addition to telling us where V1 is usually located, prior studies from V1 in macaques and humans had already suggested a high degree of retinotopic precision and a polar organization of the V1 retinotopy ((e.g., refs. 9, 10, 13, 34, 35, 46, 47), monocular dominance in the blind spot representation (29, 30, 35, 36), orientation selectivity (e.g., refs. 41–43), and even the averaged contrast gain functions (5, 48). In this sense the present study has value as a “calibration” or “confirmation” study.
However, some other aspects of the present study are more novel. The tests for orientation selectivity based on transient fMRI signals are an approach that could easily be generalized to tests for similarly coded stimulus dimensions, such as visual motion, color, etc. The tests for the blind spot representation could likewise be generalized to trace where and when other retinal inhomogenieties (e.g., rod/cone ratio changes with eccentricity, etc.) are “filled in” in cortex. For instance, it is interesting that the monocular blind spot representation does not appear in V2 (Fig. 4), although a binocular stimulus of almost equal dimensions is represented in V2 (Fig. 2). Because we are largely unconscious of these retinal variations, this issue also bears on the question of which cortical visual areas, and what functional aspects of visual processing, have access to conscious perception (e.g., ref. 49). Finally, the maps of contrast sensitivity (Fig. 5) suggest that contrast gain may be better accounted for by considering models of probability summation within progressively larger receptive fields (e.g., ref. 40), rather than in terms of predominant input from either magnocellular or parvocellular “streams” (e.g., refs. 5 and 48).
We thank Mary Foley, Terrance Campbell, William Kennedy, Bruce Rosen, and Thomas Brady for invaluable assistance during the course of this project. We are especially grateful to Dr. Stephan Brandt for helping to acquire some of the data. This work was supported by grants from the Human Frontiers Science Program and the National Eye Institute to R.B.H.T., the Swiss Fonds National de la Recherche Scientifique to N.K.H., and the McDonnell-Pew Foundation to J.D.M.
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