Ocular-following responses to broadband visual stimuli of varying motion coherence.

Manipulations of the strength of visual motion coherence have been widely used to study behavioral and neural mechanisms of visual motion processing. Here, we used a novel broadband visual stimulus to test how the strength of motion coherence in different spatial frequency (SF) bands impacts human ocular-following responses (OFRs). Synthesized broadband stimuli were used: a sum of one-dimensional vertical sine-wave gratings (SWs) whose SFs ranged from 0.0625 to 4 cpd in 0.05-log2(cpd) steps. Every 20 ms a proportion of SWs (from 25% to 100%) shifted in the same direction by ¼ of their respective wavelengths (drifting), whereas the rest of the SWs were assigned a random phase (flicker), shifted by half of their respective wavelengths (counterphase), or remained stationary (static): 25% to 100% motion coherence. As expected, the magnitude of the OFRs decreased as the proportion of non-drifting SWs and/or their contrast increased. The effects, however, were SF dependent. For flicker and static SWs, SFs in the range of 0.3 to 0.6 cpd were the most disruptive, whereas, with counterphase SWs, low SFs were the most disruptive. The data were well fit by a model that combined an excitatory drive determined by a SF-weighted sum of drifting components scaled by a SF-weighted contrast normalization term. Flicker, counterphase, or static SWs did not add to or directly impede the drive in the model, but they contributed to the contrast normalization process.