Two-pulse temporal integration functions in the fovea and peripheral retina.

Abstract

The temporal integration of luminous energy was compared in the fovea and at 7 degrees eccentricity using two-pulse stimuli and two methodologies. The two-pulse stimuli consisted of two 1-msec. light pulses separated by intervals of darkness ranging from 1 to 400 msec.; they were provided by a glow modulator tube transilluminating a 21.8' opal glass target. In Exp. 1 (equal-performance design), integration functions were generated using a forced-choice staircase procedure to estimate threshold luminance. The data for two Os showed that the critical duration (CD), and thus the period of complete integration, was briefer in the fovea than at 7 degrees. Beyond the CD, integration continued to differ for the two retinal locations. In the fovea, two-pulse stimuli beyond CD evidenced partial integration and at the longest stimulus durations no integration or inhibition. In contrast, at 7 degrees stimuli beyond CD appeared to evidence probability summation. In Exp. 2 (equal-energy design), integration functions were generated by measuring the detectability of two-pulse stimuli of different durations but equal in total luminous energy. A signal-detection procedure yielded measures of both response frequency and signal detectability, P(A). The data for two Os showed that for both measures CD was briefer in the fovea than at 7 degrees. Also, in the fovea, long two-pulse stimuli appeared to show no integration or inhibition. Both experiments then showed a foveal-peripheral difference in two-pulse measures of visual temporal integration, with the fovea evidencing less integration. In addition, the forced-choice and signal-detection procedures showed that these loci differences in integration were independent of the Os' response criterion.