Symmetry detection across the visual field

Authors: Sally, Sharon; Gurnsey, Rick

Source: Spatial Vision, Volume 14, Number 2, 2001 , pp. 217-234(18)

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Abstract:

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.

Document Type: Research article

DOI: http://dx.doi.org/10.1163/156856801300202940

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

Publication date: 2001-02-01

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