Evidence bearing on the question of whether first-order and second-order 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 patterns in which the contrast of a carrier was modulated sinusoidally in one dimension, and the modulating waveform drifted smoothly while the carrier itself remained stationary. The carrier used was in most cases static two-dimensional noise; other carriers gave similar results. Thresholds were measured in terms of amplitude of contrast modulation (modulation depth) for each of a range of envelope drift speeds and spatial frequencies in the fovea and at several viewing eccentricities. Along with direction-identification thresholds, thresholds for either simple detection of the modulation or for correctly identifying the orientation 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 thresholds 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 that found with luminance gratings. The results suggest that first-order and second-order motion are either detected by a common motion-detection mechanism or are detected by different mechanisms that use a common principle of motion detection.
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