Article,
Illusory Contours Do Not Pass through the ``Blind Spot"
Journal of Cognitive Neuroscience, 19 (1): 91--101 (2007)
Abstract
Our visual percepts are not fully determined by the physical stimulus
input. That is why we perceive crisp bounding contours even in the
absence of luminance-defined borders in visual illusions such as
the Kanizsa figure. It is important to understand which neural processes
are involved in creating these artificial visual experiences because
this might tell us how we perceive coherent objects in natural scenes,
which are characterized by mutual overlap. We have already shown
using functional magnetic resonance imaging Maertens, M., & Pollmann,
S. fMRI reveals a common neural substrate of illusory and real contours
in v1 after perceptual learning. Journal of Cognitive Neuroscience,
17, 1553-1564, 2005 that neurons in the primary visual cortex (V1)
respond to these stimuli. Here we provide support for the hypothesis
that V1 is obligatory for the discrimination of the curvature of
illusory contours. We presented illusory contours across the portion
of the visual field corresponding to the physiological "blind spot."
Four observers were extensively trained and asked to discriminate
fine curvature differences in these illusory contours. A distinct
performance drop (increased errors and response latencies) was observed
when illusory contours traversed the blind spot compared to when
they were presented in the "normal" contralateral visual field at
the same eccentricity. We attribute this specific performance deficit
to the failure to build up a representation of the illusory contour
in the absence of a cortical representation of the "blind spot" within
V1. The current results substantiate the assumption that neural activity
in area V1 is closely related to our phenomenal experience of illusory
contours in particular, and to the construction of our subjective
percepts in general.
