

the

use

of

the

haptic

system

also

allowed

the

authors

to

find

out

whether

gravitational

cues

also

played

a

role

in

the

apparition

of

the

haptic

oblique

effect

in

the

blind

as

is

the

case

with

blindfolded

persons

In

this

case

the

oblique

effect

should

be

absent

when

cues

are

reduced

and

present

when

they

are

normal

Absolute

error

analysis

showed

that

in

the

horizontal

plane

the

oblique

effect

was

absent

in

both

the

early

and

late

blind

regardless

of

gravitational

constraints

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

In

the

frontal

plane

the

oblique

effect

was

present

in

both

groups

of

blind

people

regardless

of

gravitational

constraints

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

Furthermore

no

difference

was

observed

between

the

early

and

the

late

blind

In

conclusion

these

results

confirmed

that

the

variability

of

gravitational

cues

plays

an

important

role

in

the

presence

or

absence

of

the

haptic

oblique

effect

in

the

totally

blind

although

no

visual

experience

effect

was

observed

-LSB-

first

factor

of

Appelle

and

Countryman

's

-LRB-

-RRB-

hypothesis

-RSB-

Gentaz

and

Hatwell

-LRB-

-RRB-

examined

further

the

role

of

attentional

and

cognitive

resources

on

the

haptic

perception

of

orientations

and

the

oblique

effect

by

increasing

memorization

constraints

We

know

that

the

characteristics

of

the

haptic

system

differ

in

fundamental

ways

from

those

of

the

visual

system

The

limited

size

of

the

tactile

perceptual

field

requires

that

it

be

moved

to

gain

knowledge

about

for

example

the

shape

of

an

object

In

contrast

eye

movements

play

only

a

secondary

role

in

the

visual

perception

of

the

position

or

shape

of

an

object

since

both

pieces

of

information

can

be

extracted

from

static

images

This

difference

was

recently

re-emphasized

by

Henriques

and

Soechting

-LRB-

-RRB-

who

argued

in

a

recent

review

that

the

processing

of

haptic

information

differs

fundamentally

from

visual

processing

in

that

the

former

requires

the

integration

of

information

that

evolves

in

time

as

well

as

space

It

results

therefore

from

this

quality

of

proximal

reception

of

the

haptic

system

that

haptic

processing

and

haptic

perception

are

consistently

more

sequential

than

visual

ones

Moreover

haptic

perception

needs

to

rely

more

heavily

on

working

memory

to

achieve

the

mental

synthesis

that

is

necessary

to

gain

an

unified

representation

of

the

object

-LRB-

Revesz

cf.

Hatwell

et

al.

-RRB-

In

all

previous

research

on

the

haptic

perception

of

orientations

the

delay

between

the

stimulus

exploration

phase

and

the

reproduction

phase

was

long

and

was

unoccupied

In

Gentaz

and

Hatwell

-LRB-

-RRB-

memorization

conditions

were

thus

altered

the

length

and

the

nature

of

the

gap

was

changed

in

the

two

exploratory

conditions

showing

that

the

availability

of

gravitational

cues

affected

orientation

coding

Participants

explored

a

rod

in

the

horizontal

plane

with

weak

or

natural

gravitational

cues

and

reproduced

the

rod

's

orientation

ipsilaterally

The

orientations

were

reproduced

according

to

one

of

four

memorization

conditions

after

unoccupied

gaps

or

gaps

and

gaps

occupied

by

interfering

verbal

or

motor

activities

such

as

reciting

the

alphabet

forwards

or

backwards

-LRB-

verbal

-RRB-

or

haptically

exploring

a

raised

sinuous

trajectory

-LRB-

motor

-RRB-

Absolute

error

analysis

showed

that

when

the

gap

was

unoccupied

-LRB-

regardless

of

how

long

it

was

-RRB-

haptic

orientation

treatment

and

the

oblique

effect

depended

on

the

exploratory

conditions

the

oblique

effect

was

absent

when

gravitational

cues

were

weak

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

and

present

when

they

were

natural

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

like

in

Gentaz

and

Hatwell

-LRB-

-RRB-

On

the

other

hand

when

the

gap

was

occupied

by

interfering

verbal

or

haptic

activities

the

haptic

oblique

effect

was

present

in

both

experiments

and

regardless

of

available

gravitational

cues

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

Taken

together

these

results

showed

that

high-level

factors

can

indeed

determine

the

presence

of

the

haptic

oblique

effect

since

the

reduction

of

available

attentional

or

cognitive

resources

by

an

interfering

task

caused

an

oblique

effect

in

a

condition

in

which

it

was

not

usually

observed

In

conclusion

the

presence

of

a

haptic

oblique

effect

in

blindfolded

sighted

participants

as

well

as

in

the

totally

early

blind

with

natural

exploration

and

ipsilateral

reproduction

in

the

frontal

plane

-LRB-

regardless

of

gravitational

constraints

-RRB-

invalidates

Appelle

and

Countryman

's

-LRB-

-RRB-

hypothesis

about

the

role

of

visual

representations

and

shows

the

existence

under

very

precise

conditions

of

an

oblique

effect

intrinsic

to

the

haptic

system

-LRB-

Table

-RRB-

Table

absolute

errors

in

the

studies

of

the

haptic

oblique

effect

with

blindfolded

adultsExperimental

conditionsOrientationsOblique

effectReferencesTasksPlaneDelay

-LRB-

s

-RRB-

HandV/HObliqueDifferenceReproduction

task

Free-time

exploration

and

haptically

informed

about

standard

Lechelt

et

al.

-LRB-

-RRB-

Reproduction

task

Limited-time

exploration

-LRB-

s

-RRB-

and

haptically

informed

about

standard

Lechelt

and

Verenka

-LRB-

-RRB-

Production

task

Free-time

production

and

verbally

informed

about

Appelle

and

Gravetter

-LRB-

-RRB-

Free-time

and

haptically

informed

about

Reproduction

task

Appelle

and

Countryman

-LRB-

-RRB-

Reproduction

task

Free

exploration

and

uninformed

about

Gentaz

and

Hatwell

-LRB-

-RRB-

Supported

Symmetric

Symmetric

Reproduction

task

Unsupported

Gentaz

and

Hatwell

-LRB-

-RRB-

Supported

Normal

gravity

Reduced

gravity

Reproduction

task

Early

and

late

blind

Pooled

Gentaz

and

Hatwell

-LRB-

-RRB-

Reproduction

task

Normal

unoccupied

Gentaz

and

Hatwell

-LRB-

-RRB-

Reduced

unoccupied

Normal

unoccupied

Reduced

unoccupied

Normal

verbal

Reduced

verbal

Normal

motor

Reduced

motor

Reproduction

task

Luyat

et

al.

-LRB-

-RRB-

Body

Production

task

No

context

Luyat

et

al.

