Vernier acuity, crowding and cortical magnification

Long-range dichoptic interactions in the human visual cortex in the region corresponding to the blind spot

“…the eccentricity of the center of the blind spotabout 16.5 deg), E2 is the eccentricity in deg at which cortical magniftcation has dropped to 0.5*M, (the fovea1 magnification factor in ~/deg) and k = (Mr*EZ)-'. For the rhesus monkey the following parameters have been reported (Levi et al,, 1985;Dow, Snyder, Vautin & Bauer, 1981) Another study suggested a similar expression for the variation of linear cortical magnification with eccentricity (LeVay et al, 1985) based on results reported in earlier studies (Hubel & Wiesel, 1974;Hubel & Freeman, 1977). According to this study the expression for M for the macaque monkey is given by:…”

“…It has been reported in a variety of visual tasks, e.g. letter discrimination (Bouma, 1970;Toet & Levi, 1992;Nazir, 1992), tilt discrimination (Andriessen & Bouma, 1976;Westheimer, Shimamura & McKee, 1976) and vernier acuity (Westheimer & Hauske, 1975;Levi, Klein & Aitsebaomo, 1985). It is believed to be a cortical phenomenon because performance is seen to be similar when flanking contours are placed in either the same eye as the test stimulus or in the opposite eye (Flom, Heath & Takahashi, 1963;Westheimer & Hauske, 1975).…”

“…Two-dot alignment thresholds across the blind spot are never lower than alignment thresholds at comparable locations in the visual field, when the separation of the dots is kept constant (Tripathy, Levi & Ogmen, unpublished results). Since two-dot alignment thresholds are known to decrease with reduction of the separation of the dots (Sullivan, Oatley & Sutherland, 1972;Levi, Klein & Aitsebaomo, 1985;Beck & Halloran, 1985) this result suggests that either the separation across the blind spot is not sewn-up or the separation is sewn-up, but alignment thresholds are still high because of greater positional uncertainty around the blind spot.…”

“…M can be predicted from the equation (Levi et al, 1985;Van Essen, Newsome & Maunself, 1984;Drasdo, 1991):…”

“…Using this factor to scafe the cortical extents of interaction derived for monkeys [using equation (2)], the estimates for cortical extents of interaction (for humans) at the eccentricity of the blind spot vary between 3.8 and 7.7 mm of cortex, with a mean of 6.4 mm. Another proposed estimate for the ratio human cortical ma~i~cation factor~monkey cortical magnification factor is 1.44 (Levi et at., 1985;Drasdo, 1991). Using this estimate the extents of cortical interaction for humans would be smaller than the earlier estimates by about 10%.…”