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Holger Awater, Jess R. Kerlin, Frank Tong; Cortical representation of space around the blind spot. Journal of Vision 2005;5(8):894. doi: https://doi.org/10.1167/5.8.894.
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Two hypotheses have been proposed to explain filling-in of the blind spot. The active completion theory proposes that neuronal filling-in of the blind spot only occurs when the blind spot is also perceptually filled-in, such as when a stimulus is congruently presented to opposite sides of the blind spot. Alternatively, the passive remapping hypothesis proposes that neuronal filling-in can occur in the absence of perceptual filling-in when only a single side of the blind spot is stimulated. Thus, the passive account predicts that independent stimulation applied to the left or right of the blind spot should lead to neuronal filling-in (despite no perceptual filling-in), resulting in neighboring activations in visual cortex when the blind-spot eye is stimulated, but more separated activations when the fellow eye is stimulated. Using functional MRI, we tested the remapping hypothesis in four subjects by calculating the distance between cortical activations evoked by flickering checkerboard wedges (∼5 deg) presented independently to the left or to the right of the blind spot's spatial location, 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. Distances between cortical activations were measured after identifying the activation's centroid on a cortical flat map. These distances were highly precise and reliable (mean distance 6–8 mm across subjects, SD ∼1 mm within subjects) and are not different for the blind-spot eye and the fellow eye in area V1 and in area V2/V3. These results suggest that passive remapping is not the mechanism underlying filling-in of the blind spot. Instead, our findings are consistent with the active completion theory, which proposes that neuronal filling-in of the blind spot only occurs if the blind spot is also perceptually filled-in. Follow-up experiments to investigate the active completion theory are in progress.
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