-LRB-

b

-RRB-

Congruent

haptic

Incongruent

haptic

The

table

reports

the

average

absolute

error

for

the

vertical

and

horizontal

orientations

-LRB-

V/H

column

-RRB-

for

the

and

orientations

-LRB-

Oblique

column

-RRB-

as

well

as

the

existence

of

an

oblique

effect

that

is

of

statistically

significant

difference

between

the

two

sets

of

orientation

-LRB-

marked

by

in

the

Difference

column

-RRB-

as

function

of

experimental

conditions

The

reported

studies

mainly

used

a

reproduction

paradigm

where

the

subject

explored

the

orientation

of

a

rod

with

one

hand

before

to

be

reproduced

with

the

same

-LRB-

ipsilateral

condition

-RRB-

or

other

hand

-LRB-

contralateral

condition

see

Hand

column

-RRB-

The

Plane

and

Delay

column

specify

the

plane

in

which

the

orientations

were

presented

and

reproduced

or

produced

as

well

as

the

delay

between

the

presentation

and

reproduction

The

first

column

specifies

additional

experimental

factors

that

were

manipulated

in

the

corresponding

studies

-LRB-

see

text

-RRB-

Haptic

orientations

defined

in

a

subjective

reference

frame

As

previously

indicated

the

concept

of

orientation

is

by

definition

relative

to

one

or

more

systems

of

coordinates

with

the

cardinal

vertical

and

horizontal

orientations

as

norms

of

this

system

However

on

Earth

different

verticals

-LRB-

gravitational

egocentric

-RRB-

which

can

constitute

the

norms

for

independent

references

exist

Thus

the

existence

of

an

oblique

effect

raises

the

question

of

which

kind

of

vertical

is

better

perceived

or

in

other

words

in

which

reference

frame

this

effect

is

defined

A

classical

broad

distinction

is

made

between

egocentric

-LRB-

referred

to

the

participant

's

body

-RRB-

and

allocentric

-LRB-

referred

to

environmental

cues

-RRB-

spatial

frames

-LRB-

Howard

Rock

-RRB-

The

allocentric

reference

frame

can

be

divided

into

a

gravitational

frame

defined

by

the

direction

of

the

gravity

pull

-LRB-

geocentric

frame

-RRB-

and

in

pattern-centric

reference

frames

defined

by

-LRB-

visual

or

haptic

or

both

-RRB-

contextual

cues

In

natural

conditions

the

different

reference

frames

are

in

general

congruent

Consequently

mapping

of

orientations

could

result

from

either

egocentric

or

allocentric

reference

frames

or

most

likely

from

both

In

darkness

tilting

the

body

uncouples

the

gravitational

and

the

egocentric

reference

frames

and

allows

us

to

specify

in

which

kind

of

reference

frame

orientations

are

defined

Subjective

reference

frame

In

vision

certain

studies

have

evidenced

rather

an

egocentric

-LRB-

retinocentric

-RRB-

coding

-LRB-

Banks

and

Stolartz

Chen

and

Levi

Corwin

et

al.

-RRB-

suggesting

that

the

visual

oblique

effect

could

principally

be

accounted

for

by

the

properties

of

the

orientation-selective

neurons

present

in

the

primary

visual

cortex

-LRB-

see

Introduction

-RRB-

Indeed

neurons

tuned

to

vertical

and

horizontal

retinal

orientations

are

more

numerous

and

feature

particular

response

characteristics

in

relation

to

those

tuned

to

oblique

orientations

On

the

other

hand

other

studies

have

revealed

the

existence

of

gravitational

orientation

coding

implying

that

the

origin

of

the

oblique

effect

is

probably

more

central

involving

extraretinal

information

such

as

vestibular

and

somesthetic

cues

integrated

at

a

higher

level

in

the

visual

processing

hierarchy

-LRB-

Attneave

and

Olson

Buchanan-Smith

and

Heeley

Ferrante

et

al.

Lipshits

and

McIntyre

-RRB-

However

very

little

research

has

been

done

concerning

this

topic

in

other

perceptual

modalities

In

haptics

Luyat

et

al.

-LRB-

-RRB-

tested

the

effect

of

tilting

the

body

or

the

head

on

the

haptic

oblique

effect

Blindfolded

participants

explored

a

standard

rod

in

the

frontal

plane

and

reproduced

its

orientation

with

the

same

hand

after

a

delay

Three

conditions

were

examined

upright

inclined

to

the

right

-LRB-

-RRB-

and

to

the

left

-LRB-

-RRB-

Five

orientations

were

tested

vertical

horizontal

oblique

oblique

and

the

subjective

vertical

-LRB-

SV

-RRB-

The

SV

is

the

individual

's

perception

of

the

direction

of

gravitational

force

-LRB-

cf.

Howard

Luyat

-RRB-

The

most

commonly

used

paradigm

consists

of

adjusting

a

stimulus

-LRB-

a

rod

-RRB-

to

the

physical

gravitational

vertical

The

deviation

from

the

gravitational

direction

constituted

a

measure

of

the

subjective

vertical

In

upright

posture

the

SV

is

very

close

to

the

physical

vertical

in

participants

with

no

vestibular

disease

or

parietal

lesion

However

in

darkness

lateral

head

or

body

tilt

provokes

systematic

deviations

of

the

haptic

SV

in

the

opposite

direction

to

that

of

the

head

the

Müller

effect

A

first

experimental

phase

estimated

each

participant

's

SV

in

each

postural

condition

and

this

SV

was

then

tested

in

the

exploration

reproduction

task

What

is

more

as

an

insurance

against

a

possible

fluctuation

of

the

SV

in

the

haptic

perception

of

orientation

the

SV

was

measured

again

after

the

exploration

reproduction

task

If

the

haptic

oblique

effect

is

defined

in

a

gravitational

reference

frame

then

the

reproduction

of

the

vertical

and

the

horizontal

should

be

more

accurate

than

the

reproduction

of

oblique

orientations

regardless

of

postural

conditions

On

the

other

hand

if

the

haptic

oblique

effect

is

defined

in

an

egocentric

reference

frame

then

the

reproduction

of

the

two

diagonals

-LRB-

and

-RRB-

which

are

parallel

and

perpendicular

to

the

body

tilted

to

or

should

be

more

accurate

than

the

reproduction

of

the

gravitational

vertical

and

the

horizontal

which

become

oblique

in

relation

to

the

body

Finally

knowing

the

effect

of

tilting

the

body

on

the

vertical

the

SV

could

constitute

a

reference

axis

when

the

body

is

tilted

In

this

case

the

reproduction

of

the

SV

should

be

more

accurate

than

all

the

other

orientations

in

the

tilted

body

conditions

The

results

observed

in

the

production

task

in

which

the

participants

had

to

manually

orient

the

bar

in

the

direction

that

they

thought

was

vertical

-LRB-

phase

i.e.

before

the

reproduction

task

and

phase

i.e.

