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 tested in eight healthy subjects with a von Frey hair in a forced choice point localization test. Frequencies of correct responses were higher on the left than on the right hand, consistent with a right hemispheric advantage for spatial processing. Within the hands, stimulations of the ring fingers resulted in the highest percentage of correct localizations and stimulations of the thumbs in the fewest correct responses. This superiority of the ring fingers is probably related to a higher point pressure sensitivity and does not reflect the relative size of the representational area of the different fingers in the somatosensory cortex. Mislocalizations, i.e., stimuli that were not correctly attributed to the stimulation site, were located in the vicinity of the stimulation site within the finger as well as across fingers. The distribution of mislocalization across fingers deviates from a distribution expected by chance, showing a higher frequency of mislocalizations to the neighboring fingers than to more distant fingers. This observation in humans matches well with electrophysiological evidence from animal studies that some primary somatosensory cortex neurons have receptive fields that are not restricted to a single digit, but rather cover neighboring digits.