Regularity of approaching visual stimuli influences spatial expectations for subsequent somatosensory stimuli

This study examines how the regularity of visual stimuli approaching the body influences spatial expectations of subsequent somatosensory stimuli by recording event-related brain potentials (ERPs) during a simple reaction time (RT) task involving responses to somatosensory stimuli. Twenty-one participants were instructed to put their arms on a desk, and three LEDs were placed equidistantly between their arms. Electrical stimuli were presented with a high probability (80%) of being applied to one wrist and a low probability (20%) of being applied to the opposite wrist. One trial was composed of three visual stimuli followed by one electrical stimulus. In the regular approach condition, LEDs flashed sequentially toward the wrist with the high-probability somatosensory stimulus. In the irregular approach condition, the first and second visual stimuli were presented randomly, but the third visual stimulus was invariably presented near the wrist with the high-probability stimulus. In both conditions, RTs for low-probability stimuli were slower than those for high-probability stimuli, and the low-probability stimuli elicited larger P3 amplitudes than the high-probability stimuli. Furthermore, the largest P3 amplitude was elicited by low-probability stimuli under the regular approach condition, whereas the amplitudes of contingent negative variation (CNV) elicited before the presentation of the somatosensory stimuli did not differ between conditions. These results indicate that regularity of visual stimuli approaching the body facilitates an automatic spatial expectation for subsequent somatosensory stimuli.