Abstract
Increment thresholds for a 30° circular striped or uniform test field centered at the fovea were measured against ten different background illuminances, including darkness. At photopic background levels, threshold values for coarse grating stimuli (0.99–0.07 cycle/degree) were lower than the increment threshold for the uniform test field; at scotopic levels, they were higher. The reversal of sequence took place in the zone of transition from photopic to scotopic backgrounds, for which thresholds for the two types of stimuli were similar. Results are attributed to (i) the facilitating effect of contrast during foveal cone vision; (ii) response to the illuminance of the light grating bars during mixed receptor activity; and (iii) sampling of luminous flux from both light and dark bars in peripheral rod-summation pools.
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