TEMPORAL-CODE TO RATE-CODE CONVERSION BY NEURONAL PHASE-LOCKED LOOPS

Peripheral sensory activity follows the temporal structure of input si gnals. Central sensory processing uses also rate coding, and motor out puts appear to be primarily encoded by rate. I propose here a simple, efficient structure, converting temporal coding to rate coding by neur onal phase-locked loops (PLL). The simplest form of a PLL includes a p hase detector (that is, a neuronal-plausible version of an ideal coinc idence detector) and a controllable local oscillator that are connecte d in a negative feedback loop. The phase detector compares the firing times of the local oscillator and the input and provides an output who se firing rate is monotonically related to the time difference. The ou tput rate is fed back to the local oscillator and forces it to phase-l ock to the input. Every temporal interval at the input is associated w ith a specific pair of output rate and time difference values; the hig her the output rate, the further the local oscillator is driven from i ts intrinsic frequency. Sequences of input intervals, which by definit ion encode input information, are thus represented by sequences of fir ing rates at the PLL's output. The most plausible implementation of PL L circuits is by thalamocortical loops in which populations of thalami c ''relay'' neurons function as phase detectors that compare the timin gs of cortical oscillators and sensory signals. The output in this cas e is encoded by the thalamic population rate. This article presents an d analyzes the algorithmic and the implementation levels of the propos ed PLL model and describes the implementation of the PLL model to the primate tactile system.