after

the

reproduction

task

-RRB-

showed

systematic

deviations

of

the

haptic

SV

in

the

direction

opposite

to

that

of

the

tilted

body

-LRB-

Müller

effect

-RRB-

The

comparison

of

phases

and

showed

that

SV

perception

was

stable

and

faithful

over

time

thus

justifying

its

study

in

the

exploration

reproduction

task

Absolute

error

analysis

in

the

exploration

reproduction

task

showed

that

tilting

the

body

affects

the

precision

of

reproduction

particularly

of

the

gravitational

vertical

and

the

horizontal

Thus

an

oblique

effect

was

present

in

the

upright

condition

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

but

absent

in

tilted

conditions

-LRB-

vertical

horizontal

M

and

obliques

M

-RRB-

It

should

be

noted

that

the

same

results

were

observed

in

a

complementary

experiment

in

which

only

the

head

was

tilted

In

conclusion

tilting

affected

vertical

perception

in

the

production

task

and

also

in

orientation

reproduction

This

means

that

in

a

tilted

body

condition

the

gravitational

vertical

and

horizontal

orientations

no

longer

appear

to

act

as

reference

norms

as

they

were

not

better

reproduced

than

the

oblique

orientations

Consequently

the

hypothesis

of

a

pure

gravitational

reference

frame

underlying

the

haptic

oblique

effect

can

be

rejected

In

the

same

way

the

results

did

not

confirm

the

hypothesis

of

a

pure

egocentric

reference

frame

as

-LRB-

or

-RRB-

oblique

orientations

-LRB-

which

are

vertical

or

horizontal

in

relation

to

the

body

-RRB-

were

not

better

reproduced

than

the

gravitational

vertical

and

the

horizontal

On

the

other

hand

results

showed

that

the

SV

was

better

reproduced

than

the

other

four

orientations

particularly

in

the

tilted

body

conditions

-LRB-

M

-RRB-

In

these

conditions

the

SV

could

constitute

a

norm

for

a

subjective

vertical

reference

frame

Finally

these

results

questioned

the

nature

of

the

processes

responsible

for

the

oblique

effect

across

sensory

modalities

-LRB-

Hatwell

Thinus-Blanc

and

Gaunet

-RRB-

Several

studies

in

the

spatial

domain

show

that

visual

representations

largely

feed

tactile

representations

and

vice

versa

Thus

Luyat

and

Gentaz

-LRB-

-RRB-

examined

whether

the

visual

oblique

effect

is

also

coded

in

a

subjective

gravitational

reference

frame

analogous

to

that

used

in

haptics

The

effect

of

inclining

the

whole

body

on

the

visual

oblique

effect

was

studied

with

the

same

paradigm

as

that

proposed

in

haptics

The

classic

oblique

effect

observed

in

the

upright

body

condition

disappeared

in

the

inclined

body

conditions

because

of

a

reduction

in

the

accuracy

of

the

reproduction

of

the

gravitational

vertical

and

the

horizontal

In

inclined

conditions

the

subjective

vertical

appears

to

be

the

best

perceived

orientation

Thus

the

visual

oblique

effect

seems

also

to

be

defined

within

a

subjective

vertical

reference

frame

However

results

in

the

production

task

showed

that

inclining

the

body

produces

systematic

deviations

to

the

visual

subjective

vertical

-LRB-

VSV

-RRB-

in

the

direction

of

the

body

incline

axis

whereas

deviation

is

in

the

direction

opposite

to

body

inclined

in

the

haptic

modality

In

another

study

Gentaz

et

al.

-LRB-

-RRB-

examined

the

oblique

effects

in

the

visual

haptic

and

somato-vestibular

systems

by

asking

adults

to

reproduce

-LRB-

after

a

delay

-RRB-

an

orientation

presented

in

the

frontal

plane

In

the

visual

modality

participants

reproduced

the

orientation

of

a

luminous

rod

presented

in

a

dark

room

In

the

haptic

modality

blindfolded

participants

explored

a

rod

with

one

hand

and

reproduced

its

orientation

ipsilaterally

In

the

somato-vestibular

modality

blindfolded

participants

reproduced

the

incline

of

their

body

Results

revealed

similar

oblique

effects

in

the

three

tasks

However

no

significant

correlation

between

the

magnitude

of

the

visual

haptic

and

somato-vestibular

oblique

effects

was

observed

Taken

together

these

results

suggest

the

existence

of

both

processes

specific

to

the

haptic

system

-LRB-

different

subjective

reference

frames

in

haptics

and

in

vision

-RRB-

and

processes

similar

to

the

corresponding

visual

processes

-LRB-

access

to

a

subjective

reference

frame

-RRB-

but

not

common

across

sensory

modalities

-LRB-

absence

of

correlations

between

modalities

-RRB-

Let

us

now

look

at

the

effect

of

contextual

cues

on

the

haptic

perception

of

orientation

in

adults

by

comparing

it

to

the

corresponding

visual

one

Haptic

contextual

reference

frame

In

previous

experiments

the

stimulus

consisted

of

a

single

rod

without

any

contextual

cues

around

it

In

this

case

it

was

not

possible

to

determine

whether

the

orientation

perception

in

natural

conditions

could

also

be

coded

-LRB-

at

least

partially

-RRB-

in

a

pattern-centric

reference

frame

-LRB-

defined

by

contextual

cues

-RRB-

In

vision

it

is

well

known

that

spatial

orientation

is

enhanced

when

vertical

contextual

cues

are

available

and

as

a

consequence

it

is

strongly

affected

by

roll

or

pitch

of

single

tilted

lines

or

by

a

more

structured

context

such

as

a

room

-LRB-

Groen

et

al.

Luyat

-RRB-

Recently

Luyat

et

al.

-LRB-

-RRB-

showed

that

the

oblique

effect

decreased

with

visual

contextual

cues

tilted

Another

characteristic

of

the

haptic

system

is

that

the

shape

and

size

of

the

tactile

perceptual

field

can

vary

according

to

the

mode

of

exploration

adopted

by

the

observer

-LRB-

e.g.

with

one

or

several

fingers

-RRB-

This

observation

stands

in

sharp

contrast

with

the

visual

system

where

the

shape

and

size

of

visual

field

are

essentially

invariant

The

ability

of

the

haptic

system

to

perceive

contextual

cues

might

vary

drastically

in

accordance

with

the

number

of

contacts

that

are

established

with

the

environment

For

example

two-handed

exploration

might

enlarge

the

haptic

perceptual

field

sufficiently

to

facilitate

the

perception

of

contextual

cues

-LRB-

on

the

role

of

reference

cues

cf.

