Metric for separation discrimination by the human visual system

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Abstract
It is now clear that models of positional coding for elements sufficiently separated to permit individual identification require not only a first stage of linear filtering but also a second stage of representation that is preceded by a rectifying type of nonlinearity. To address the issue of the metric of this second stage we measure separation discrimination for Gabor stimuli of different sizes and of different peak spatial frequencies and separation, with and without different types of lateral distractors. Our results show that there is only a weak dependence of separation discrimination on the spatial frequency of equidetectable, spatially narrow-band stimuli; however, carrier spatial frequency can affect the influence that lateral distractors have on separation judgments. We conclude that (1) the second-stage representation is a space-size one consistent with the fact that there are scaled distributions of energy detectors of different sizes and (2) the influence of distractor elements suggests a spatial-frequency influence either at the second-stage representation or at a site beyond this second stage.

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