Enhancement of Orientation Discrimination with Stimuli in Apparent Motion

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Form and motion have been considered to be analysed separately in different streams of the visual system and then recombined at a later stage of computation of forms in motion. Nevertheless, motion analysis is required in visual tasks like breaking of camouflage and this suggests that motion mechanisms respond directly to form attributes. This second possibility was investigated by using stimuli in apparent motion, consisting of two frames in which a target element segregates by an orientation gradient from a texture background of uniformly oriented line elements. Frame-duration thresholds for target-orientation discrimination were measured to assess if they depend on: (1) whether the target is static or moving horizontally (left/right); (2) whether the target is oriented in parallel or orthogonally to the direction of motion; (3) the amount of dynamic noise present in the background texture (dynamic jitter). Thresholds for direction of motion discrimination were also measured as a function of orientation and dynamic jitter. Thresholds for orientation discrimination were lower for a moving than for a static target. Thresholds for orientation discrimination were lower than those for discrimination of motion direction. Thresholds for orientation and direction discriminations were lower when the target was oriented orthogonally to the direction of motion. Dynamic noise interfered only at high levels of background jitter. These results suggest that orientation discrimination occurs before motion-direction discrimination on the basis of a directionally selective mechanism also responding to form attributes. Form discrimination is likely to be enhanced by the motion component carried by contour motion processing similar to that involved in second-order motion which cannot specify direction of motion at its absolute threshold.