Orientation discrimination for objects defined by relative motion and objects defined by luminance contrast.
A bar-shaped area within a pattern of random dots was demarcated by moving the dots within the bar at a velocity equal and opposite to the velocity of dots outside the bar. Orientation discrimination for this motion-defined dotted bar was compared with orientation discrimination for a contrast-defined dotted bar that was created by switching off all dots outside the bar. Orientation discrimination was approximately as acute (approx. 0.5 deg) for a motion-defined bar as for a contrast-defined dotted bar, provided that dot contrast and speed were both high. Furthermore, this 0.5 deg discrimination compares with the most acute values reported for sharp-edged lines and sinewave gratings. For the motion-defined bar discrimination fell off rapidly when dot contrast was reduced, but remained acute for the contrast-defined bar for a further reduction of 0.6 log units. Thus, there was a 4:1 range of contrasts over which discrimination had collapsed for the motion-defined bar but remained acute for the contrast-defined bar. For the motion-defined bar discrimination also fell off rapidly at low dot speeds, but was almost unaffected by speed for the contrast-defined bar. These findings bear on the question whether orientation of motion-defined and contrast-defined bars are analyzed by the same or by different neural mechanisms, and pose a challenge for current theories of orientation discrimination.
Regan
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Department of Psychology BSB, York University, Ontario, Canada.
D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D008027:Light; D008297:Male; D008875:Middle Aged; D009039:Motion Perception / Q000502:physiology*; D009949:Orientation / Q000502:physiology*; D013028:Space Perception / Q000502:physiology; D014795:Visual Pathways