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Takao Sato, Kazushi Maruya; Local motion speed affects the perceived speed of motion-defined motion. Journal of Vision 2005;5(8):930. doi: 10.1167/5.8.930.
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© ARVO (1962-2015); The Authors (2016-present)
It has been known that the perceived speed of luminance-defined(LD) stimuli increases as the contrast increases. Similar results have been reported for motions of contrast-modulated stimuli, but little is known as for the parameters that affect perceived speed for the other second-order motions. In this study, we examined the effect of local motion speed on perceived speed of motion defined(MD) motion by using comparison against that of LD motion. The carrier for LD and MD stimuli was 1024 random dots scattered within a 4(H) × 10(V) deg field. The LD stimuli were square wave gratings of 0.2 c/d created by modulating the dot luminance. The contrast between light and dark dots(dot contrast) was fixed at 0.75. This corresponds approximately to 0.6 of the space average luminance contrast for dark and light areas. The MD gratings were generated by moving the dots in either up or down direction within the areas corresponding to the dark and light bars of LD gratings. The dots were shifted up or down by either 100, 200, 300, or 400 min/sec. The frame-rate for this local motion for pattern generation was 50 Hz. Clearly segregated patterns were observed for all these stimuli. The motion defined patterns were then shifted horizontally by 3.5 deg once every 120 ms to generate MDM. The speed of LDM was varied in 6 steps around that of MDM to obtain an equivalent speed by a successive comparison method. It was found that the perceived speed of MDM increases by 30% as its local motion speed increases from 100 to 400 min/sec. The MDM with 400 min/sec appears faster than LDM. We conducted a similar experiment using randomly refreshed incoherent dots as carriers for LD stimuli to examine effect of temporal frequency component, and found the same results. Thus the LDM/MDM speed difference seems intrinsic to the nature of motion types. This is a first report of a parameter that modulates the perceived speed of MD motion and provides a useful method for motion studies.
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