Cortical Neuronal Mechanisms for Frequency Discrimination in the Somesthetic Sense of Flutter

Abstract

We summarize here the results of a study whose aim was to define the postcentral neural signals essential for frequency discrimination by primates in the somesthetic sense of flutter. We have made new psychophysical measures of this discriminative capacity in humans and monkeys, and combined experiments in monkeys in which we recorded the activity of neurons of the postcentral somatic sensory cortex evoked by the stimuli discriminated by the monkeys. The primate capacity for frequency discrimination is invariant across a range of frequencies including both flutter and vibration: from 20 Hz to 200 Hz the Weber fractions are in the range of 4-10%. Difference limens are similar whether stimuli are received passively upon the glabrous skin of the hand, or acquired actively by manipulation. Neurons of the postcentral areas 3b and 1 are entrained to periodic discharge by stimuli in the frequency range of flutter. The periods in the neuronal activity match exactly the cycle lengths of the stimuli that evoke them. The neural discriminanda for frequencies are the differences in the neural period lengths in the activity evoked by the discriminable stimuli. The neural periodicity depends upon the serial order in which neurons discharge impulses. Analysis showed that discrimination for frequency cannot be made on a basis of a neural rate code. The evoked responses of neurons of areas 3b and 1 of the postcentral somatic cortex are invariant, whether the stimuli evoking them are relevant for behavior, as in the discrimination task, or irrelevant as guides for behavior. This indicates that the differential effects of attention, so powerful in influencing discrimination performance, must be imposed upon the relevant neural processing networks at some point ‘more central’ than areas 3b and 1.