Sensory and motor systems 467

Distinct neural processes of bodily awareness in crossed
fingers illusion
Takayasu Sekinea,b and Ken Mogia,b
The tactile reassignment process supports the flexible anddynamic changes of body schema in various situations
such as those involving tool use. Here, we show that
there exist two distinct neural processes in the dynamicalreassignment process. One process is involved in
identifying the body part where the tactile stimuli areapplied, whereas the other is involved in the assignmentof the tactile stimuli in the external space including one’s
body. These processes, combined together, would facilitate
the quick and appropriate acquisition of information fromthe environment, resulting in the speedy spatial perception
and execution of motor activities. In addition, we showthat the body posture affects the accuracy of tactile
localization in the crossed fingers illusion. NeuroReport

Introduction
Interaction through the tactile modality plays an essential
role in the execution of many daily tasks. Tactile
localization not only requires the information abouttactile stimuli on the body surface but also the spatial
information in the external spatial frame of reference
including one’s own body. The external frame ofreference pertaining to the environment as well as thesomatotopic frame of representation is involved in thetactile perception and localization. The interplay between
these two elements, partially independent and partiallyinterdependent, is of particular interest when one tries tounderstand how the body interacts with the environment.
Investigation of the nature of illusory perception facilitates
the understanding of perception in general, as the neuralmechanisms involved in normal and illusory perceptions
are closely related and often share common processes.
Dynamic reassignments of localization of tactile stimuli
have been revealed for crossed fingers in the so-called
Aristotle’s illusion [1], where stimuli applied to the crossed
portion of the fingers by rubbing a single finger againstthem is interpreted as consisting of separate stimuli on
both sides of the finger. Studying the properties of tactile
relocalization in such illusory perceptions provides unique
opportunities for clarifying the nature of tactile perception
in normal perceptual situations.
The tactile localization processes have been shown toextend to tools held in hands. Earlier studies haverevealed the existence of reassignment processes ofstimulus localization in the tactile perception for crossed
0959-4965 c 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins

20:467–472 c 2009 Wolters Kluwer Health | Lippincott
Williams & Wilkins.
NeuroReport 2009, 20:467–472
Keywords: Aristotle’s illusion, body schema, finger, posture, tactileaDepartment of Computational Intelligence and System Science, Tokyo Instituteof Technology, Nagatsuta-cho, Midori-ku, Yokohama and bSony Computer
Science Laboratories, Higashi-gotanda, Shinagawa-ku, Tokyo, Japan

Correspondence to Dr Ken Mogi, PhD, Sony Computer Science Laboratories,
Takanawa-Muse Building, 3-14-13 Higashi-gotanda, Nerima-ku,
Tokyo 141-0022, Japan
Tel: + 81 3 5448 4380; fax: + 81 3 5448 4273; e-mail: kenmogi@csl.sony.co.jp
Received 1 December 2008 accepted 21 December 2008

tools held in uncrossed hands [2], and the reversal of
temporal order judgment of two tactile stimuli applied insuccession to crossed hands or to the tips of crossed tools
held in uncrossed hands [2,3]. These studies focusing on
the temporal properties of tactile perception [4] showedthat the tactile localization process not only requires
the representation of one’s body surface in the primarysomatosensory cortex (as was classically depicted in theso-called Penfield’s homunculus [5]) but also spatial
information about the body posture in the external space.
Perception based on the combination of heterogeneous
cognitive processes has been reported, for example asshown in the dissociation between the ‘where’ and ‘what’
pathways [6–9] in the visual system. Compared with therelatively well-studied visual system, the nature of thefunctional segregation (if any) of cortical areas involvedin the processing of body schema is yet to be clarified.
Here, we study differential cognitive processes underlying
tactile localization in reference to the body surface andthe external space by means of the illusory perception
induced by crossed fingers. In the literature, it has
been reported that the bodily posture affects tactile
perception [3,10,11], active touch [12,13], tool use[2,14], and this aspects of bodily perception. In viewof this evidence, we investigate the role of posture in theprocess of tactile perception.

