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CPYRGHT
KELLY AND SAVOIE
whether the "pause” switch was set).
The response history section
is the heart ot the t-Lb
program
: Fig. 7 shows a flow chart of this section and its
relation to the other parts of the program. Each staircase
follows
the
algorithm
according
to
its own
history,
independent of the other staircases. The next contrast to
he presented (for the selected staircase)
is calculated on
the
basis of the
present
contrast, the present response
the previous two responses, the contrast increment, and
whether
the
contrast was changed
after
the
previous
response. The contrast algorithm
is as follows: (1) When
S
makes
a wrong (W)
response,
the
contrast
of
that
pattern
is increased
for
its next
presentation.
After he
makes two
successive
right (R) responses to the same
pattern,
its contrast
is decreased
(this
sets the
average
end
point
of each
staircase
at about 75% correct). The
new contrast
is obtained by multiplying (or dividing) the
preceding contrast by
a fixed ratio
(i.e., a constant log
increment).
(2) Each time the sequence W,R,R
occurs,
the
size
of
this
increment
is
decreased.
(This
is most
Likely
to occur when the contrast increases from below
to
above
the
75%
correct
level.)
The
successive
increment
sizes
are:
8X,
2X,
1.3X.
(3) After
the
sequence W,R,R has occurred three
times with
a given
pattern,
the
staircase terminates with
three more, trials
using
the
smallest
increment,
and
that
pattern
is not
presented
again.
The
session
ends when
all
staircases
have terminated.
The
last
eight
contrasts
from
each
staircase
are
averaged to obtain the threshold. The minimum number
of trials before termination but
after the second W,R,R
sequence
is eight. Therefore,
all contrasts being averaged
were
necessarily presented
during the
period when the
smallest contrast increment was in effect.
REFERENCES
Bekesy. G.
v. A new audiometer. Acta Oto-laryngologica, 1947,
35/411-422.
i
T
.
Blackwell. H. R. Contrast thresholds of the human eye. Journal
of the Optical Society of America, 1946, 36, 624-643.
Campbell.
F.
W., &
Green,
D.
G.
Optical
and
retinal factors
affecting visual resolution. Journal of Physiology, 1965, 181,
576-593.
j
,
.
.
Comsweet,
T.
N.
The
staircase
method
in
psychophysics.
American Journal of Psychology, 1963,75,483-491.
Davidson.
M.
A
perturbation
analysis
of
spatial
brightness
interaction in flashed visual fields. Doctoral thesis. University
of California. Berkeley, 1966.
Heinemann,
E.
G. The
relation
of apparent brightness to
the
threshold
for
differences
in
luminance.'
Journal
of
Experimental Psychology, 1961, 61, 389-399.
Kelly. D. H. Frequency doubling in visual responses. Journal ot
the Optical Society of America, 1966, 56, 1628-1633.
Kelly, D. H. Effects of sharp edges on the visibility of sinusoidal
gratings. Journal of the Optical Society of America. 1970, 60,
98-103.
Kelly,
D.
H.
Transient
effects on the
visibility
ot low
spatial
frequencies. Journal of the Optical Society of America, 1971,
61, 1576 (A).
.
,
.
Kelly,
D.
H. Adaptation
effects on
spatio-temporal
sine-wave
thresholds. Vision Research, 1972, 12. 89-101.
Kelly,
D.
H.
Lateral
inhibition
in human colour mechanisms.
Journal of Physiology, 1973, 228, 55-72.
Kohayakawa, Y. Contrast -difference
thresholds with
sinusoidal
gratings. Journal of the Optical Society of America, 1972, 62.
584-587.
.
.
,
Rose, R. M., Teller, D. Y., & Rendleman, P. Statistical properties
of* staircase estimates. Perception & Psychophysics, 1970,
8.
199-204.
,
,
.
.
,
VanMeeteren, A. Spatial sinewave response of the visual system
(a
critical
literature
sun’ey).
Soesterberg:
Institute
for
Perception RVO-TNO, 1966.
Wertheimer,
G.
Modulation
thresholds
for
sinusoidal
light
distributions on the retina. Journal of Physiology, 1960, 152.
67-74.
NOTE
1. Because
the FCS
sessions are long and tiresome, a number
of preliminary
experiments were
conducted
to determine the
most
suitable
compromise
between
speed
and
end-point
stability We found that the variability shown in Fig. 5 could not
be
decreased
significantly without
prohibitive increases
in run
length. The number and
sizes of our increments were governed
by
the
following
constraints
(see Comsweet,
1963):
(a) For
maximum
efficiency,
the
smallest increment must be
fixed
at
about
the
same
size
as
the
contrast-difference
threshold
tor
sinusoidal
gratings;
at
a
spatial
frequency
of
2.1
cycles/deg,
Kohayakawa
(1972)
obtained
jnds
of
30%
to
60%
with
low-contrast gratines. (Note that this is much larger than the jnd
of
luminance.)
(b) All
staircases
must
start
at
a common
baseline; for
this purpose, we chose 100% contrast. (To start at
equal
distances
from
threshold
would
require
an
a prion
assumption
of the curve shape we
are
measuring, which could
bias
the
result.)
(c) If
the
contrast
is
to
approach
threshold
quickly, the
initial increment must be much larger than specified
in
(a). The increment should decrease as the contrast approaches
threshold, but
not
so
rapidly
as
to run
a high
risk of being
'‘trapped’
in
a
small increment far from threshold,
(d) Subject
to
constraint
(c),
the
total number
of increments should
be
minimized.
(Received for publication January 16, 1973;
accepted March 15. 1973;
revision received March 29, 1973.)
Approved For Release 2003/04/18
: CIA-RDP96-00791R000200040003-0