Visual motion gradient sensitivity shows scale invariant spatial frequency and speed tuning properties

We investigated sensitivity to motion gradients psychophysically using a band-pass filtered white noise stimulus with two superimposed components moving in opposite directions and spatially modulated with out of phase periodic functions. An optimum sensitivity ratio of the carrier to the modulator frequency of about 11 was measured. Tuning for speed was also observed, with sensitivity falling off at higher speeds in a trend showing scale invariance, consistent with temporal frequency tuning. Similar tuning properties were observed for both luminance and motion contrast thresholds. These findings are consistent with local and global processes in striate and extra-striate cortex respectively and suggest the scale of second stage low frequency integration is broad and matched to the spatiotemporal scale of the sensitivity of first stage local filters. The finding of scale invariance over a large range in stimulus size of 4.6–37° of visual angle suggest a general property of integrating neural mechanisms, which was identified here because of the use of narrowband stimuli.