DIFFERENT PROCESSES UNDERLIE THE DETECTION OF 2ND-ORDER MOTION AT LOWAND HIGH TEMPORAL FREQUENCIES

The aim of this project was to determine whether first- and second-ord er motion stimuli are detected by the same mechanism. We began by meas uring the temporal contrast sensitivity function (CSF) for the directi onal discrimination of 0.3 c deg(-1) drifting spatial beat patterns an d luminance modulated gratings. The CSF for beat patterns was bimodal, with sensitivity maximal near 1-2 Hz and 10-12 Hz. In contrast, the C SF for luminance gratings had a single peak near 10 Hz. Separate adapt ation experiments were then done using all permutations of beat patter ns and luminance gratings as the test and adaptation stimuli. In gener al, sensitivity to a test pattern of fixed temporal frequency (2, 4, 8 or 16 Hz) was measured both before and after adaptation to patterns w hose temporal frequency varied over a wide range. The rationale for th ese experiments was that if first- and second-order stimuli are proces sed by the same mechanisms, the adaptation tuning curves should all be similar. Our results show that this is the case at high temporal freq uencies (> 4 Hz), but not at low temporal frequencies. The post-adapta tion sensitivity functions for test patterns with temporal frequencies of 8 Hz and 16 Hz were bandpass, with maximal adaptation near 12 Hz, and showed evidence of beat-specific adaptation; for 2 Hz test pattern s the sensitivity functions were lowpass, adaptation declining above 2 0 Hz, but there was no beat-specific adaptation. The shape of the CSFS and the results of the adaptation experiments show that separate proc esses mediate the detection of drifting beat patterns at low and high temporal frequencies. The results are consistent with the hypothesis t hat fast second-order motion is detected by Fourier-type mechanisms, p receded by a nonlinearity, and slow second-order motion is detected by a process involving a comparison of local luminance features.