Distribution of mislocalizations of tactile stimuli on the fingers of the human hand

Sensitivity of the fingers of the two hands to faint tactile stimuli were t ested in eight healthy subjects with a von Frey hair in a forced choice poi nt localization test. Frequencies of correct responses were higher on the l eft than on the right hand, consistent with a right hemispheric advantage f or spatial processing. Within the hands, stimulations of the ring fingers r esulted in the highest percentage of correct localizations and stimulations of the thumbs in the fewest correct responses. This superiority of the rin g fingers is probably related to a higher point pressure sensitivity and do es not reflect the relative size of the representational area of the differ ent fingers in the somatosensory cortex. Mislocalizations, i.e., stimuli th at were not correctly attributed to the stimulation site, were located in t he vicinity of the stimulation site within the finger as well as across fin gers. The distribution of mislocalization across fingers deviates from a di stribution expected by chance, showing a higher frequency of mislocalizatio ns to the neighboring fingers than to more distant fingers. This observatio n in humans matches well with electrophysiological evidence from animal stu dies that some primary somatosensory cortex neurons have receptive fields t hat are not restricted to a single digit, but rather cover neighboring digi ts.