Second-order motion perception in the peripheral visual field.

In motion perception, luminance-defined stimuli (first-order motion) are distinguished from stimuli defined by more complex attributes (second-order motion), because they differ in their processing requirements. For instance, a two-layer model with the output of an array of elementary motion detectors (EMD's) feeding into a second array of EMD's has been proposed to account for seeing the movement of motion-defined objects. The question is raised whether this processing scheme is operating across the whole visual field or whether second-order motion perception is restricted to the fovea. The detection, orientation discrimination, and motion direction discrimination of oblique, vertically moving bars was tested at horizontal eccentricities between 0 degree and 16 degrees. Bars were defined on a dynamic noise background by an area of static dots (drift-balanced motion) or by coherent dot motion either in the direction of the bar motion (Fourier motion) or in the orthogonal direction (theta motion). Coherence thresholds for direction discrimination are severely impaired in the periphery for both types of second-order motion but not for Fourier motion, whereas orientation discrimination and detection marginally decline for all three bar types when the stimuli are presented further out in the periphery. In a control experiment it is shown that this result cannot be due entirely to the changes in spatial scale of the peripheral visual system. The facts that motion-defined objects can be detected in the periphery and that their orientation can be detected, but not their direction of motion, supports the view that the two-layer system suggested for the processing of theta motion is restricted to the central region of the visual field.