Cortical Representation of Space Around the Blind Spot

Abstract

. The neural mechanism that mediates perceptual filling-in of the blind spot is still under discussion. One hypothesis proposes that the cortical representation of the blind spot is activated only under conditions that elicit perceptual filling-in and requires congruent stimulation on both sides of the blind spot. Alternatively, the passive remapping hypothesis proposes that inputs from regions surrounding the blind spot infiltrate the representation of the blind spot in cortex. This theory predicts that independent stimuli presented to the left and right of the blind spot should lead to neighboring/overlapping activations in visual cortex when the blindspot eye is stimulated but separated activations when the fellow eye is stimulated. Using functional MRI, we directly tested the remapping hypothesis by presenting flickering checkerboard wedges to the left or right of the spatial location of the blind spot, either to the blind-spot eye or to the fellow eye. Irrespective of which eye was stimulated, we found separate activations corresponding to the left and right wedges. We identified the centroid of the activations on a cortical flat map and measured the distance between activations. Distance measures of the cortical gap across the blind spot were accurate and reliable (mean distance: 6 -8 mm across subjects, SD ϳ1 mm within subjects). Contrary to the predictions of the remapping hypothesis, cortical distances between activations to the two wedges were equally large for the blind-spot eye and fellow eye in areas V1 and V2/V3. Remapping therefore appears unlikely to account for perceptual filling-in at an early cortical level.

Illusory Contours Do Not Pass through the “Blind Spot”

“…We attribute the impaired curvature discrimination performance observed here to the absence of a cortical representation of the illusory contour in the blind spot region rather than to a failure in the curvature readout process. This absence of a cortical representation of the contralateral portion of the visual field corresponding to the blind spot has been reported specifically for V1 (Awater et al, 2005;Tong & Engel, 2001;Horton et al, 1990;Ts'o et al, 1990;Tootell & Hamilton, 1989;Tootell et al, 1988). We therefore conclude that V1 neurons play a critical role in generating the illusory contour representation which served as the basis of curvature discrimination in the current task.…”

“…In histological preparations of macaque (Tootell, Switkes, Silverman, & Hamilton, 1988) and human V1 (Horton, Dagi, McCrane, & DeMonasterio, 1990), the blind spot can be identified in the input layer IV as an elliptical monocular region, about four ocular dominance columns wide. Functionally, V1 neurons within the retinotopic representation of the blind spot (referred to as ''blind spot representation'' from now on) are driven entirely by the ipsilateral eye (Awater, Kerlin, Evans, & Tong, 2005;Tong & Engel, 2001). Results from a functional magnetic resonance imaging study in humans suggest that the cortical blind spot representation is confined to visual area V1 (Awater et al, 2005;Tootell et al, 1998) and lost later on.…”

“…Functionally, V1 neurons within the retinotopic representation of the blind spot (referred to as ''blind spot representation'' from now on) are driven entirely by the ipsilateral eye (Awater, Kerlin, Evans, & Tong, 2005;Tong & Engel, 2001). Results from a functional magnetic resonance imaging study in humans suggest that the cortical blind spot representation is confined to visual area V1 (Awater et al, 2005;Tootell et al, 1998) and lost later on. Most of the time we are completely unaware of this circumscribed blind region.…”

fMRI of the rod scotoma elucidates cortical rod pathways and implications for lesion measurements

“…The blind spots, although they are omnipresent, naturally occurring scotomas, are located in the midperiphery, where there is less cortex devoted to visual analysis, and the RFs are larger relative to the fovea (4, 5, 10). Not surprisingly, fMRI studies of the blind spot can only localize a small area of V1 corresponding to the blind spot, which does not make it ideally suited for these questions of ectopic responses and plasticity (36,37). Artificial, reversible scotomas caused by stabilization of the stimulus on the retina are transient and most effective in the periphery (38), making them difficult to measure in early visual areas with fMRI because of the comparatively slow temporal resolution of fMRI and the relatively smaller amount of cortex devoted to peripheral processing (10,11,39,40).…”

Temporal Limitations in Object Processing Across the Human Ventral Visual Pathway

“…Retinotopic visual areas of each subject were delineated in a separate experimental session using well-established methods (DeYoe et al 1996;Engel et al 1997;Sereno et al 1995). Details of our specific procedures for retinotopic mapping and cortical flattening can be found in previous reports from our laboratory (Awater et al 2005). In brief, subjects maintained fixation while viewing "traveling wave" stimuli consisting of rotating wedges and expanding rings, which were used to construct phaseencoded retinotopic maps of polar angle and eccentricity, respectively.…”