Abstract
Early visual cortex responds to illusory contours in which abutting lines or collinear edges imply the presence of an occluding surface, as well as to occluded parts of an object. Here we used functional magnetic resonance imaging (fMRI) and population receptive field (pRF) analysis to map retinotopic responses in early visual cortex using bar stimuli defined by illusory contours, occluded parts of a bar, or subtle luminance contrast. All conditions produced retinotopic responses in early visual field maps even though signal-to-noise ratios were very low. We found that signal-to-noise ratios and coherence with independent high-contrast mapping data increased from V1 to V2 to V3. Moreover, we found no differences of signal-to-noise ratios or pRF sizes between the low-contrast luminance and illusion conditions. We propose that all three conditions mapped spatial attention to the bar location rather than activations specifically related to illusory contours or occlusion.
Highlights
To interpret a visual scene an observer has to differentiate between foreground and background, for instance by detecting the edges separating one triangle overlapping another
A more recent functional magnetic resonance imaging (fMRI) study demonstrated that the orientation of illusory contours is encoded across visual cortex, including V11
While higher visual areas are preferentially activated by illusory contour stimuli, the lateral occipital complex (LOC) activation itself might reflect global completion processes that are independent of the illusory contour percept
Summary
To interpret a visual scene an observer has to differentiate between foreground and background, for instance by detecting the edges separating one triangle overlapping another. There is conflicting evidence that V1 and V2 respond to both illusory and occlusion stimuli[11] It remains unclear whether a separate or common neural mechanism mediates these different perceptual completion processes within early visual cortex. While disrupting neural processing in LOC eliminates the illusory contour percept shortly after stimulus onset, TMS over early visual cortex does so at a later time This suggests that feedback signals from LOC into early visual areas are required for the phenomenal experience of illusory contours to arise. This concurs with the response properties of neurons in early visual cortex: V1/V2 neurons have small receptive fields[19,20] that are selective for edge orientation[21,22]. It seems plausible that it extracts shape and surface properties and is only indirectly involved in detecting illusory contours
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