frequency-dependent synaptic depression, Hebbian pairing, and synchrony in cortical pyramidal neurons

[Stephen Grossberg]

The following article is now available at 
http://www.cns.bu.edu/Profiles/Grossberg in HTML, PDF, and  Gzipped 
Postscript.

Okatan, M. and Grossberg, S.
Frequency-Dependent Synaptic Potentiation, Depression, and Spike
Timing Induced by Hebbian Pairing in Cortical Pyramidal Neurons.
Neural Networks

ABSTRACT: Experiments by Markram and Tsodyks (1996) have suggested that
Hebbian pairing in cortical pyramidal neurons potentiates or 
depresses the transmission of a subsequent pre-synaptic spike train 
at steady-state depending on whether the spike train is of low 
frequency or high frequency, respectively. The frequency above which 
pairing induced a significant decrease in steady-state synaptic 
efficacy was as low as about 20 Hz and this value depends on such 
synaptic properties as probability of release and time constant of 
recovery from short-term synaptic depression. These characteristics 
of cortical synapses have not yet been fully explained by neural 
models, notably the decreased steady-state synaptic efficacy at high 
pre-synaptic firing rates. This article suggests that this decrease 
in synaptic efficacy in cortical synapses was not observed at 
steady-state, but rather during a transition period preceding it 
whose duration is
frequency-dependent. It is shown that the time taken to reach 
steady-state may be frequency-dependent, and may take considerably 
longer to occur at high than low frequencies. As a result, the 
pairing-induced decrease in synaptic efficacy at high pre-synaptic 
firing rates helps to localize the firing of the post-synaptic neuron 
to a short time interval following the onset of high frequency 
pre-synaptic spike trains. This effect may "speed up the time scale" 
in response to high frequency bursts of spikes, and may contribute to 
rapid synchronization of spike firing across cortical cells that are 
bound together by associatively learned connections.

Key Words: synaptic potentiation, synaptic depression, frequency-dependent
synaptic plasticity, cortical pyramidal cells, Hebbian pairing, 
cortical synchronization