Millar

Millar

and

Al-Attar

-RRB-

The

haptic

perceptual

field

remains

greatly

reduced

compared

to

the

visual

field

even

when

the

two

hands

are

used

with

active

exploratory

movements

As

a

result

the

perceptual

cues

which

are

relevant

in

a

task

-LRB-

or

useful

as

a

spatial

exocentric

reference

frame

-RRB-

are

less

available

in

the

haptic

than

in

the

visual

modalities

Moreover

voluntary

movements

must

be

made

to

compensate

for

the

smallness

of

the

haptic

perceptual

field

As

a

result

haptic

perception

is

highly

sequential

-LRB-

cf.

Introduction

-RRB-

However

this

latter

property

permits

contextual

effects

to

be

reduced

in

haptic

perception

compared

to

visual

perception

Indeed

it

is

not

the

case

in

vision

because

of

the

presence

of

simultaneous

peripheral

-LRB-

contextual

cues

-RRB-

and

central

-LRB-

the

target

-RRB-

stimulation

The

effect

of

contextual

cues

on

the

perception

of

orientations

was

investigated

first

by

Walker

-LRB-

-RRB-

in

a

short

report

in

which

the

participant

had

to

position

a

rod

to

the

vertical

while

passing

the

right

hand

through

a

tilted

grid

This

author

reported

an

effect

but

unfortunately

both

precise

data

and

important

methodological

aspects

as

the

amount

of

tilt

used

were

not

provided

More

recently

Luyat

et

al.

-LRB-

-RRB-

studied

this

question

by

asking

blindfolded

participants

to

position

a

rod

with

their

right

dominant

hand

to

three

different

spatial

orientations

in

their

fronto-parallel

plane

to

the

vertical

and

along

two

oblique

orientations

respectively

at

to

the

left

and

to

the

right

of

the

vertical

one

This

production

task

was

carried

out

in

four

different

backgrounds

-LRB-

-RRB-

in

the

absence

of

contextual

cues

-LRB-

smooth

-RRB-

-LRB-

-RRB-

in

presence

of

congruent

contextual

cues

-LRB-

stripes

parallel

to

the

orientation

to

be

produced

-RRB-

-LRB-

-RRB-

in

presence

of

tilted

stripes

context

to

the

left

and

-LRB-

-RRB-

in

presence

of

tilted

stripes

context

to

the

right

of

the

orientation

to

be

produced

Using

at

the

same

time

the

right

and

left

hands

with

one

hand

exploring

the

context

and

the

other

-LRB-

dominant

hand

-RRB-

adjusting

the

stimulus-rod

to

the

expected

orientation

allows

a

simultaneous

stimulation

by

both

the

context

and

the

stimulus

The

analysis

of

the

precision

of

adjustments

-LRB-

variable

errors

-RRB-

revealed

that

the

oblique

effect

obtained

in

the

control

condition

-LRB-

no

context

-RRB-

-LRB-

vertical

M

and

obliques

M

-RRB-

was

similar

to

that

obtained

previously

by

Gentaz

et

al.

-LRB-

-RRB-

with

a

similar

method

of

direct

estimation

of

orientations

-LRB-

production

task

-RRB-

Indeed

most

experiments

on

the

haptic

oblique

effect

have

been

carried

out

with

a

reproduction

task

which

requires

memorizing

an

unknown

but

previously

scanned

orientation

This

experiment

confirmed

that

a

haptic

oblique

effect

in

the

fronto-parallel

plane

can

be

evidenced

with

a

production

task

which

involves

a

cognitive

representation

of

orientations

probably

based

on

an

internal

model

of

gravity

More

interesting

is

the

fact

that

tactile

contextual

cues

had

an

influence

on

the

production

of

oblique

and

vertical

orientations

particularly

on

the

oblique

effect

When

the

context

was

congruent

with

the

standard

orientations

the

precision

of

adjustments

was

significantly

improved

-LRB-

vertical

M

and

obliques

M

-RRB-

an

effect

very

similar

to

the

effect

exerted

by

visual

vertical

cues

on

spatial

orientation

During

noncongruent

context

scanning

the

precision

of

orientations

particularly

the

obliques

also

tended

to

be

enhanced

and

as

a

consequence

the

oblique

effect

was

significantly

weakened

However

this

improvement

does

not

mean

that

the

accuracy

was

enhanced

since

analysis

on

constant

error

revealed

that

deviations

erred

in

the

direction

of

the

stripes

Thus

the

presence

of

a

context

congruent

or

not

diminishes

the

variability

of

estimations

and

therefore

enhances

the

precision

of

the

production

of

orientations

Furthermore

the

mismatch

between

the

orientation

to

be

produced

and

the

context

-LRB-

noncongruent

condition

-RRB-

revealed

a

tendency

to

adjust

the

rod

in

the

direction

of

this

context

In

conclusion

these

results

suggest

that

the

haptic

oblique

effect

is

defined

in

a

subjective

reference

frame

which

may

integrate

not

only

vestibular

and

somesthetic

inputs

-LRB-

head

or

whole

body

tilt

-RRB-

but

also

haptic

contextual

cues

according

to

task

conditions

Moreover

it

should

be

emphasized

that

tasks

such

as

pointing

to

remembered

visual

targets

have

also

been

found

to

involve

processing

information

in

multiple

reference

frames

-LRB-

see

Carrozzo

et

al.

McIntyre

et

al.

b

-RRB-

We

propose

that

the

subjective

vertical

and

the

subjective

horizontal

-LRB-

probably

-RRB-

constitute

the

norms

of

this

subjective

vertical

reference

frame

At

the

neurophysiological

level

the

questions

of

how

and

where

vestibular

or

contextual

information

dedicated

to

spatial

cognition

-LRB-

specifically

to

the

subjective

perception

of

what

is

up

-RRB-

is

processed

in

the

cortex

is

far

from

perfectly

known

Research

in

humans

with

functional

magnetic

resonance

imaging

-LRB-

fMRI

-RRB-

-LRB-

Fasold

et

al.

-RRB-

or

electrical

stimulation

in

patients

with

epilepsy

-LRB-

Kahane

et

al.

