Frequency discrimination in the sense of flutter: psychophysical measurements correlated with postcentral events in behaving monkeys.

The capacities of humans and monkeys to discriminate between the frequencies of mechanical sinusoids delivered to the glabrous skin of the hand have been measured in psychophysical experiments. The 2 primates have similar capacities; they make discriminations with Weber fractions that change little over the frequency range from 20 to 200 Hz. The discriminatory capacities are similar whether stimuli are received passively or acquired actively. Combined experiments have been made in monkeys in which the electrical signs of the activity of quickly adapting (QA) and slowly adapting (SA) neurons of postcentral areas 3b and 1 were recorded, both in the working state as the animal made discriminations and in the irrelevant state in which the stimuli did not guide behavior. The neuronal responses were analyzed in terms of discharge rates, periodicities in the neuronal discharges, and harmonic contents. It was shown that discriminatory capacity depends upon the period lengths in the sets of periodically entrained activity evoked by stimuli readily discriminated, and not upon the small differences in rates of discharge evoked by those stimuli. The periodicities were shown by harmonic analysis to be sharply limited to stimulus frequencies. Low-frequency stimuli evoke periodicities at the second and third harmonics in some neurons, in addition to strongly periodic signals at the fundamental frequency of the stimuli. Their presence does not appear to interfere with frequency discrimination. Neuronal responses recorded in the stimulus-irrelevant state were not distinguishable from those recorded as monkeys made discriminations. The responses of SA neurons, recorded under similar conditions, resembled those of QA neurons in almost every feature, but reasons are given for concluding that the SA system plays no role in frequency discrimination in the sense of flutter.