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Abstract:
Abstract: The aim of the study was to investigate the neural
mechanisms subserving visual perception of illusory contours by
adopting a selective attention paradigm. Eight students
participated in this study as paid volunteers. Forty different
equiluminant black and white configurations were randomly presented
forty times for 200 msec in the foveal field in different runs
(1600 total stimuli). Half of them produced the illusory perception
of a Kanizsa square; the remaining were obtained by rotating
outward the inductors and did not elicit any illusory percept. For
each of the two categories, half of them had white inductors on a
black background and viceversa. Again, for each of the 40 stimuli,
half of them were symmetrical and the other half asymmetrical. The
task consisted in deciding about the presence of the illusory
square by pressing one out of two buttons. Event-related potentials
(ERPs) recorded from 30 scalp sites. Results showed larger
lateral-occipital P85 and N145 responses to symmetrical than
asymmetrical figures. N145 was larger to stimuli producing an
illusory percept. P300 was much more sensitive to illusory contours
on a black than white background. This might suggest that bright
inducers on a black background produced a better stimulation for
visual neurons. P300 was larger to stimuli with illusory contours
only when symmetrical, thus indicating the crucial role of edges
alignment in figure/ground segmetation.
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