Methods
Ten healthy adults (six male and four female, 21–55
years old, average=26.8 years old) participated in the
DOI: 10.1097/WNR.0b013e3283277087

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468 NeuroReport 2009, Vol 20 No 5

was adjusted so that the two marked stimulation points
were placed at 15 mm apart in the external space,a distance at which the participants could keep thefingers crossed at ease. In the ‘fingers uncrossed’ posture,
the fingers were adjusted so that the stimulation points
were placed at 25 mm apart, a distance that could bemaintained without difficulty. The sides of the index andmiddle fingers did not touch each other in the ‘fingers
uncrossed’ posture. The stimuli were applied manuallyto stimulation points by the experimenter with a 500-ms
interval by touching with a thin wooden stick of 2 mmdiameter. The timing was cued to the experimenter bya digital metronome (‘Auftakt’ by K. Kimura, Shokei
University, Japan) run on a notebook computer (IBMThinkPad X23, New York, USA). Calibration experimentssuggested that the temporal accuracy of the manualapplication of the stimuli by the experimenter was
reasonably high, with a standard deviation of 31 ms.
The participant’s eyes remained closed during thewhole procedure.

experiment. Six individuals participated in both the‘fingers crossed’ and ‘fingers uncrossed’ conditions. Fourindividuals participated only in the ‘fingers crossed’
condition. All participants were right handed. The
experiment was conducted in accordance with the
Declaration of Helsinki. The participants were instructed
about the general conditions of experiments and gave
written informed consent. The experimental procedure
was approved by the institutional ethics committee.
In experiment 1, we investigated the illusion caused bystimuli applied to the index and middle fingers when
they were crossed. Preliminary experiments have foundthat the illusion of essentially the same nature occurrs
for both the left and right hands. In this experiment, wereport the results for left hands. The participant wasseated in a chair and was instructed to keep his or her left
hand either in the ‘palm up’ or ‘palm down’ posture, with
their eyes closed. As our preliminary investigations haveindicated that the hand posture (‘palm up’ or ‘palm
down’) affects the accuracy of tactile localization [15], we
compared the participant’s performance between the two
postures. The hand was held horizontally in front of theparticipant’s midline, about 30 cm away from the chest,
without an aid such as a desk to rest the arm on, while theelbow naturally touched the side belly (Fig. 1a). Whencrossed, the index finger was put on top of the middle
finger when seen in the ‘palm up’ posture, that is, withthe ventral side of the middle finger touching the dorsalside of the index finger (Fig. 1a). Stimulation points weremarked with a pen as black spots on the middle line offingers on the ventral side, at 10 mm from the fingertip on
the finger pad. The configuration of the crossed fingers

Before the test session, the participants practiced thejudgment of which finger was to be touched second. This
practice session was found to be necessary for theparticipant to be able to report in a manner faithful towhat they were actually perceiving, which became
possible after a transient period. Feedback (correct/
wrong) was provided so that the participants could adjust
their response. The participant was required to practice
until they successfully made the correct ‘finger judgment’for 10 consecutive times, to ensure that the transientperiod has passed. Then the test phase followed. For eachof the four combinations of finger (crossed or uncrossed)

Fig. 1

(a)

(b)

1

10 mm

2

1

2

(c)

