((U. Polat & C.W. Tyler)) Smith-Kettlewell Eye Research Institute, San Francisco
Although sensitivity improves with increasing stimulus area, detection efficiency peaks at about 3 cycles1
, with slower sensitivity improvement thereafter. Current psychophysical models are based on the responses of arrays of localized, orientation selective spatial filters that are typically assumed to have a circularly symmetric envelope. Since each filter assumed to respond independently of the others, sensitivity improvement as a function of stimulus area is explained in terms of probability summation. If so, improvement should be uniform regardless of stimulus configuration. By contrast, recent studies2
have shown sensitivity facilitation specific for collinear patterns that is explained by long-range interactions. To address this distinction, we asked how the visual system pools responses for elongated envelopes of collinear and orthogonal gratings and what is the optimal spatial filter that could account for the data?
Methods. We used 2AFC to measure contrast threshold detection for human fovea and near periphery for either circular or elongated Gabor patches, controlling uncertainty effects by evaluating psychometric slope. Patch elongation increased either collinearly with, or orthogonally to, carrier orientation; aspect ratios varied from 1:1 to 12:1. Patches were presented either for 40 msec or in a 1 sec cosine bell.
Results. Sensitivity improved with increasing aspect ratio for collinear configurations much more than orthogonal configurations. Efficiency peaked for the collinear configuration at aspect ratios of 1.5:1 – 3:1. For orthogonal configurations, the improvement was less than probability summation (i.e., efficiency declined throughout).
The pattern of results is consistent with either elongated collinear filters or orientation-specific, long-range collinear interactions. The length for maximal efficiency implied an optimal filter elongated along the orientation axis with an aspect ratio of about 2:1. In terms of long-range interactions, the optimal improvement is consistent with the maximal collinear facilitation found at 3 wavelengths2
1. Watson et al. (Nature 302, 419, 1983). 2. Polat & Sagi (Vision Research. 33, 993, 1994)
CWT Supported by NIH 7890 C5