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Abstract

When redundant sources of sensory information are available, integrating these sources is beneficial
for a system in order to decrease the perceptual noise and so to increase the accuracy
of the overall percept [1]. However, a potential cost may come from incorrect binding of the
different sources, which will generally evoke perceptual illusions [2].
Here we ask whether humans take advantage of a bimanual size estimate originating from
an object of constant size. The stimulus was a cylindrical object that subjects felt with their
left and right hand simultaneously. To display the bimanual haptic stimulus we used two
PHANToM force-feedback devices and measured size discrimination performance using a
2IFC paradigm. Subjects’ task was to decide which of two bimanual stimuli was bigger. From
the resulting psychometric functions we determined the JND for bimanual vs unimanual size
discrimination and the point of subjective equality (PSE).
There was no difference in the bimanual vs the unimanual JNDs which indicates that subjects
did not benefit from having available two size estimates in the bimanual situation. We
therefore conclude that there is no integration of size estimates between hands when an object
is touched bimanually. We may speculate that this failure of integration to occur results from
the fact that naturally the object is touched at slightly different spatial locations and so information
is not truly redundant even though subjects were told to touch an object of constant
size. Not being able to integrate information that comes from two different spatial locations
may help to prevent misbinding of different sensory sources.