-RRB-

have

shown

several

vestibular

cortex

areas

in

particular

insulo-parietal

-LRB-

analogous

to

the

Parieto-Insular

Vestibular

Cortex

in

monkeys

-RRB-

and

temporal

areas

with

a

right

hemispheric

dominance

The

integration

of

gravitational

and

contextual

cues

in

the

oblique

effect

suggests

that

these

associative

areas

with

multimodal

neurons

could

be

implicated

A

last

question

raised

in

current

haptic

studies

is

the

presence

of

the

haptic

oblique

effect

in

space

in

adults

Haptic

orientation

in

space

In

all

previous

experiments

we

considered

the

haptic

perception

of

orientations

on

a

plane

rather

than

in

space

An

orientation

in

space

is

defined

by

two

parameters

such

as

its

azimuth

and

elevation

This

observation

raises

the

questions

of

whether

the

haptic

oblique

effect

is

still

present

in

the

absence

of

any

spatial

constraint

when

the

participant

needs

to

focus

on

at

least

two

independent

parameters

to

perform

the

task

More

importantly

given

the

fact

that

there

is

a

much

larger

set

of

alternative

reference

frames

which

can

be

used

to

code

an

orientation

in

space

than

on

a

plane

the

perception

of

orientations

in

space

raises

the

question

of

which

reference

frame

is

most

adequate

to

describe

the

pattern

of

errors

To

examine

this

question

Baud-Bovy

and

Gentaz

-LRB-

-RRB-

used

a

haptic

display

to

present

the

stimuli

and

record

the

participant

's

hand

movements

to

remove

if

desired

the

usual

planar

constraint

imposed

by

the

experimental

apparatus

used

in

most

prior

haptic

studies

In

a

first

experiment

the

haptic

reproduction

of

vertical

horizontal

and

diagonal

orientations

on

a

plane

and

in

space

was

compared

Thus

participants

were

asked

to

explore

the

orientation

of

a

virtual

rod

with

to-and-fro

movements

on

the

frontal

plane

and

then

they

were

asked

to

respond

in

two

different

conditions

In

the

reproduction

condition

the

finger

movements

were

constrained

to

the

frontal

plane

like

in

the

exploration

phase

In

the

reproduction

condition

the

movements

of

the

finger

were

unconstrained

and

could

move

freely

in

space

The

first

finding

was

that

the

vertical

and

horizontal

orientations

were

better

reproduced

than

the

diagonal

orientations

in

the

condition

Thus

removing

the

constraint

during

the

reproduction

phase

did

not

prevent

the

occurrence

of

the

haptic

oblique

effect

Decomposing

the

angular

error

in

the

condition

in

an

in-plane

component

and

in

an

out-of-plane

component

revealed

that

the

anisotropy

in

this

condition

concerned

mostly

the

in-plane

component

As

a

matter

of

fact

the

error

pattern

in

the

condition

was

very

similar

to

the

in-plane

component

of

the

condition

while

the

analysis

of

out-plane

errors

showed

that

their

distribution

was

much

more

uniform

across

orientations

This

finding

suggests

first

that

an

orientation

in

space

is

coded

by

two

parameters

-LRB-

i.e.

the

angle

inside

the

frontal

plane

and

the

angle

between

target

orientation

and

the

frontal

plane

-RRB-

and

second

that

these

two

parameters

are

processed

independently

To

examine

how

orientations

are

coded

in

space

in

the

absence

of

any

sort

of

reference

to

a

plane

Baud-Bovy

and

Gentaz

-LRB-

-RRB-

tested

thirteen

target

orientations

which

belonged

to

different

planes

-LRB-

three

principal

axes

vertical

sagittal

and

lateral

axes

the

two

and

diagonals

in

the

frontal

sagittal

and

horizontal

plane

respectively

and

four

diagonals

-RRB-

As

previously

blindfolded

participants

were

asked

to

explore

the

orientation

of

a

virtual

rod

with

to-and-fro

movements

and

to

respond

in

the

condition

i.e.

without

any

constraint

on

the

finger

movements

during

the

reproduction

phase

The

haptic

processing

of

orientations

was

clearly

anisotropic

The

vertical

orientation

was

reproduced

most

precisely

Once

again

the

accuracy

and

consistency

with

which

participants

reproduced

this

orientation

was

striking

For

the

other

orientations

the

error

pattern

depended

in

a

complex

manner

on

participants

stimulus

and

error

components

which

makes

its

interpretation

complex

On

the

one

hand

the

three

principal

axes

were

most

accurately

reproduced

when

all

error

components

were

pooled

together

This

observation

has

a

straightforward

consequence

in

a

plane-by-plane

analysis

as

it

implies

that

the

horizontal

and

vertical

axes

in

the

frontal

and

sagittal

planes

as

well

as

the

lateral

and

sagittal

axes

in

the

horizontal

plane

are

more

accurately

reproduced

than

the

diagonal

orientations

In

other

words

this

observation

is

akin

to

a

demonstration

of

the

classic

oblique

effect

in

each

one

of

these

planes

Regarding

the

frontal

plane

the

results

of

this

experiment

were

similar

to

those

of

the

previous

experiment

The

analysis

of

the

different

error

components

showed

a

clear

oblique

effect

in

both

experiments

with

the

vertical

and

horizontal

orientations

being

most

accurately

reproduced

even

in

the

absence

of

any

planar

structure

in

the

design

of

the

experiment

and

of

any

constraint

during

the

reproduction

phase

On

the

other

hand

the

presence

of

the

classical

oblique

effect

was

much

less

obvious

when

we

considered

the

various

error

components

separately

The

average

angular

error

inside

the

horizontal

and

sagittal

planes

did

not

exhibit

the

expected

pattern

Decomposing

the

in-plane

errors

in

systematic

intersubject

and

intrasubject

error

components

confirmed

this

negative

finding

Further

studies

are

needed

to

understand

these

complex

patterns

of

results

observed

in

space

Let

us

now

look

at

the

ontogenetic

development

of

the

haptic

oblique

effect

by

comparing

it

to

the

corresponding

visual

one

Developmental

aspect

of

the

oblique

effects

The

oblique

effects

in

children

and

infancy

In

vision

Leehey

et

al.

-LRB-

-RRB-

tested

the

oblique

effect

in

infants

aged

from

to

weeks

using

contrast

sensitivity

The

stimulus

was

a

circular

grating

consisting

of

light

and

dark

bars

uniformly

alternated

Bars

were

of

five

different

widths

When

spatial

frequency

was

above

threshold

discrimination

the

pattern

appeared

contrasted

whereas

it

looked

like

a

homogeneous

gray

field

when

spatial

frequency

was

below

threshold

Two

gratings

of

the

same

spatial

frequency

differing

in

orientations

were

simultaneously

proposed

Each

trial

consisted

of

an

oblique

grating

-LRB-

or

-RRB-

and

a

vertical

or

a

horizontal

grating

presented

in

a

random

fashion

As

infants

looked

longer

at

the

more

contrasted

of

two

patterns

a

forced-choice

judgment

was

used

to

determine

which

grating

the

young

subject

preferred

Given

that

the

two

stimuli

have

the

same

spatial

frequency

an

oblique

effect

would

be

present

if

for

a

specific

bar

width

infants

could

discriminate

the

vertical

or

the

horizontal

lines

but

not

the

oblique

lines

For

this

particular

spatial

frequency

infants

should

look

preferentially

at

vertical

or

horizontal

gratings

rather

than

at

oblique

gratings

Results

revealed

an

oblique

effect

If

the

critical

value

of

spatial

frequency

progressively

decreased

with

age

according

to

the

development

of

visual

acuity

an

oblique

effect

was

systematically

observed

at

all

ages

These

findings

were

replicated

by

Gwiazda

et

al.