2

10 mm
15 mm

15 mm

30 cm

Spatial reversal
2

Index and middle
Posture1

StimuliIndex and ring1

Temporal reversal
Perception

Two possible mechanisms of illusion leading to a reversal in the direction judgment. (a) The posture of the participant. A schematic representation of
the body posture is shown while the participant sits with the index and middle fingers of the left hand crossed either in the ‘palm up’ or ‘palm down’posture. (b) Stimuli applied. The index and middle fingers (experiment 1), or the index and ring fingers (experiment 2) of the participant’s left hand iscrossed in the ‘palm up’ posture. The Arabic numerals represent the order of stimuli applied, whereas the arrow indicates the direction of motion. The
distance between the two stimulation points is 15 mm in both experiments. (c) Perception. When the participants correctly recognize the temporalorder of two stimuli while the finger crossing fails to be correctly represented in the somatosensory system, the direction judgment would again bereversed (‘spatial reversal’). When the finger crossing is correctly represented in the somatosensory system while the temporal order of the two
stimuli is reversed, the direction judgment would be reversed (‘temporal reversal’). The two fingers represent the index and middle fingers (experiment 1)
or the index and ring fingers (experiment 2).

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Distinct neural processes in illusion Sekine and Mogi 469

and hand (palm up or palm down) postures, anexperimental block consisted of 10 sets of stimuli being
applied to the fingers followed by either the direction
or finger judgment. Three experimental blocks wereconducted for the direction and finger judgments each,resulting in 24 blocks for six participants (‘fingers crossed’and ‘fingers uncrossed’ conditions), or 12 blocks forfour participants (‘fingers crossed’ condition). Six
experimental blocks were conducted in one sequence,lasting for about 2 min, followed by a break of 2 min.
Given the short duration of the experiment, keeping
the instructed arm and finger configurations was notstrenuous for the participants.
In the direction judgment, the participant is essentiallybeing asked to report the direction of the vector of motion[leftward (right-to-left) or rightward (left-to-right)], which
entails both the spatial and temporal dimensions. There
are therefore two possible reasons for which theparticipants make mistakes in the direction judgment.
When the participant correctly judges the temporal orderof the two applied stimuli while their respective
spatial locations are interchanged, this would result inthe misjudgment of the direction of motion (‘spatial
reversal’). Alternatively, if the participant correctly
recognizes the locations of applied stimuli in space while
the temporal order of two stimuli was misrepresented,
this would again induce a reversal in the direction
judgment (‘temporal reversal’) (Fig. 1c). These tworeversals would be indistinguishable based on the rateof correct judgment alone.
To separate these two possibilities of cognitive failure, we
made the interval between the two stimuli to be 500 ms,
which was longer than the 30-ms interval, necessary forthe correct judgment of temporal order of two stimuli
applied on the body surface [16] and the 300-ms interval
below which a reversal on temporal order judgment isincurred in the crossed hands condition [2]. Under thiscondition, a reversal on the temporal order judgment is
excluded, so that a reversal in the direction judgment
would be automatically translated into one in spatial
judgment. If the participant correctly reports in the finger
judgment while reversely reporting in the direction
judgment, it would suggest that cognitive processes
underlying the finger and location judgments are separate.
It is interesting to investigate the nature of interaction
between the judgment in the external spatial frame ofreference and one in the somatotopic frame of reference.
For example, would the amount of interference increaseas the distance between fingers increases in the external
space, as measured with the fingers uncrossed? How
would the performance be different when the index andring fingers are crossed compared with the case where theindex and middle fingers are crossed, while the distance

between the tips of the crossed fingers is kept constantin the external space (Fig. 1b)? If the direction judgment
depends on the distance between two fingers in theexternal frame of reference alone, there would be nodifference between these cases.
To investigate this point, we executed a further seriesof experiment (experiment 2) with the same participants.
The stimulation points were marked in the same manneras in experiment 1. The index and ring fingers werecrossed, with the ventral side of the ring finger touching
the dorsal side of the index finger in the space ventral tothe middle finger. The configuration of the hands was
adjusted so that the distance between the two stimuli
in the physical space was maintained at the same distance(15 mm) as that between the index and middle fingers
in experiment 1. The participants held the hands either
in the palm up or the palm down posture. When thefingers were uncrossed, the stimulation points wereplaced at 50 mm apart between the index and ring fingers
to maintain a natural posture, with the sides of the index,
middle, and ring fingers not touching each other. The
procedures were otherwise the same as in experiment 1.

