The Impact of Synaptic Unreliability on the Information Transmitted
by Spiking Neurons.
Anthony Zador.
Salk Institute MNL, La Jolla, CA 92037.
APStracts 4:333N, 1997.
ABSTRACT
The spike generating mechanism of cortical neurons is highly reliable, able to
produce spikes with a precision of a few milliseconds or less. The excitatory
synapses driving these neurons are by contrast much less reliable, subject
both to release failures and quantal fluctuations. This suggests that synapses
represent the primary bottleneck limiting the faithful transmission of
information through cortical circuitry. How does the capacity of a neuron to
convey information depend on the properties of its synaptic drive? We address
this question rigorously in an information theoretic framework. We consider a
model in which a population of independent unreliable synapses provides the
drive to an integrate-and-fire neuron. Within this model, the mutual
information between the synaptic drive and the resulting output spike train
can be computed exactly from distributions that depend only on a single
variable, the interspike interval. The reduction of the calculation to
dependence on only a single variable greatly reduces the amount of data
required to obtain reliable information estimates. We consider two factors
that govern the rate of information transfer: the synaptic reliability, and
the number of synapses connecting each presynaptic axon to its postsynaptic
target (i.e. the connection redundancy, which constitutes a special form of
input synchrony). The information rate is a smooth function of both
mechanisms; no sharp transition is observed from an ``unreliable'' to a
``reliable'' mode. Increased connection redundancy can compensate for synaptic
unreliability, but only under the assumption that the fine temporal structure
of individual spikes carries information. If only the number of spikes in some
relatively long time window carries information (a ``mean rate'' code), an
increase in the fidelity of synaptic transmission results in a seemingly
paradoxical decrease in the information available in the spike train. This
suggests that the fine temporal structure of spike trains can be used to
maintain reliable transmission with unreliable synapses.
Received 12 September 1997; accepted in final form 19 November 1997.
APS Manuscript Number J757-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997