-LRB-

-RRB-

According

to

Held

and

his

colleagues

findings

suggest

that

the

visual

oblique

effect

depends

on

endogenous

maturation

rather

than

exposure

to

a

carpentered

world

as

assumed

by

other

authors

-LRB-

Annis

and

Frost

Ross

-RRB-

Annis

and

Frost

-LRB-

-RRB-

studied

the

oblique

effect

in

Euro-Canadians

raised

in

a

carpentered

environment

characterized

by

the

preponderance

of

cardinal

contours

as

well

as

in

Cree

Indians

living

in

a

traditional

setting

presenting

more

heterogeneous

arrays

of

orientations

A

classical

matching

task

was

used

subjects

having

to

turn

a

test-rod

until

it

matched

the

given

standard-orientation

The

four

standard

axes

were

tested

vertical

horizontal

and

the

two

diagonals

Prior

knowledge

of

the

tested

orientations

was

given

The

expected

oblique

effect

was

observed

in

Euro-Canadians

but

it

was

absent

in

Cree

Indians

In

the

same

vein

Ross

-LRB-

-RRB-

showed

that

its

extent

increases

in

an

almost

linear

fashion

between

the

ages

of

and

years

In

haptics

the

oblique

effect

has

been

studied

in

school-age

children

and

in

infants

In

children

-LRB-

from

years

old

to

years

old

-RRB-

results

showed

that

the

oblique

effect

may

be

present

under

certain

conditions

-LRB-

like

in

adults

-RRB-

and

that

its

extent

did

not

change

-LRB-

Gentaz

and

Hatwell

-RRB-

In

infancy

Gentaz

and

Streri

-LRB-

-RRB-

initially

examined

whether

the

babies

are

capable

of

haptically

discriminating

-LRB-

without

visual

control

-RRB-

between

a

vertical

rod

and

a

oblique

rod

positioned

in

the

fronto-parallel

plane

The

authors

offered

each

baby

a

familiarization

phase

for

one

oriented

rod

After

this

phase

a

discrimination

phase

was

proposed

in

which

the

two

orientations

were

alternately

presented

By

definition

haptic

discrimination

was

said

to

occur

when

there

was

a

significant

difference

between

manual

holding

times

in

the

discrimination

phase

for

the

familiar

orientation

-LRB-

the

one

proposed

in

the

familiarization

phase

-RRB-

and

for

the

novel

orientation

-LRB-

for

a

discussion

about

the

direction

of

preference

see

Kerzerho

et

al.

-RRB-

Results

confirmed

these

predictions

and

thus

showed

that

babies

were

capable

of

haptically

discriminating

between

two

orientations

-LRB-

vertical

and

oblique

-RRB-

Then

Gentaz

and

Streri

-LRB-

-RRB-

studied

whether

a

haptic

oblique

effect

is

present

from

the

age

of

months

To

answer

this

question

the

authors

used

a

method

based

on

a

critical

angular

value

as

did

Leehey

et

al.

's

works

in

vision

in

babies

A

haptic

oblique

effect

is

said

to

be

present

when

the

discrimination

between

two

orientations

does

not

only

depend

on

their

angular

difference

but

also

on

the

value

of

the

tested

orientation

In

this

case

the

authors

predicted

that

with

the

same

angular

difference

-LRB-

-RRB-

discrimination

should

be

observed

between

the

vertical

and

a

oblique

orientation

whereas

no

discrimination

should

be

observed

between

a

oblique

and

a

oblique

Results

confirmed

these

predictions

These

results

showed

that

an

oblique

effect

may

be

present

when

the

shoulder

arm

hand

system

of

the

infants

from

the

age

of

months

could

actively

move

freely

in

the

air

to

hold

and

to

slightly

explore

the

rod

positioned

in

the

frontal

plane

with

the

finger

movements

This

effect

could

be

explained

partially

by

the

presence

of

the

gravitational

cues

provided

by

the

arm

hand

system

These

cues

could

reinforce

gravitational

-LRB-

vertical

-RRB-

direction

as

an

important

axis

which

could

be

used

by

infants

as

a

reference

axis

to

define

spatial

orientations

as

it

is

the

case

in

children

and

in

adults

-LRB-

Gentaz

and

Hatwell

-RRB-

Subjective-reference

frame

in

infancy

As

in

adults

the

effect

of

body

tilt

on

the

oblique

effect

was

investigated

in

the

visual

and

haptic

systems

in

infants

In

vision

Jouen

-LRB-

-RRB-

showed

a

classical

oblique

effect

in

the

upright

body

position

of

infants

with

longer

looking

at

the

vertical

and

horizontal

gratings

than

at

the

and

gratings

In

contrast

in

the

tilted

body

position

infants

looked

longer

at

oblique

stimuli

aligned

with

their

body

orientation

These

results

showed

that

the

visual

oblique

effect

in

infants

is

not

defined

in

a

gravitational

reference

frame

but

rather

in

a

retinotopic

reference

frame

Consequently

this

suggests

that

this

effect

would

be

directly

related

to

the

properties

of

orientation-selective

neurons

of

the

low-levels

of

the

visual

system

where

the

retinotopical

mapping

of

most

orientation-selective

neurons

is

preserved

-LRB-

cf.

Introduction

-RRB-

In

the

haptic

modality

Kerzerho

et

al.

