Multisensory Integration and the Body Schema: Close to Hand and Within Reach
http://www.icn.ucl.ac.uk/WebSpace/users/attentiongrp/pubs/Jon%20Driver/Author/Original%20Research%20Papers/2003/Current%20Biology%2013%20R531%20R539.pdf
For instance, a multisensory neuron with a tactile receptive
field (RF) on one hand will typically respond to visual
stimulation near that hand, thereby increasing its rate
of firing as the visual stimulus approaches the tactile
RF, and declining as it is moved away.
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Incongruent visual
distractors have been shown to delay tactile judgments
and to produce more erroneous responses, leading to a
crossmodal congruency effect, which is defined as the
performance difference between incongruent versus
congruent trials. Importantly, this crossmodal congruency
effect is more pronounced for a visual distractor
near the tactually stimulated hand than for one near the
other hand, or elsewhere
In addition to such effects of discrete visual precueing
events on tactile judgments (or vice versa), some
other recent studies [33–35] have shown that continuous
rather than discrete vision of the hand or arm can
also modulate tactile performance for the corresponding
body part, even when no additional information
about the position of the tactile stimulus or its identity
is provided by vision
the activation of a subset of such multisensory neurons
by a spatial stimulus in one sensory modality might
lead to an enhanced response from the same neurons
to a second stimulus, which is presented at the same
(or similar) external location, but in a different sensory
modality and to which the activated neurons also
respond
crossmodal congruency effect
(B) The
magnitude of this effect (shown numerically)
is reduced when the stuffed rubber
gloves are placed in an anatomically
implausible posture.
When the hand was occluded below a
screen, neurons responded more to the
vision of a taxidermied monkey arm (dark
brown) aligned over the real arm, if the
dummy arm matched the real arm (C), but
less if a taxidermied contralateral arm was
placed at the same location (D).
However, the rubber
hands produced no such modulatory effect when
placed in an anatomically implausible posture (Figure
2B) that was totally inconsistent with the real hands’
actual posture
Thus, while purely visual information
(i.e. sight of the rubber hands) can dominate slightly discrepant
proprioception (Figure 2A), proprioception may
reduce the impact of vision when the visual information
about hand position is inconsistent with proprioception
(Figure 2B). In agreement with this notion, in the total
absence of vision of any hands (real or dummy), proprioceptive
information about current hand posture (e.g.
crossed or uncrossed hands, or hands placed near
versus far from one another) has been shown to modulate
crossmodal interference effects [29]
These results indicate that visual information about
body position seems to strongly influence ‘body-partcentred’
multisensory spatial representations [54].
These representations, at least in area 5, may even be
detailed enough to incorporate visual discrimination
between a left or right hand [55]. But if the arm of a
trained monkey is actively or passively moved underneath
an occluding screen, so that no arm — neither
real nor dummy — is visible, some remapping can still
be shown to occur in the anterior bank of the intraparietal
sulcus (IPS) [56] and/or premotor cortex [54], with
the visual RF tending to shift along with the unseen
arm as its position changes. Thus, if sensory modalities
are in conflict (e.g. when viewing dummy hands),
plausible visual information about arm or hand location
can dominate proprioception, perhaps due to the
greater spatial acuity of vision. It is, however, also clear
that proprioceptive/kinaesthetic information can play
some role, as shown in the absence of visual information
about limb position. The same point applies to the
crossmodal congruency effects reported in human
performance, which can show visual dominance when
a dummy hand is seen in a possible location for the
real hand, but can still be modulated by proprioceptive
information about actual hand location under conditions
of occlusion or darkness
Figure 3. Apparent expansion of the
representation of peripersonal space
around the hand following tool-use, or
when viewing the hand only indirectly via a
distant mirror-reflection.
