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The Effect of Contrast and Size Scaling on Face Perception in Foveal and Extrafoveal Vision

From the Department of Optometry and Vision Sciences, Cardiff University, United Kingdom.

PURPOSE. To determine whether face perception can be equalized across the visual field by scaling size and contrast simultaneously.

METHODS. Contrast sensitivities were measured for detection (N = 1) and identification (N = 2–8) of a target face as a function of size (0.4^o–10^o) across eccentricities (E = 0^o–10^o).

RESULTS. In all conditions contrast sensitivity first increased and then saturated, as a function of stimulus size. Maximum sensitivity (S_max) decreased, whereas critical size (where S = S_max/) increased with eccentricity and set size (N). At each set size, sensitivities from all eccentricities could be equated by double scaling—i.e., translation in horizontal (size) and vertical (contrast) dimensions on log–log axes. Similarly, at each eccentricity, data from all set sizes could be superimposed using double scaling. Furthermore, all data could be superimposed onto the foveal detection curve when double scaled according to the equation F = 1 + E/E_2i + logN/logN_2i + E(logN)/K, where i is horizontal or vertical. This equation incorporates the eccentricity (E₂) and set size (N₂), where contrast and size double, as well as the interaction term (K).

CONCLUSIONS. Double scaling superimposes data. Not only is this possible across set sizes or eccentricities separately, but by combining their effects, a function is provided that collapses all data to a single curve, explaining all performance variation across eccentricity and set size. Our results support the proposition based on numeral recognition that failures of spatial scaling across eccentricities may simply reflect the need for scaling both size and contrast.

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