-LRB-

-RRB-

investigated

the

effect

of

whole

body

tilt

on

the

haptic

oblique

effect

by

using

the

same

method

as

Gentaz

and

Streri

-LRB-

-RRB-

Two

postural

conditions

were

tested

upright

body

and

body

tilted

to

the

left

Two

orientations

-LRB-

vertical

or

left

oblique

-RRB-

defined

gravitationally

were

proposed

in

the

familiarization

phase

and

four

-LRB-

vertical

or

left

oblique

-RRB-

in

the

test

phase

The

results

showed

that

the

body

tilt

had

an

effect

on

the

infant

's

haptic

discrimination

of

orientations

The

understanding

of

this

effect

was

complex

because

it

acted

on

both

the

spatial

orientation

discriminated

and

the

direction

preference

of

holding

times

The

pattern

of

results

observed

in

the

upright

body

position

the

presence

of

a

discrimination

between

vertical

and

left-oblique

rods

and

a

failure

between

left

oblique

and

left

oblique

rods

confirmed

Gentaz

and

Streri

's

-LRB-

-RRB-

previous

observations

and

were

in

line

with

those

found

in

vision

by

Held

and

his

colleagues

However

the

pattern

of

results

observed

in

the

tilted

conditions

showed

the

presence

of

discrimination

between

vertical

and

left-oblique

rods

and

between

left

oblique

and

left

oblique

rods

In

other

words

the

haptic

oblique

effect

found

in

the

upright

posture

disappeared

when

the

body

was

tilted

This

effect

of

the

body

tilt

on

the

patterns

of

results

clearly

showed

that

the

hypothesis

of

a

purely

gravitational

reference

frame

underlying

the

haptic

oblique

effect

is

not

supported

Similarly

the

results

do

not

favor

a

purely

egocentric

reference

frame

since

a

successful

haptic

discrimination

was

observed

in

the

body

tilted

condition

both

-LRB-

-RRB-

between

the

oblique

rod

that

was

vertical

by

reference

to

the

body

axis

in

the

tilted

position

and

the

oblique

rod

and

-LRB-

-RRB-

between

the

gravitational

vertical

rod

and

oblique

rod

In

summary

the

results

suggest

that

spatial

orientations

are

not

defined

only

in

a

single

reference

frame

The

results

indicated

that

the

haptic

discrimination

of

the

gravitational

vertical

-LRB-

with

the

oblique

-RRB-

occurred

in

the

two

body

positions

This

result

suggests

that

in

tilted

position

gravitational

vertical

seemed

still

to

play

the

role

of

a

reference

axis

at

least

partially

In

contrast

the

haptic

discrimination

of

the

oblique

rod

depended

on

the

body

position

It

was

absent

in

the

upright

position

when

the

oblique

rod

was

oblique

by

reference

to

the

body

axis

whereas

it

was

present

in

the

tilted

body

position

when

the

oblique

rod

was

vertical

by

reference

to

the

body

axis

This

result

suggests

that

in

the

tilted

position

the

egocentric

vertical

also

seemed

to

have

the

role

of

a

reference

axis

at

least

partially

In

conclusion

these

results

support

the

hypothesis

of

a

reference

frame

which

integrates

not

only

gravitational

information

but

also

egocentric

information

As

indicated

before

this

mixed-reference

frame

hypothesis

underlying

the

orientation

perception

has

already

been

proposed

in

the

haptic

-LRB-

Kappers

-RRB-

and

visual

-LRB-

Lipshits

et

al.

Lipshits

and

McIntyre

Luyat

et

al.

-RRB-

systems

in

adults

General

discussion

and

perspectives

This

review

of

experimental

studies

of

the

haptic

oblique

effect

has

showed

that

the

perception

of

spatial

orientations

can

be

isotropic

or

anisotropic

depending

on

various

factors

such

as

the

presence

of

gravitational

cues

the

plane

in

which

orientations

are

presented

the

modality

of

response

-LRB-

ipsilateral

or

contralateral

hand

-RRB-

etc.

One

of

the

most

striking

findings

is

the

absence

of

an

oblique

effect

in

some

conditions

which

demonstrates

that

the

haptic

system

can

process

all

orientations

isotropically

This

observation

stands

in

sharp

contrast

with

vision

where

an

oblique

effect

is

observed

much

more

systematically

probably

because

some

form

of

directional

anisotropy

already

emerges

at

the

lowest

levels

of

the

visual

system

Our

general

interpretation

of

these

studies

is

that

the

haptic

oblique

effect

is

a

classic

Class

oblique

effect

which

occurs

relatively

late

in

the

processing

of

sensory

information

More

precisely

we

believe

that

these

studies

globally

indicate

that

the

oblique

effect

occurs

when

the

sensory

motor

traces

associated

with

the

exploratory

movements

are

transformed

into

a

more

abstract

representation

at

the

cognitive

level

where

presumably

the

perceived

orientation

is

related

to

a

frame

in

reference

that

favors

the

vertical

and

horizontal

orientations

While

the

high-level

representation

would

introduce

some

degrees

of

anisotropy

it

would

also

be

less

taxing

from

a

memory

point

of

view

and

be

more

robust

to

motor

interference

tasks

In

this

framework

gravity

would

facilitate

the

transformation

from

the

low-level

to

the

high-level

representation

by

adding

information

about

the

direction

of

the

movement

relative

to

the

vertical

in

the

sensory-motor

traces

In

absence

of

these

cues

the

oblique

effect

would

be

smaller

or

absent

because

the

axes

of

the

reference

frames

would

be

known

with

less

certainty

In

addition

studies

conducted

in

the

haptic

and

visual

modalities

where

the

geo-centered

and

ego-centered

reference

frames

was

dissociated

by

tilting

the

body

have

shown

that

the

axes

of

this

reference

frame

were

influenced

by

vestibular

and

somesthetic

inputs

Thus

these

findings

suggest

that

orientations

might

be

represented

in

a

frame

of

reference

based

on

the

subjective

vertical

and

horizontal

that

mixes

ego

and

geo-centric

cues

While

it

is

tempting

to

postulate

that

this

subjective

reference

frame

is

shared

between

the

two

sensory

modalities

a

detailed

analysis

of

the

results

revealed

a

lack

of

correlation

between

the

sizes

of

the

oblique

effect

obtained

in

each

modality

-LRB-

Luyat

et

al.

see

detailed

review

in

Haptic

orientations

defined

in

a

subjective

reference

frame

-RRB-

Additional

studies

are

necessary

to

determine

if

this

lack

of

correlation

reflects

only

a

difference

of

degree

of

anisotropy

present

in

the

visual

and

haptic

processing

of

orientations

or

if

it

is

due

to

a

different

positioning

of

the

subjective

axes

in

the

modalities

In

either

case

the

absence

of

a

purely

gravitational

or

egocentric

frame

of

reference

is

a

strong

argument

in

favor

of

a

high-level

haptic

oblique

effect

This

review

shows

that

gravity

is

a

sufficient

but

not

necessary

condition

for

the

occurrence

of

an

oblique

effect

in

the

haptic

modality

As

a

matter

of

fact

the

haptic

oblique

effect

can

occur

in

absence

of

any

gravitational

cues

Moreover

the

haptic

oblique

effect

can

also

be

strengthened

by

factors

other

than

gravity

For

example

some

studies

have

shown

the

presence

of

an

oblique

effect

with

reduced

gravitation

cues

when

different

hands

were

used

in

the

exploration

and

reproduction

phases

-LRB-

e.g.

the

contralateral

condition

in

Appelle

and

Countryman

-RRB-

To

understand

this

result

it

should

be

noted

that

the

reproduction

by

the

other

hand

of

an

oblique

requires

a

complex

transformation

of

the

sensory-motor

trace

of

the

scanning

movement

given

the

symmetric

arrangement

of

the

two

upper

limbs

It

is

therefore

plausible

that

the

transformation

from

the

low-level

to

the

high-level

representation

is

forced

in

this

case

because

the

sensory-motor

trace

associated

with

the

scanning

arm

is

of

little

use

for

the

reproduction

phase

and

the

haptic

system

must

rely

on

the

more

abstract

representation

of

the

orientation

-LRB-

see

Baud-Bovy

and

Viviani

for

a

similar

hypothesis

in

pointing

context

-RRB-

Other

studies

have

shown

that

an

interference

task

during

the

delay

between

the

presentation

and

the

recall

of

the

orientation

-LRB-

with

the

same

hand

-RRB-

is

responsible

for

an

oblique

effect

in

reduced

gravity

conditions

or

that

it

strengthens

in

normal

conditions

-LRB-

Gentaz

and

Hatwell

-RRB-

The

idea

is

that

higher

attentional

or

cognitive

demands

decrease

the

amount

of

resources

available

to

the

haptic

system

Such

a

situation

would

induce

the

transformation

of

sensory-motor

trace

or

give

more

weight

to

a

more

abstract

representation

of

the

stimulus

Work

in

neighboring

fields

is

consistent

with

this

hypothesis

For

example

Rossetti

et

al.

