VERNIER ACUITY WITH NONSIMULTANEOUS TARGETS - THE CORTICAL MAGNIFICATION FACTOR ESTIMATED BY PSYCHOPHYSICS

The eccentricity at which peripheral thresholds double their foveal va lue (E(2)) may relate to the visual system's anatomical organization, Using a variety of experimental approaches, previous estimates of E(2) for vernier acuity have ranged fi-om less than 0.1 deg to greater tha n 15.0 deg. This broad range of values seems to challenge the usefulne ss of E(2) for determining visual topography, We explain that the vary ing contributions from at least two different regimes, spatial filter and local sign, may explain the broad range of E(2) values found previ ously, We attempt to limit responses to the local sign regime, where i t may be possible to determine the psychophysical analog to the gradie nt of the cortical spatial grain, In our experiments we measure how ve rnier task performance falls off with eccentricity, We hypothesize tha t if the vernier features are adequately separated in time, they will fall outside of the spatial filter's temporal integration span and the local sign regime would then predominate for precise positional proce ssing. Using an interstimulus interval ranging from 20 to 200 msec bet ween the two vernier features, we estimate that vernier thresholds in the local sign regime double at about 0.8 +/- 0.2 deg eccentricity, wh ich is similar to anatomical estimates of the eccentricity at which th e linear spacing of human cortical units doubles. Copyright (C) 1996 E lsevier Science Ltd