Researchers delve into the mystery of how the hand percieves what the eye cannot
THE world, as we perceive it through our
senses, appears to be unitary: the final
mental picture we have of the world, at
any moment, is synthetic, built up from
sensory impressions, received through
sight and hearing primarily, as well as
smell, touch and taste.
In daily life, we do not have two
different impressions of the world at
the same time. S Aglioti, J G X DeSouza
and M A Goodale of the Universities
of Verona and Western
Ontario have carried out
experiments showing how
the hand can recognise reality even when the eye is
fooled (Current Biology 5,
(6): 679-685).
What is strange is that
the information available to
the subject is purely visual,
but how the subject interprets it depends on whether
he or she merely describes
what is seen or proceeds to
perform a physical action.
The experiments involved a
common optical illusion
known as the Titchener circles, one that we can try out
for ourselves. Draw two,iden-
tical circles on a piece of
paper at some distance from
each other. Surround the
first circle by a number of
smaller circles, and the second one by large circles. If
you look at the paper now,
the first circle appears to be
larger than the second .
The usual explanation for the phenomenon is that the brain )udges the
size of a seen object on the basis of a
comparison with the nearest available
'standard' in the neighbourhood. Thus,
the first circle is judged to be larger than
the second even though the two are, in
fact, identical.
There is an interesting variant of this
iflusion: a circle which is actually larger
than another can be made to appear to
be of the same size if it is nested within
larger circles, whereas the smaller one is
in the middle of still smaller circles (see
Fig 2). Aglioti and his colleagues set up
both illusions, not by drawing circles on
paper but by using poker chips (circular
discs), varying in diameter from 27 to 33
m. To be judged equal in size, the disc ifi
the array of larger circles had to be, on
an average, 2.5 mm wider than the disc
in the array of smaller circles.
After presenting the two discs
(along with their surrounding circles)
for three seconds, subjects were told to
pick up the one on the left if they
thought the two discs to be of the same
size, or to pick up the one on the right if
they thought otherwise. Which disc a
subject chose was an indicator of
whether he or she judged them to be
identical in size or different.
What the subject did was monitered;
the movements of the wrist and fingers
were monitored during the act of picking up a disc. Scientists measured the
maximum size of the grip and the opening between the index finger and the
thumb during grasping. Earlier studies
had shown that maximum grip size,
typically achieved about 70 per cent of
the way through a grasping movement,
is proportional to the size of the object
that is picked up.
At the perceptual level, the illusion
was successful. All subjects
treated discs that were physically different in size as perceptually equivalent, and
discs that were physically
identical were judged to be
perceptually different.
Remarkably, these illusory
perceptions did not affect
the manner in which the
subjects scaled their grasp.
On the contrary, the maximum grip size was larger
than average in the case of
discs whose true size was
larger than average, and correspondingly, smaller than
average when the true size
was smaller.
What do the observations imply? That the control of a skilled movement
(such as grasping) is dependent on perceptual information. At the same time, the
results of these experiments
indicate that the brain is
capable of making distinct
judgements about the external world
depending on whether or not it has to
act on the basis of the judgements. The
kinds of computations performed by
the brain before the performance of
skilled motor actions differ from those
computations that lead to the making of
perceptual judgements about the world.
Such a disjunction between visual perception and visual control of skilled
motor acts has also been observed in
patients with brain damage.
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