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The purpose of the present study is to investigate whether perceptual sensitivity to digital nerve stimulation is modulated by the afferent volley from the digital nerve of a contralateral finger. Fifteen healthy humans participated in this study. A test stimulus was given to the right-hand index finger, and a conditioning stimulus was given to one of the five fingers on the left hand 20, 30, or 40 ms before the test stimulus. The perceptual threshold of the finger stimulation was measured. The perceptual threshold of the test stimulus was significantly increased by a conditioning stimulus to the left-hand index finger given 40 ms before the test stimulus. In contrast, the threshold was not significantly changed by a conditioning stimulus to any finger other than the index finger. Perceptual sensitivity to digital nerve stimulation is suppressed by the afferent volley from the digital nerve of the contralateral homologous finger. This means that the afferent volley from the digital nerve suppresses the homologous finger representation in the ipsilateral somatosensory areas. These findings can be explained by the view that the afferent volley from the digital nerve of the index finger projects to the index finger representation in the contralateral primary sensory cortex and that the interhemispheric transcallosal inhibitory drive is provided from the secondary sensory cortex to the homologous finger representation in the contralateral secondary sensory cortex.

Under certain conditions, a tactile stimulus to the head induces the movement of the head away from the stimulus, and this is thought to be caused by a defense mechanism. In this study, we tested our hypothesis that predicting the stimulus site of the head in a quiet stance activates the defense mechanism, causing a body to sway to keep the head away from the stimulus. Fourteen healthy male participants aged 31.2 ± 6.8 years participated in this study. A visual cue predicting the forthcoming stimulus site (forehead, left side of the head, right side of the head, or back of the head) was given. Four seconds after this cue, an auditory or electrical tactile stimulus was given at the site predicted by the cue. The cue predicting the tactile stimulus site of the head did not induce a body sway. The cue predicting the auditory stimulus to the back of the head induced a forward body sway, and the cue predicting the stimulus to the forehead induced a backward body sway. The cue predicting the auditory stimulus to the left side of the head induced a rightward body sway, and the cue predicting the stimulus to the right side of the head induced a leftward body sway. These findings support our hypothesis that predicting the auditory stimulus site of the head induces a body sway in a quiet stance to keep the head away from the stimulus. The right gastrocnemius muscle contributes to the control of the body sway in the anterior–posterior axis related to this defense mechanism.

The present study examined whether an internal or external attentional focus would affect participants’ feet-in-place balance response to postural stance perturbations. A movable platform automatically slid forward or backward while healthy participants stood on it and (a) performed no cognitive activity (control), (b) focused on the pelvis or upper body sway (internal focus), (c) memorized a number displayed immediately before the platform slid (external focus), or (d) kept the equilibrium of an unstable cylinder over the arm (external focus). The forward displacement of the pelvis induced by the platform sliding forward was smaller when participants focused on their pelvic sway, although such effect was absent when they focused on their upper body sway, indicating that the internal focus was effective for the postural response when attention was paid to the pelvic sway. Regarding an external attention focus, the forward displacement of the pelvis induced by the platform sliding forward was smaller when participants focused on the equilibrium of an unstable object over the arm, but this effect was absent when they focused on the number, indicating that an external focus was only effective when the unstable object focused upon was relevant to the equilibrium of one’s own body. No attentional intervention was effective during backward sliding of the support surface, indicating that central set for responding to postural perturbation depends on the direction of the postural perturbation.