Central mechanisms for two-point discrimination in humans.

We studied the cognitive mechanisms for two-point discrimination (TPD) in 11 normal subjects, using electrical pulses. We used six ball-shaped electrodes placed in line on the dorsal surface of the left hand, and two-point was stimulated by two electrodes randomly selected. We measured the reaction time for TPD and calculated the percentage of correct responses for each two-point stimulation. The subjects' response was significantly affected by the preceding stimuli as well as the distance of the stimuli: for a two-point stimulus condition, subjects tended to feel the stimuli as two-point when the distance between the stimuli was longer than that of preceding stimuli, whereas they felt the stimuli as one-point when the distance was shorter than that of the preceding stimuli. The present results indicate that the TPD process involved evaluation of the distance between the stimuli relatively to that of the preceding stimuli, as well as evaluation of absolute distance between the stimuli.