Difference in perceptual and oculomotor responses revealed by apparent motion stimuli presented with an interstimulus interval

Abstract

To understand the mechanisms underlying visual motion analyses for perceptual and oculomotor responses and their similarities/differences, we analyzed eye movement responses to two-frame animations of dual-grating 3 f5 f stimuli while subjects performed direction discrimination tasks. The 3 f5 f stimulus was composed of two sinusoids with a spatial frequency ratio of 3:5 (3 f and 5 f), creating a pattern with fundamental frequency f. When this stimulus was shifted by 1/4 of the wavelength, the two components shifted 1/4 of their wavelengths and had opposite directions: the 5 f forward and the 3 f backward. By presenting the 3 f5 f stimulus with various interstimulus intervals (ISIs), two visual-motion-analysis mechanisms, low-level energy-based and high-level feature-based, could be effectively distinguished. This is because response direction depends on the relative contrast between the components when the energy-based mechanism operates, but not when the feature-based mechanism works. We found that when the 3 f5 f stimuli were presented with shorter ISIs (<100 ms), and 3 f component had higher contrast, both perceptual and ocular responses were in the direction of the pattern shift, whereas the responses were reversed when the 5 f had higher contrast, suggesting operation of the energy-based mechanism. On the other hand, the ocular responses were almost negligible with longer ISIs (>100 ms), whereas perceived directions were biased toward the direction of pattern shift. These results suggest that the energy-based mechanism is dominant in oculomotor responses throughout ISIs; however, there is a transition from energy-based to feature-tracking mechanisms when we perceive visual motion.