In addition to the goal-directed preplanned control, which strongly governs reaching movements, another type of control mechanism is suggested by recent findings that arm movements are rapidly entrained by surrounding visual motion. It remains, however, controversial whether this rapid manual response is generated in a goal-oriented manner similarly to preplanned control or is reflexively and directly induced by visual motion. To investigate the sensorimotor process underlying rapid manual responses induced by large-field visual motion, we examined the effects of contrast and spatiotemporal frequency of the visual-motion stimulus. The manual response amplitude increased steeply with image contrast up to 10% and leveled off thereafter. Regardless of the spatial frequency, the response amplitude increased almost proportionally to the logarithm of stimulus speed until the temporal frequency reached 15-20 Hz and then fell off. The maximum response was obtained at the lowest spatial frequency we examined (0.05 cycles/°). These stimulus specificities are surprisingly similar to those of the reflexive ocular-following response induced by visual motion, although there is no direct motor entrainment from the ocular to manual responses. In addition, the spatiotemporal tuning is clearly different from that of perceptual effects caused by visual motion. These comparisons suggest that the rapid manual response is generated by a reflexive sensorimotor mechanism. This mechanism shares a distinctive visual-motion processing stage with the reflexive control for other motor systems yet is distinct from visual-motion perception.

Information pertaining to visual motion is used in the brain not only for conscious perception but also for various kinds of motor controls. In contrast to the increasing amount of evidence supporting the dissociation of visual processing for action versus perception, it is less clear whether the analysis of visual input is shared for characterizing various motor outputs, which require different kinds of interactions with environments. Here we show that, in human visuomotor control, motion analysis for quick hand control is distinct from that for quick eye control in terms of spatiotemporal analysis and spatial integration. The amplitudes of implicit and quick hand and eye responses induced by visual motion stimuli differently varied with stimulus size and pattern smoothness (e.g., spatial frequency). Surprisingly, the hand response did not decrease even when the visual motion with a coarse pattern was mostly occluded over the visual center, whereas the eye response markedly decreased. Since these contrasts cannot be ascribed to any difference in motor dynamics, they clearly indicate different spatial integration of visual motion for the individual motor systems. Going against the overly unified hierarchical view of visual analysis, our data suggest that visual motion analyses are separately tailored from early levels to individual motor modalities. Namely, the hand and eyes see the external world differently.

We investigated a visuomotor mechanism contributing to reach correction: the manual following response (MFR), which is a quick response to background visual motion that frequently occurs as a reafference when the body moves. Although several visual specificities of the MFR have been elucidated, the functional and computational mechanisms of its motor coordination remain unclear mainly because it involves complex relationships among gaze, reaching target, and visual stimuli. To directly explore how these factors interact in the MFR, we assessed the impact of spatial coincidences among gaze, arm reaching, and visual motion on the MFR. When gaze location was displaced from the reaching target with an identical visual motion kept on the retina, the amplitude of the MFR significantly decreased as displacement increased. A factorial manipulation of gaze, reaching-target, and visual motion locations showed that the response decrease is due to the spatial separation between gaze and reaching target but is not due to the spatial separation between visual motion and reaching target. Additionally, elimination of visual motion around the fovea attenuated the MFR. The effects of these spatial coincidences on the MFR are completely different from their effects on the perceptual mislocalization of targets caused by visual motion. Furthermore, we found clear differences between the modulation sensitivities of the MFR and the ocular following response to spatial mismatch between gaze and reaching locations. These results suggest that the MFR modulation observed in our experiment is not due to changes in visual interaction between target and visual motion or to modulation of motion sensitivity in early visual processing. Instead the motor command of the MFR appears to be modulated by the spatial relationship between gaze and reaching.

The body midline provides a basic reference for egocentric representation of external space. Clinical observations have suggested that vestibular information underpins egocentric representations. Here we aimed to clarify whether and how vestibular inputs contribute to egocentric representation in healthy volunteers. In a psychophysical task, participants were asked to judge whether visual stimuli were located to the left or to the right of their body midline. Artificial vestibular stimulation was applied to stimulate the vestibular organs. We found that artificial stimulation of the vestibular system biased body midline perception. Importantly, no effect was found on motor effector selection. We also ruled out additional explanations based on allocentric visual representations and on potential indirect effects caused by vestibular-driven movements of the eyes, head and body. Taken together our data suggest that vestibular information contributes to computation of egocentric representations by affecting the internal representation of the body midline.