Vernier acuity, crowding and cortical magnification.

When a vernier target is flanked by optimally positioned lines, foveal vernier discrimination is strongly degraded (Westheimer and Hauske, 1975). We confirmed this observation (Experiment I) and have mapped out a 2 dimensional "perceptive field" for crowding in the fovea using a 2 dot target (Experiment II). Crowding was also measured in peripheral vision, using either small flanking dots as masks (Experiment III), or using repetitive vernier gratings (Experiment IV). The results showed that when scaled in proportion to recent estimates of the cortical magnification factor, vernier acuity is as good in the periphery as it is centrally. Both centrally and peripherally, there appears to be a psychophysical processing module which we term a "perceptive hypercolumn". At all eccentricities vernier thresholds were found to be approximately 1/40 of the size of a perceptive hypercolumn and were elevated if interfering contours are present in the same (or adjacent) hypercolumns.