Results
In the test phase of experiment 1, the participants
could make the finger judgment accurately, aided by
the completion of the test phase. The correct rates(mean ± SEM) for the finger judgment were 96 ± 2.1 and96.3 ± 1.6% for the palm down and palm up postures,
respectively. There was no significant difference between
these data by paired t-test (P=0.89, two tails, Fig. 2).
Thus, the participants were able to accurately perceive
which finger was being touched, after the passage ofthe transient period of confusion. The sufficiently large
interval between the stimuli (500 ms) effectively ensured
that a reversal in the temporal order judgment wasabsent. Thus, if the tactile localization of fingers
supported by a knowledge of the crossed state of fingers
automatically translates into the judgment of localizationin external space, the direction judgment would exhibit
a comparable level of accuracy.
However, the correct rates (mean ± SEM) of direction
judgment for stimuli applied to the crossed index andmiddle fingers were 41.3 ± 12.9 and 80.3 ± 9.4% for thepalm down and palm up postures, respectively, exhibitinga significant difference from the finger judgment[F(1,9)=9.939, P=0.012]. In addition, the correct rateof judgments was significantly higher for the palm up
compared with the palm down posture (paired t-test,
P=0.010, two tails, Fig. 2), replicating the result ofour previous research [15]. This ‘posture effect’ is an
important element to be considered in clarifying theneural mechanisms involved.

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470 NeuroReport 2009, Vol 20 No 5

Fig. 2

Fig. 3

100

100
Index and middle
Index and ring80

Correct rate (%)

Correct rate (%)

80

60

40

60

40

20
20
0
0
Palm down

Palm up
PostureThe effect of the distance between fingers in the direction judgment.
Black and white bars indicate the correct rate of direction judgment forthe crossing of index and middle fingers and index and ring fingers,
respectively (% correct response ± SEM). There are significant
differences between finger combinations (index and middle/index andring) and between hand postures (‘palm up’ or ‘palm down’).

These results indicate the existence of two distinct
neural processes. One process would be involved in
identifying the body part where the tactile stimuli areapplied, whereas the other process would be involvedin the assignment of the tactile stimuli in the external
space including one’s body. Note that when the fingers
were uncrossed these two processes could not be
separated, as the correct rate for the direction judgment
was 100%, without a single instance of error occurring
during the experiment. Studying illusory direction
judgment in crossed fingers thus enables us to separate
these two processes.
In experiment 2, when the index and ring fingers
were crossed, the correct rates (mean ± SEM) for finger
judgment were 94 ± 4.7 and 95 ± 2.1% for the palm down
and palm up postures, respectively, with no significant
difference between the two postures (paired t-test,
P=0.79, two tails). The correct rates (mean ± SEM) of
direction judgment were 24.5 ± 11.9 and 58.3 ± 13.3%
for the palm down and palm up postures, respectively,
exhibiting a significant difference (paired t-test,
P=0.028, two tails, Fig. 3). There was a significant
difference between the correct rates for the direction
judgment and finger judgment [F(1,9)=19.614, P=0.002],
suggesting the existence of dissociable neural processes.
We conducted two-way analysis of variance for the
performance on direction judgment, where the factors

Palm up
Palm down
Finger judgment

Palm down
Palm up
Direction judgment

Correct rates for finger and direction judgments when the index andmiddle fingers are crossed. White and black bars show theperformances in the finger judgment and direction judgment,
respectively (% correct response ± SEM). The correct rates for the
direction judgment are significantly different between the two handpostures (‘palm up’ or ‘palm down’).

were the ‘finger combination’ (index and middle vs. index
and ring) and the ‘hand posture’ (palm down vs.
palm up). There was a significant effect of finger
combination [F(1,9)=10.576, P=0.010] and hand posture[F(1,9)=8.473, P=0.017]. The interaction between
finger combination and hand posture was not significant[F(1,9)=0.652, P=0.218] (Fig. 3). Thus, the pairingof fingers in the crossed posture affects the direction
judgment, indicating an interference between theexternal frame of reference and the somatotopic frameof reference in the neural processes involved.

