Neural Responses in the Retinotopic Representation of the Blind Spot in the Macaque V1 to Stimuli for Perceptual Filling-In

Abstract

When visual stimuli that cover the entire blind spot are presented monocularly, the color and brightness of the surrounding field are seen within the blind spot, although it receives no retinal input. Important questions about such perceptual filling-in are whether neurons in the visual system representing visual field locations within the blind spot are activated when filling-in occurs and, if so, what the properties of these neurons are. To address these questions, we recorded the activities of single neurons in the primary visual cortex (V1) of the awake monkey. We first identified the area of V1 representing the region of the blind spot and then assessed neural responses to stationary visual stimuli of various size. We found that there are neurons in layer 4 and deeper laminae, particularly layer 6, that respond to large stimuli covering the blind spot which induces perceptual filling-in. Most of these neurons had very large binocular receptive fields that extended outside the blind spot. These neurons also preferred relatively large stimuli and exhibited color selectivity. These results indicate that when a large uniform surface is presented on the blind spot, neurons at the V1 region representing the blind spot transmit signals essential for filling-in that inform of the presence of a large surface as well as the absence of smaller stimuli at the blind spot.

The primary visual cortex fills in color

“…The present finding, though limited to brain activity corresponding to color or brightness filling-in and illusory contours of an illusory transparent surface, is consistent with other findings that showed V1 activity for uniform surface and brightness (9,10,54) in the sense that roles of V1 neurons are not restricted to be a simple orientation filter of lines (8). V1 seems to play a greater variety of roles than was once thought.…”

“…Although recent findings have shown that V1 in the monkey brain is activated for surface processing (9,10,40,41), it has been suggested that this activation results solely from feedback from higher cortical stages. However, when we controlled for attention, which could give feedback signals from the higher-order visual areas, significant fMRI activity was observed only in the part of V1 that retinotopically corresponds to a color filled-in area of an illusory transparent surface.…”

“…However, the present finding of V1 being activated with filling-in for an illusory transparent surface may not be applied to all of the filling-in effects. Activity for filling-in across the blind spot was found to be in V1 of monkeys and humans (10,50) and has been attributed to activity of V1 cells with large receptive fields encompassing the inducing contours across the blind spot. The color filling-in in the present study is spatially too extensive (Ϸ10°) to be explained by activity of the cells with large receptive fields.…”

“…It has been suggested that the surface representation level resides at a midstage of visual processing, beyond the level of local filtering (2-7). However, contradicting empirical and theoretical reports have also been proposed (8)(9)(10)(11)(12). One reason for this controversy may be the tacit assumption that surface representation is accomplished by single processing rather than multiple processing.…”

No Functional Magnetic Resonance Imaging Evidence for Brightness and Color Filling-In In Early Human Visual Cortex

“…Neurophysiological studies of monkey and cat visual cortex provide some support for the filling-in hypothesis. A small proportion of neurons, responding to the interiors of achromatic surfaces, exhibit properties consistent with aspects of human brightness constancy (MacEvoy and Paradiso, 2001), brightness induction (Rossi et al, 1996;Rossi and Paradiso, 1999;Kinoshita and Komatsu, 2001;Peng and Van Essen, 2005), the Craik-Cornsweet-O'Brian brightness illusion (Hung et al, 2001; Roe et al, 2005), and surface completion of the retinal blind spot (Komatsu et al, 2000(Komatsu et al, , 2002. No neurophysiological study, however, has yet revealed evidence of a topographic cortical representation corresponding to uniform surface regions von der Heydt et al, 2003).…”