The Journal of Neuroscience, April 1, 2003, 23(7):3006
On the Transmission of Rate Code in Long Feedforward Networks
with Excitatory-Inhibitory Balance
Vladimir
Litvak1,
Haim
Sompolinsky1, 2,
Idan
Segev1, 3, and
Moshe
Abeles1, 4
1 The Interdisciplinary Center for Neural Computation,
2 The Racah Institute of Physics, 3 Department
of Neurobiology, Institute of Life Sciences, and
4 Department of Physiology, Hadassah Medical School, The
Hebrew University, Jerusalem 91904, Israel
The capability of feedforward networks composed of multiple layers
of integrate-and-fire neurons to transmit rate code was examined.
Synaptic connections were made only from one layer to the next, and
excitation was balanced by inhibition. When time is discrete and the
synaptic potentials rise instantaneously, we show that, for random
uncorrelated input to layer one, the mean rate of activity in deep
layers is essentially independent of input firing rate. This implies
that the input rate cannot be transmitted reliably in such feedforward
networks because neurons in a given layer tend to synchronize partially
with each other because of shared inputs. As a result of this
synchronization, the average firing rate in deep layers will either
decay to zero or reach a stable fixed point, depending on model
parameters. When time is treated continuously and the synaptic
potentials rise instantaneously, these effects develop slowly, and rate
transmission over a limited number of layers is possible. However, the
correlations among neurons at the same layer hamper reliable assessment
of firing rate by averaging over 100 msec (or less). When the synaptic potentials develop gradually, as is the realistic case, transmission of
rate code fails. In a network in which inhibition only balances the
mean excitation but is not timed precisely with it, neurons in each
layer fire together, and this volley successively propagates from layer
to layer. We conclude that the transmission of rate code in
feedforward networks is highly unlikely.
Key words:
rate code; temporal code; synfire chain; network
models; excitation-inhibition balance; synchrony; bistability; correlation; synaptic integration; information transmission
Copyright © 2003 Society for Neuroscience 0270-6474/03/2373006-10$05.00/0