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NEUROREPORT
STANFORD AND STEIN
many cases, linear combination) of information on unisensory
channels, it is not surprising that the function
relating the behavioral products of multisensory integration
to stimulus intensity is qualitatively similar to that for
unisensory stimuli. For unisensory stimuli, the relationship
between stimulus intensity and stimulus detection or
reaction time is also one of diminishing returns; psychometric
functions of stimulus detection probability versus
stimulus intensity display the characteristic sigmoid shape,
positively accelerating near threshold and negatively accelerating
near maximal performance. Likewise, reaction time
decrements with stimulus intensity are described by an
inverse power function (Pieron’s Law) that prescribes large
reaction time decrements for near-threshold intensity increments
and little to no effect for increases in the highintensity
range [30,31].
The analogy drawn between increases in activity owing to
increments in within-modal stimulus intensity and those
due to the addition of stimuli from a second modality (i.e.
multisensory integration) is instructive; it reminds us that,
from the point of view of circuits that must ‘read out’
superior colliculus activity to produce behavior, the particular
mechanism that gave rise to an increase in activity is
irrelevant. With this in mind, we emphasize that superadditivity
is but one of several computations through which
multisensory integration enhances the neural representation
of sensory signals and the behaviors that depend on them.
Acknowledgement
Grant support: NS36916 and NS22543.
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