Spatial and Luminance Determinants of the Increment Threshold under Monoptic and Dichoptic Viewing*†

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Abstract
Increment thresholds for a 5-ms, 3.5′ test flash (TF) were measured as a function of conditioning-flash (CF) diameter at four retinal positions for photopic and scotopic luminances. CF was either 200 or 50 ms, synchronized with TF onset (transient) or continuously exposed (steady state). TF was presented to the right eye, CF either concentrically to the right eye (monoptic), or to the homotopic contralateral retinal location (dichoptic). A four-channel maxwellian-view optical system was used. When TF luminance at threshold was plotted against CF diameter, inverted U-shaped functions were obtained for both monoptic and dichoptic conditions, the peaks occurring at progressively larger CF diameters with more peripheral stimulation. For any given retinal position, monoptic and dichoptic peaks occurred over the same CF diameter. Monoptic thresholds were higher than dichoptic, except for foveal photopic data where both showed over 10-fold maximum threshold change. The transient CF produced higher threshold elevation than the steady-state CF. Dichoptic stimulation with the transient CF often produced over 10-fold threshold hanges, especially for scotopic stimulation. For any nonfoveal retinal position, the scotopic peak occurred over a larger CF diameter than the corresponding photopic condition. The results are discussed in terms of receptive-field organization.

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