Discrimination of time: comparison of foveal and peripheral sensitivity.

Temporal processing is different in the periphery than in the fovea. Two measures of temporal sensitivity, increment thresholds for onset asynchrony and temporal modulation thresholds (flicker), were studied in the fovea and at 20 deg in the lower visual field. The ability to detect incremental changes in the time between the onsets of two vertical lines was generally more precise in the fovea than at the peripheral locus, at least for asynchronies ranging from 10 to 80 msec. Flicker thresholds were measured from 1 to 40 Hz with a vertical-line target used for the asynchrony thresholds. The peripheral flicker function peaks at a higher temporal frequency (15-20 Hz) than the foveal function (8-10 Hz); this suggests that the peripheral temporal impulse response follows a faster time course than the foveal impulse response. Asynchrony discrimination would potentially provide a simple estimate of the temporal sensitivity underlying velocity discrimination. We find that asynchrony discrimination is not a good predictor of velocity thresholds; it fails at the high velocities at which velocity discrimination is most precise.