DECODING TEMPORALLY ENCODED SENSORY INPUT BY CORTICAL OSCILLATIONS AND THALAMIC PHASE COMPARATORS

The temporally encoded information obtained by vibrissal touch could b e decoded ''passively,'' involving only input-driven elements, or ''ac tively,'' utilizing intrinsically driven oscillators, A previous study suggested that the trigeminal somatosensory system of rats bees not o bey the bottom-up order of activation predicted by passive decoding, T hus, we have tested whether this system obeys the predictions of activ e decoding, We have studied cortical single units in the somatosensory cortices of anesthetized rats and guinea pigs and found that about a quarter of them exhibit clear spontaneous oscillations, many of them a round whisking frequencies (approximate to 10 Hz), The frequencies of these oscillations could be controlled locally by glutamate. These osc illations could be forced to track the frequency of induced rhythmic w hisker movements at a stable, frequency-dependent, phase difference, D uring these stimulations, the response intensities of multiunits at th e thalamic recipient layers of the cortex decreased, and their latenci es increased, with increasing input frequency, These observations are consistent with thalamocortical loops implementing phase-locked loops, circuits that are most efficient in decoding temporally encoded infor mation like that obtained by active vibrissal touch, According to this model, and consistent with our results, populations of thalamic ''rel ay'' neurons function as phase ''comparators'' that compare cortical t iming expectations with the actual input timing and represent the diff erence by their population output rate.