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Form from motion parallax and form from luminance contrast: Vernier discrimination

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image of Spatial Vision
For more content, see Multisensory Research and Seeing and Perceiving.

Some objects are perfectly camouflaged when stationary, but are clearly visible when moving; the boundaries of such an object are defined entirely by motion parallax. Little is known about the eye's ability to make spatial discriminations between motion-defined objects. In this study, subjects viewed a pseudorandom pattern of dots within which a camouflaged bar was made visible by relative motion of dots. Vernier acuity for the motion-defined bar was 27-45 sec arc for three subjects, much less than the interdot separation of 360 sec arc, much less than the 2 deg receptive field size for motion, and comparable with the foveal intercone separation of 30 sec arc. It is proposed that an opponent-orientation process and an opponent-position process can both contribute to vernier judgements for motion-defined objects. Real-world motion contrast commonly confounds the following cues for figure-ground segregation: (1) different texture velocities on either side of the figure's boundary; (2) in any given time interval, texture in figure and ground moves different distances; and (3) texture continually appears and disappears along the figure's boundary. When cues (2) and (3) were eliminated, thus ensuring figure-ground segregation was achieved entirely by motion-sensitive neural elements, vernier acuity was 44 ± 5 sec arc compared with 36 ± 8 sec arc for a dotted bar defined by luminance contrast. Conclusion: Vernier acuity for a dotted bar whose boundary was defined entirely by motion-sensitive neural elements was similar to vernier acuity for a dotted bar whose boundary was defined by luminance contrast.

Affiliations: 1: Department of Ophthalmology, Dalhousie University, Gerard Hall, 5303 Morris Street, Halifax, Canada B3J 1B6


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