Effect of contrast and adaptation on the perception of the direction and speed of drifting gratings

Abstract

Three experiments were conducted to analyse the effect of contrast and adaptation state on the ability of human observers to discriminate the motion of drifting gratings. In the first experiment, subjects judged the direction of briefly presented gratings, which slowly drifted leftward or rightward. The test gratings were enveloped in space by a raised cosine function and in time by a Gaussian. The centre of the spatial envelope was either 2 deg left or right of the fixation point. An adaptive staircase procedure was used to find the velocities, at which the observer judged the motion direction in 75% of the presentations as leftwards or rightwards, respectively. In the second experiment, subjects judged the relative speed of two simultaneously presented gratings. Stimulus contrast was varied in both experiments from 0.01 to 0.32. Discrimination threshold vs contrast functions were measured before and after adaptation to a high-contrast (0.4) grating drifting at rates between 2 and 32 Hz. In a third experiment, subjects matched, before and after adaptation, the relative speed of a test stimulus, which had a constant contrast (0.04 or 0.08) and a variable speed, to that of a reference stimulus having a variable contrast but a constant speed. The results indicate that, before adaptation, direction and speed discrimination thresholds are independent of test contrast, except when test contrast approaches the detection threshold level. Adaptation to a drifting grating increases the lower threshold of motion (LTM) and the speed discrimination threshold (ΔV/V) for low test contrasts. In addition, the point of subjective stationarity (PSS) shifts towards the adapted direction and this shift is more pronounced for low test contrasts. The perceived speed of a drifting grating increases with increasing contrast level. Adaptation to a drifting grating shifts the perceived speed vs log contrast function downwards and to the right (toward higher contrast levels) and this shift is greatest for adaptation frequencies between 8 and 16 Hz. We further explored the effects of adaptation contrast (0.04, 0.4 and 0.9) and adaptation drift direction (iso- or contra-directional) on the perceived speed versus contrast function. The effect of adaptation is greatest for iso-directional drift and increases with increasing adaptation contrast. The results are discussed in terms of a contrast gain control model of adaptation.