Abstract
Purpose: Motion- and luminance-defined motions (MDM/LDM) were perceived misaligned when they were presented physically with a same speed and in phase. MDM is motion of patterns defined by local motion direction. In this study, we examined the effects of global motion speed (physical and perceptual) of MDM and LDM on the amount of this offset to explore the origin of this phenomenon. Method: Stimuli were two types of square wave patterns. Each of them was consisted of 1024 random dots in the stimulus field of 120 × 300 pixels (4 × 10 deg). The square wave pattern was generated by modulating these dots' motion direction (MDM) or luminance (LDM). MDM and LDM were generated by shifting these square waves once every 120 ms. These patterns were moved at the same speed, and the phase misalignment between the two stimuli was measured by finding the null point while varying the physical phase offset between LDM and MDM using the method of constant stimuli. The speed of global motion was varied in five steps between 1.7 and 6.9 deg/sec. Subjects' task was to judge the offset direction of LDM with a 2-AFC method. Results and discussions: The null point for the apparent offset was calculated with a probit analysis. The calculated offset increased as the physical global motion speed increased. In contrast, no systematic change was found when the perceived speed of LDM or MDM was varied by manipulating the dots contrast or local motion speed. Theoretically, the spatial distance is the product of speed and time. So, the apparent spatial offset could be accounted for by the difference between perceived speed or processing time for LDM and MDM. The present results reject the former hypothesis. The offset phenomenon reported here, therefore, can be interpreted as the spatio-temporal conversions of differences in processing time for LDM and MDM.