EFFECT OF CONTRAST AND ADAPTATION ON THE PERCEPTION OF THE DIRECTION AND SPEED OF DRIFTING GRATINGS

Three experiments were conducted to analyse the effect of contrast and adaptation state on the ability of human observers to discriminate th e motion of drifting gratings. In the first experiment, subjects judge d the direction of briefly presented gratings, which slowly drifted le ftward or rightward. The test gratings were enveloped in space by a ra ised cosine function and in time by a Gaussian. The centre of the spat ial 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, su bjects judged the relative speed of two simultaneously presented grati ngs. Stimulus contrast was varied in both experiments from 0.01 to 0.3 2. Discrimination threshold vs contrast functions were measured before and after adaptation to a high-contrast (0.4) grating drifting at rat es between 2 and 32 Hz. In a third experiment, subjects matched, befor e and after adaptation, the relative speed of a test stimulus, which h ad 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 acid speed di scrimination 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 t he speed discrimination threshold (Delta V / V) for low test contrasts . In addition, the point of subjective stationarity (PSS) shifts towar ds the adapted direction and this shift is more pronounced for low tes t 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 adaptati on frequencies between 8 and 16 Hz. We further explored the effects of adaptation contrast (0.04, 0.4 and 0.9) and adaptation drift directio n (iso- or contra-directional) on the perceived speed versus contrast function. The effect of adaptation is greatest for iso-directional dri ft and increases with increasing adaptation contrast. The results are discussed in terms of a contrast gain control model of adaptation.