Ocular following responses of monkeys to the competing motions of two sinusoidal gratings

Abstract

Ocular following responses (OFRs) were elicited in monkeys at short latencies (∼50 ms) by applying motion in the form of successive 1/4-wavelength steps to each of two overlapping vertical sine-wave gratings that had different spatial frequencies. In the first experiment, the two sine waves had spatial frequencies in the ratio 3:5 and moved in opposite directions. The initial OFRs showed a highly nonlinear dependence on the relative contrasts of the competing sine waves. On average, when the contrast of one was less than a third of that of the other then the one with the lower contrast became ineffective – as though suppressed – and the OFR was entirely determined by the sine wave of higher contrast: winner-take-all. In a second experiment, the two sine waves had spatial frequencies in the ratio 3:7 and moved in the same direction (though at different speeds). The initial OFRs again showed a highly nonlinear dependence on the relative contrasts of the competing sine waves, with a winner-take-all outcome when the contrasts of the two sine waves were sufficiently different. In both experiments, the nonlinear dependence on the relative contrasts of the competing sine waves was well described by a contrast-weighted-average model with just two free parameters. These findings were very similar to those of [Sheliga, B.M., Kodaka, Y., FitzGibbon, E.J., Miles, F.A., 2006c. Human ocular following initiated by competing image motions: evidence for a winner-take-all mechanism. Vision Res. 46, 2041–2060] on the human OFR, indicating that the monkey is a good animal model for studying the nonlinear interactions that emerge when competing motions are used.