Abstract
Surrounding visual motion during arm reaching induces an ultra short latency manual response in the direction of visual motion. Such short latency manual response (named MFR) is almost proportional to the stimulus log-speed up to the stimulus temporal frequency around 15 Hz (Gomi et al. 2006). However, effect of the spatial integration of visual motion on the MFR is not well understood. In this study, we investigated the influence of change in stimulus size to the MFR. In the experiment, vertical sinusoidal grating pattern (85×70, or 46×36 deg) with 50% image contrast was shown on the screen. Experimental protocol was almost the same to that in our previous study. The MFR amplitude was quantified by the difference, between the rightward and leftward visual motion conditions, in the hand acceleration averaged over a brief period after the visual motion onset. The four kinds of visual stimuli were randomized within each experimental block. The MFR amplitudes for 0.02cpd with 80deg/s were significantly smaller than for the 0.125cpd with the same stimulus speed for all subjects in each stimulus size. For the small size stimuli, MFR was not greater for 0.02cpd 500deg/s than for 0.125cpd 80deg/s and 0.05cpd 200deg/s, in spite of higher speed stimulus. For the large size stimuli, however, MFR was greater for 0.02cpd 500deg/s than for the other stimuli in some subjects. These results suggest that the MFR amplitude cannot be represented only by stimulus speed in the very low spatial frequency range, and that the spatial integration characteristics varied by the stimulus spatiotemporal frequency is different in individual subjects. This subject dependent MFR modulation was highly correlated to the modulation of magnetoencephalography responses observed in the similar kind of experiment, which were evoked with a short latency (∼60ms) after the visual motion onset, suggesting a MFR related brain activity.