A neuronal phase-locked loop (NPLL) that can decode temporally-encoded
information and convert it to a rate code is based on an algorithm similar
to that of the electronic PLL, but is a stochastic device, implemented by
neural networks (real or simulated). The simplest embodiment of the NPLL
includes a phase detector (that is, a neuronal-plausible version of an
ideal coincidence detector) and a controllable local oscillator that are
connected in a negative feedback loop. The phase detector compares the
firing times of the local oscillator and the input and provides an output
whose firing rate is monotonically related to the time difference. The
output rate is fed back to the local oscillator and forces it to
phase-lock to the input. Every temporal interval at the input is
associated with a specific pair of output rate and time difference values;
the higher the output rate the further the local oscillator is driven from
its intrinsic frequency. Sequences of input intervals, which, by
definition, encode input information, are thus represented by sequences of
firing rates at the NPLL's output. The NPLL is an adaptive device which
can deal with signals whose exact characteristics are not known in advance
and can adapt to changing conditions.