Temporal frequency discrimination above threshold

Temporal frequency discrimination was measured above threshold with a two-alternative spatial forced-choice procedure. Stimuli were two 1 deg homogeneous fields modulated around a mean luminance of 3.7 log trolands. Observers determined which of the two stimuli was modulated at a higher frequency. To avoid differences in apparent modulation depth as a cue for discrimination, all stimuli were matched in apparent modulation depth to an 11 Hz standard that was 0.5 log units above its threshold. Adaptation, caused by repeated presentation of suprathreshold stimuli, was avoided by using a 15 sec inter-trial interval. The relative difference thresholds (delta f/f) were a non-monotonic function of frequency. Discrimination was best near 1.5, 4.0 and 30.0 Hz (delta f/f = 0.08) and worst near 20.0 Hz (delta f/f = 0.50). Control experiments showed that the improvement in discrimination beyond 20.0 Hz was not an artifact of mismatches in apparent modulation depth. These results demonstrate the existence of multiple channels sensitive to different ranges of temporal frequency.