Signal combination in vibration perception

While the brain’s integration of auditory and visual inputs has been extensively investigated, the mechanisms underlying the combination of somatosensory signals remain less explored. Here, we investigated vibrotactile summation across fingers using psychophysical and electrophysiological methods. In Experiment 1, discrimination thresholds for 26 Hz vibrations were measured using a two-alternative forced-choice (2AFC) task. Thresholds exhibited a 'dipper' pattern when plotted against pedestal intensity. Detection thresholds decreased by approximately 1dB when all ten fingers were stimulated ('dekadactyl' condition) compared to when each alternate finger was stimulated ('pentadactyl' condition), suggesting a process of probability summation. When a target stimulus was presented to five digits and a pedestal stimulus to the remaining digits ('dichodactyl' condition) thresholds increased, consistent with suppression between digit representations. In Experiment 2, steady-state somatosensory evoked potential (SSSEP) signals showed approximately a 1.4-fold amplitude increase in dekadactyl compared to pentadactyl conditions, indicating a summation effect. Conversely, SSSEP amplitudes decreased when targets and masks were presented at different frequencies, providing additional evidence of suppression. These results are consistent with a model featuring inhibition between digits and reveal that the weight of suppression is intermediate between that observed in binocular vision and binaural hearing.