-LRB-

-RRB-

showed

that

in

proprioceptive

pointing

tasks

memorization

leads

participants

to

stop

using

sensory-motor

representation

to

produce

a

movement

and

commits

them

to

using

a

semantic

spatial

representation

In

haptics

a

similar

shift

in

perception

of

parallelity

was

also

observed

by

Zuidhoek

et

al.

-LRB-

-RRB-

These

authors

tested

the

effect

of

a

delay

between

the

perception

of

a

reference

bar

and

the

parallel

setting

of

a

test

bar

in

the

horizontal

plane

They

observed

that

a

delay

significantly

improved

performance

They

suggest

that

a

shift

from

the

egocentric

towards

the

allocentric

reference

frame

during

the

delay

period

would

lead

to

different

spatial

processing

-LRB-

for

discussion

see

also

Faineteau

et

al.

Gentaz

and

Gaunet

-RRB-

The

nature

of

the

more

abstract

representation

and

underlying

processes

is

still

an

open

question

One

possibility

is

that

the

orientations

would

be

mapped

to

one

or

more

topologically

organized

layers

of

processing

units

and

that

the

perceived

orientation

would

correspond

to

the

most

active

units

in

one

of

these

layers

The

DFT

of

spatial

cognition

developed

by

Spencer

and

colleagues

assumes

such

representational

scheme

-LRB-

Spencer

et

al.

-RRB-

According

to

this

theoretical

framework

the

stimulus

would

propagate

from

perceptual

maps

to

the

working-memory

map

where

the

oblique

effect

would

progressively

occur

under

the

influence

of

neighboring

units

and

of

the

long-term

memory

map

While

it

is

out

of

scope

to

enter

into

the

details

of

this

model

here

it

is

important

to

note

that

the

dynamic

properties

of

these

maps

could

explain

the

presence

of

a

systematic

bias

toward

or

away

from

the

reference

axes

as

well

as

the

increase

of

precision

observed

along

the

reference

axes

Work

on

this

model

has

so

far

focused

on

the

visual

modality

and

has

not

yet

addressed

issues

specific

to

the

haptic

modality

such

as

the

role

of

gravity

or

the

coding

of

an

orientation

in

space

In

particular

the

problem

of

extracting

an

initial

representation

of

the

orientation

from

the

complex

afferent

and

efferent

signals

that

correspond

to

the

sensory-motor

trace

of

the

scanning

movement

seems

daunting

but

this

observation

also

holds

true

for

all

other

explanations

of

the

haptic

oblique

effect

Another

possibility

is

that

the

orientation

would

be

represented

in

categorical

terms

-LRB-

Huttenlocher

et

al.

see

also

the

work

on

Categorical

Perception

Harnard

-RRB-

As

described

in

Hypotheses

on

the

origins

of

the

oblique

effects

the

Category-Adjustment

model

assumes

the

existence

of

a

double

representation

of

the

stimulus

According

to

the

CA

model

the

oblique

effect

occurs

at

a

late

stage

of

processing

when

both

representations

are

combined

with

possible

different

weights

to

produce

the

response

From

a

theoretical

point

of

view

such

a

double

representation

can

be

advantageous

when

the

reliability

of

the

low-level

representation

decreases

though

it

might

introduce

biases

In

this

theoretical

framework

variations

of

the

strength

of

the

oblique

effect

could

be

understood

in

terms

of

a

relative

reliability

of

abstract

representation

of

the

sensory

motor

traces

The

idea

of

a

double

representation

could

also

be

useful

to

explain

the

complex

error

patterns

observed

in

the

reproduction

of

orientations

showing

that

a

participant

responded

using

a

heterogeneous

set

of

cues

that

included

the

low-level

sensory-motor

trace

as

well

as

some

more

abstract-coding

scheme

based

on

the

two

angles

between

the

orientation

and

the

sagittal

or

frontal

plane

-LRB-

Baud-Bovy

and

Gentaz

-RRB-

However

a

problem

with

the

CA

model

is

that

it

predicts

an

increase

of

the

variability

of

response

at

the

category

boundaries

-LRB-

G.

Baud-Bovy

A

critical

commentary

of

the

category

adjustment

model

and

the

oblique

effect

Psychol

Rev

unpublished

-RRB-

This

observation

is

problematic

insofar

that

it

is

usually

assumed

that

the

category

boundaries

correspond

to

the

horizontal

or

vertical

orientations

A

possible

remedy

would

be

to

assume

the

existence

of

narrow

categories

around

the

vertical

and

horizontal

orientations

in

addition

to

the

usual

oblique

categories

centered

on

the

diagonals

To

conclude

it

is

important

to

note

that

both

the

DFT

and

the

CA

model

were

originally

conceived

to

explain

the

presence

of

systematic

biases

toward

the

closest

diagonal

in

various

location-recall

or

position-recall

tasks

in

the

visual

modality

which

has

not

yet

been

observed

in

the

haptic

modality

As

a

matter

of

fact

the

analysis

of

the

constant

errors

in

studies

of

the

haptic

oblique

effect

did

not

reveal

systematic

directional

errors

This

observation

holds

true

whether

the

reproduction

method

-LRB-

Appelle

and

Countryman

Gentaz

and

Hatwell

Lechelt

et

al.

Lechelt

and

Verenka

-RRB-

or

the

production

method

-LRB-

Appelle

and

Gravetter

Gentaz

et

al.

Luyat

et

al.

-RRB-

was

used

to

assess

the

perceived

position

of

the

orientation

in

the

haptic

modality

This

negative

result

does

not

however

invalidate

these

models

since

the

aforementioned

studies

of

the

haptic

oblique

effect

have

used

a

limited

set

of

stimuli

-LRB-

the

vertical

horizontal

and

the

two

main

diagonals

-RRB-

that

precludes

the

observation

of

a

bias

toward

the

closest

diagonal

since

it

can

be

observed

only

with

oblique

orientations

that

differ

from

the

diagonals

The

question

of

the

accuracy

of

the

perception

as

opposed

to

its

precision

has

been

so

far

neglected

in

studies

of

the

haptic

oblique

effect

and

needs

to

be

further

investigated

in

future

studies

haptic
space
orientations
oblique effect
hand
reference frame
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