In normal viewing, the visual system effortlessly assigns
approximately constant attributes of color and shape to perceived objects.
A fundamental component of this process is the compensation for illuminant
variations and intervening media to recover reflectance properties of
natural surfaces. We exploited the phenomenon of transparency perception
to explore the cortical regions implicated in such processes, using fMRI.
By manipulating the coherence of local color differences around a region
in an image, we interfered with their global perceptual integration and
thereby modified whether the region appeared transparent or not. We found
the major cortical activation due to global integration of local color
differences to be in the anterior part of the parahippocampal gyrus.
Regions differentially activated by chromatic versus achromatic
geometric patterns showed no significant differential response related to
the coherence/incoherence of local color differences. The results link
the integration of local color differences in the extraction of a
transparent layer with sites activated by object-related properties of an
image.