Vibrotactile temporal summation: probability summation or neural integration?

Temporal summation, a decrease in the detection threshold that occurs when either the duration of a stimulus or the number of stimuli in a sequence is increased, has been attributed to the operations of either the mechanism o f neural integration or of probability summation. Our experiments indicate that under certain conditions, both mechanisms may operate, but that the pr ocess of neural integration is an exclusive characteristic of the Pacinian (P) channel. The P channel was isolated by applying 250 Hz stimuli through a 1.5 cm(2) contactor to the thenar eminence of the hand and the NPII chann el was isolated by applying the stimuli through a 0.01 cm(2) contactor. The finding that the slopes of the psychometric functions were the same within both channels indicated that probability summation could not account for t emporal summation for stimulus durations less than 1 s. The finding that th e threshold for the detection of multiple-pulse stimuli increased as the in terpulse interval increased indicated that, for time intervals less than 80 0 ms, temporal summation results from neural integration. But for interstim ulus intervals greater than 800 ms, probability summation accounts for temp oral summation.