DIRECTION IDENTIFICATION THRESHOLDS FOR 2ND-ORDER MOTION IN CENTRAL AND PERIPHERAL-VISION

Evidence bearing on the question of whether first-order and second-ord er motion are detected by use of the same or different principles has been sought. This question was approached by measuring thresholds for correctly identifying the direction of motion of various second-order motion patterns. The patterns used were contrast-modulated noise patte rns in which the contrast of a carrier was modulated sinusoidally in o ne dimension, and the modulating waveform drifted smoothly while the c arrier itself remained stationary. The carrier used was in most cases static two-dimensional noise; other carriers gave similar results. Thr esholds were measured in terms of amplitude of contrast modulation (mo dulation depth) for each of a range of envelope drift speeds and spati al frequencies in the fovea and at several viewing eccentricities. Alo ng with direction-identification thresholds, thresholds for either sim ple detection of the modulation or for correctly identifying the orien tation of the modulation were simultaneously measured. Thresholds for direction identification were generally somewhat higher than those for simple detection. However, they were in most cases very similar to th resholds for identification of orientation, as found for conventional luminance gratings. Contrary to some reports, sensitivity to contrast- modulated patterns declines with eccentricity at a similar rate to tha t found with luminance gratings. The results suggest that first-order and second-order motion are either detected by a common motion-detecti on mechanism or are detected by different mechanisms that use a common principle of motion detection.