Identifying possible neural codes for vibrotaction using the theory of temporal summation

Abstract

The theory of temporal summation refers to the psychophysical phenomenon in which threshold decreases with increasing stimulus duration. The effect is found in audition and is central. Vibrotactile thresholds mediated by the Pacinian-corpuscle (PC) population (the “P” channel) also exhibit temporal summation. Assuming that the tactile summator is also central, it may be possible to determine the neural code for threshold used by the P channel. Theoretically, temporal summation predicts a decrease in threshold at a rate of −3 dB/doubling of duration from 10 to 100 ms. Thus by integrating the response from single fibers for various response criteria, those not having the potential for temporal summation can be rejected as possible coding schemes. PCs isolated from cat mesentery were stimulated with bursts of vibrations (300 Hz) having durations from 10 to 1000 ms, the intensity of the stimuli adjusted for fixed criterion responses of 1, 2, and 4 impulses/burst and 1 impulse/cycle. Data were sampled over the entire burst duration, in effect analyzing the response with an ideal summator. The decreases in intensity for increases in duration from 10 to 100 ms were −0.9, −1.2, −2.3, and −0.2 dB/doubling, respectively, with neither the 1 impulse/burst nor the impulse/cycle criteria showing significant summation. The other two criteria show modest summation, indicating that they have the potential for being the neural code for threshold in the P channel. [Work supported by NS23933.]