No local cancellation between directionally opposed first-order and second-order motion signals.
Despite strong converging evidence that there are separate mechanisms for the processing of first-order and second-order motion, the issue remains controversial. Qian, Andersen and Adelson (J. Neurosci., 14 (1994), 7357-7366) have shown that first-order motion signals cancel if locally balanced. Here we show that this is also the case for second-order motion signals, but not for a mixture of first-order and second-order motion even when the visibility of the two types of stimulus is equated. Our motion sequence consisted of a dynamic binary noise carrier divided into horizontal strips of equal height, each of which was spatially modulated in either contrast or luminance by a 1.0 c/deg sinusoid. The modulation moved leftward or rightward (3.75 Hz) in alternate strips. The single-interval task was to identify the direction of motion of the central strip. Three conditions were tested: all second-order strips, all first-order strips, and spatially alternated first-order and second-order strips. In the first condition, a threshold strip height for the second-order strips was obtained at a contrast modulation depth of 100%. In the second condition, this height was used for the first-order strips, and a threshold was obtained in terms of luminance contrast. These two previously-obtained threshold values were used to equate visibility of the first-order and second-order components in the third condition. Direction identification, instead of being at threshold, was near-perfect for all observers. We argue that the first two conditions demonstrate local cancellation of motion signals, whereas in the third condition this does not occur. We attribute this non-cancellation to separate processing of first-order and second-order motion inputs.
Scott-Samuel
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Department of Psychology, Royal Holloway, University of London, Egham TW20 0EX, UK. ness@astra.vision.mcgill.ca
Smith
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AT
D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D008029:Lighting; D009039:Motion Perception / Q000502:physiology*; D014795:Visual Pathways / Q000502:physiology*