Discussion
In this study, we reported an illusion that occurs while
the fingers of the participant was crossed. In order toseparately evaluate the tactile localization in the external
space and the judgment of which body part is being
touched, the participants were asked to conduct two
types of cognitive tasks for the same set of stimuli. In the
Aristotle’s illusion, the process of assignment of tactile
stimuli on fingers is disrupted. In the illusion reported
here, such a disruption is not observed. The accurate
perception of which fingers are touched, combined withthe absence of a reversal in temporal order judgment(ensured by a sufficiently long interval between thestimuli) would seem to result in a correct judgment in
direction, provided that the knowledge that the fingers
are crossed is properly used in the cognitive process. Our
results suggest that this is not the case. The direction
judgment remains disrupted while the finger judgment is

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Distinct neural processes in illusion Sekine and Mogi 471

correctly made. It is thus indicated that there are two
distinct neural processes involved. One is concerned withthe position judgment on the body surface, whereas
the other is concerned with the direction judgment in thephysical space. The cognitive processes in the brain
concerned with these two processes are not necessarily
consistent with each other, where one process successfully
makes a use of the knowledge of the fact that the fingers
are crossed, whereas the other fails to do so. These
processes are likely to operate in parallel and enable theexecution of automatic and reflective behaviors based on
the body schema [17].
In the visual system, a segregation between the ‘what’
and ‘where’ pathways has been reported [6–9]. The
coexistence of distinctive neural pathways that contributedifferentially to cognition might thus be a property sharedby different sensory modalities such as the visual andsomatosensory processes.
There is a significant difference between the correct
rates in the direction judgment between the ‘palm up’and ‘palm down’ postures. The fact that the hand postureaffects the verbally reported tactile perception isconsistent with the observation that the neural activity
leading to the conscious perception of body schema isin the parietal associated area including BA5 [18],whereas there is an evidence that suggests that theactivity in the primary somatosensory area reflects thebodily awareness [19].

‘default’ hand posture with the fingers uncrossed affects
the spatial judgment of the tactile stimuli. Such aninteraction reveals the nature of the integrated processingof information on the bodily surface with one on
the external space. Cortical areas responsible for the
representation of fingers including the primary somatosensory cortex [23] are likely to be involved in such an
integration process.

Conclusion
In this study, we have demonstrated the existence ofdistinct neural processes in bodily perception. One isassociated with the judgment of which body part isbeing touched, whereas the other is concerned with thetactile localization in the physical space. These processes,
combined together, are likely to support the flexible anddynamic changes of bodily awareness in response to thevarious modes of interaction with the environment,
where the hand posture affects the accuracy of tactile
localization.

Acknowledgements
The authors are grateful to H. Nagashima, K. Omata, andT. Yanagawa for helpful discussions. They also thankS. Nozawa and M. Kato for constructing some of thedigital images used in this study and are indebted for oneof the section editors for critical comments on an earlier
version of this study.

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It is interesting to consider the underlying brain
mechanisms as well as the functional implications of
posture. It is known that the somatosensory activity issuppressed during motor activity [20], whereas a change
in posture affects the motor area [21]. In the palm
down posture, it is possible that the activity in thesomatosensory cortex is suppressed because of the motor
activity and the accuracy of tactile perception is therefore
compromised. Some functional considerations mightsuggest the rationale for the prominence of the ‘palm
up’ posture. In many situations of action execution indaily life, the palm down posture is the norm, where theelements involved in the sensorimotor coordination areprocessed unconsciously (e.g. writing with a pen, typing
with the computer keyboard, etc.). In contrast, handling
objects in the palm up posture is unstable in many
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fingers than for the index and ring fingers would indicatethat the distance on the body surface as measured in the

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