Evidence for different spatiotemporal mechanisms using duration thresholds: An individual differences approach

The study of motion perception through classical psychophysical methods has suggested that independent spatiotemporal filters acting over specific locations in retinal images carry out early motion processing. On the other hand, individual differences approaches have been able to identify a structure of spatiotemporal filters too. In this same fashion-through an individual differences approach-the present study aims to uncover a structure of spatiotemporal frequency selective motion mechanisms. This is done, for the first time, using supra-threshold contrast stimuli in a motion direction discrimination task. Two experiments were performed measuring duration thresholds for drifting 2D Gabor gratings of 0.25, 0.5, 0.75, 1, 1.5, 2, 3 and 6 c/deg. They moved with a speed of 2 deg/sec, with Michelson contrasts of 0.1 or 0.9 (Experiment 1) or had a contrast of 0.9 drifting with a temporal frequency of 2 Hz or 8 Hz (Experiment 2). Principal component analyses uncover three factors in each of four conditions. When Varimax-rotated, these are seen to be selective to spatial frequencies lower than 0.5 c/deg, intermediate ones from 0.5 to 1-1.5 c/deg, and frequencies greater than 1-1.5 c/deg. Direct Oblimin rotations indicate that factors are moderately correlated. Further analyses show very slight differences in the correlational structures between contrast conditions (0.1 vs. 0.9), and no differences between temporal frequency conditions (2 Hz vs. 8 Hz). To conclude, the idea of a three-factor structure in motion processing for low, intermediate, and high spatial frequencies is supported.