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Symmetry detection across the visual field

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image of Spatial Vision
For more content, see Multisensory Research and Seeing and Perceiving.

Humans are extremely sensitive to symmetry when it is foveated but sensitivity drops as a symmetrical region of a fixed size is moved into the periphery. A psychophysical study was undertaken to determine if eccentricity dependent sensitivity loss could be overcome by magnifying stimuli at each eccentricity (E) by a factor F = 1 + E/E2, where E2 indicates the eccentricity at which the size of a stimulus must be doubled, relative to a foveal standard, to achieve equivalent performance. The psychophysical task required subjects to decide on each trial in which of two intervals a symmetrical stimulus had been presented. Stimuli were presented at a range of sizes and eccentricities (0 to 8°) and the probability of a correct discrimination was computed for each condition. In Experiment 1, thresholds were measured with stimuli set to maximum available contrast and, in Experiment 2, stimuli were presented at a constant multiple of contrast detection threshold. In both experiments, a single scaling function removed most of the eccentricity dependent variation from the data. However, the E2 value recovered for one subject tested in both experiments was larger by about 65% when stimuli were not equated for visibility. We conclude that symmetry detection can be equated across a range of eccentricities by scaling stimuli with an E2 in the range of 0.88 to 1.38°. Failure to equate for visibility across all viewing conditions may result in an inflated estimate of E2.

Affiliations: 1: Department of Psychology, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec, H4B 1R6, Canada


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