![](bg4.png)
Figure 6: The evoked activities and inputs dynamics in the
visual (blue line) and in the auditory (red line) areas of the
model, in case of a cross-modal stimulation (a single flash
and two beeps presented to the network, as depicted in the
lower panel B) but without the visual illusion.
6), in which the network was stimulated with the
same pattern of external stimuli, but we used
auditory stimuli slightly weaker.
In this case, the second beep is not able to enhance
the visual input enough to overcome the visual
threshold, and to elicit a sufficient activity to
produce the perceptual visual illusion.
4 CONCLUSIONS
The present results match with the neuroimaging and
psychophysical findings present in literature about
the Shams illusion (Watkins et al., 2006, 2007).
These works have studied this phenomenon by using
the same cross-modal stimulation (one flash, two
beeps), and comparing the evoked potentials in the
visual area in case of perception of the visual
illusion, and in case the illusion was not present
(subjects correctly perceived just one flash). The
interesting finding was that only in the first case the
illusory perception was paired with an increase of
the visual cortex activity, in agreement with the
results in Fig. 5 and 6. In our model the fundamental
point that can lead to the illusory perception is the
ability of the auditory activity to enhance the visual
input over the activation threshold, to drive an
additional peak of activity in the cortex.
Moreover, by comparing these results it is worth
to note that the illusory activity in the visual area is
comparable, in terms of strength and duration, with
the activity evoked by a real visual stimulus. This
result supports the idea this illusion is a perceptual
phenomenon involving the primary visual areas.
The model suggests that the mechanisms
underlying multisensory interactions in early cortical
areas are based on direct excitatory synapses among
these regions, and do not need feedback projections
from higher-order integrative regions.
Furthermore, model ascribes the Shams illusion to
the better temporal resolution of the auditory
processing compared with the visual one. Similarly,
the better spatial resolution of visual processing can
explain the ventriloquism effect (not shown here for
briefness), with the same model structure and the
same parameter values. Future works will be
devoted to analyse if the same neural mechanisms
can explain further auditory-visual interactions too,
such as the fusion effect and the temporal
ventriloquism. Moreover, future model versions may
include a more precise characterization of the time
delays involved in the visual and auditory pathways,
in order to provide an accurate simulation of
electrophysiological data.
REFERENCES
Schroeder, C. E., Foxe, J., 2005. Multisensory
contributions to low-level ‘unisensory’ processing.
Curr. Opin. Neurobiol. 15, 454– 458.
Shams, L., Kamitani, Y., Shimojo, S. 2002. Visual
illusion induced by sound. Cogn. Brain Res. Vol. 14,
pp. 147–152.
Stein, B. E., Meredith, M. A., 1993. The merging of the
senses, The MIT Press. Cambridge MA.
Watkins, S., Shams, L., Tanaka, S., Haynes, J.D., Rees,
G., 2006. Sound alters activity in human V1 in
association with illusory visual perception.
NeuroImage 31, 1247–1256.
Watkins, S., Shams, L., Josephs, O., Rees, G., 2007.
Activity in human V1 follows multisensory
perception. NeuroImage 37, 572–578.
IJCCI2012-InternationalJointConferenceonComputationalIntelligence
642