PubMed · 7269337
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A M Derrington, G B Henning. 1981. Pattern discrimination with flickering stimuli.. https://doi.org/10.1016/0042-6989(81)90107-3
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The smooth pursuit system is usually studied using single moving objects as stimuli. However, the visual motion system can respond to stimuli that must be integrated spatially and temporally (Williams DG, Sekuler R. Vision Res 1984;24:55-62; Watamaniuk SNJ, Sekuler R, Williams DW. Vision Res 1989;29:47-59). For example, when each dot of a random-dot cinematogram (RDC) is assigned a new direction of motion each frame from a narrow distribution of directions, the whole field of dots appears to move in the average direction (Williams and Sekuler, 1984). We measured smooth pursuit eye movements generated in response to small (10 deg diameter) RDCs composed of 250 dynamic random dots. Smooth eye movements were assessed by analyzing only the first 130 ms of eye movements after pursuit initiation (open-loop period). Comparing smooth eye movements to RDCs and single spot targets, we find that both targets generate similar responses confirming that the signal supplied to the smooth pursuit system can result from a spatial integration of motion information. In addition, the change in directional precision of smooth eye movements to RDCs with different amounts of directional noise was similar to that found for psychophysical direction discrimination. These results imply that the motion processing system responsible for psychophysical performance may also provide input to the oculomotor system.
We measured Weber fractions for discriminating the speed of cyclopean gratings and Weber fractions for discriminating the speed of luminance gratings. Of our 14 observers, five were unable to see the cyclopean grating sufficiently well to discriminate its speed. One observer experienced great difficulty in discriminating the speed of cyclopean gratings, even though her threshold for detecting cyclopean gratings was low, and even though she discriminated the speed of luminance gratings on the basis of the task-relevent variable. But several observers based their speed discriminations on trial-to-trial variations of the task-relevent variable while ignoring associated trial-to-trial variations in all task-irrelevant variables (specifically: displacement; temporal frequency; spatial frequency; and presentation duration). We conclude that the visual systems of these observers contain a specialized neural mechanism for the speed of cyclopean gratings that supports acute discriminations of speed (Weber fractions were as low as 0.05-0.07).
The motion of an object can be described by a single velocity vector, or equivalently, by direction and speed separately. Similarly, our ability to see subtle differences in the motion of two objects could be constrained by either a velocity-based sensory response, or separate sensory responses to direction and speed. To distinguish between these possibilities we investigated whether direction discrimination and speed discrimination were differentially affected by changes in the axis-of-motion. Psychophysical data from 12 naive observers indicated that direction discrimination depended on axis-of-motion, but speed discrimination did not. The difference suggests that a velocity-based sensory response is not the limiting factor on the two tasks. Instead, the results imply that the sensory response which constrains speed discrimination is at least partially independent from the sensory response which constrains direction discrimination.