Frequency-dependent synaptic potentiation, depression and spike timing induced by Hebbian pairing in cortical pyramidal neurons.
Experiments by Markram and Tsodyks (Nature, 382 (1996) 807-810) 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.
Okatan
M
M
Department of Cognitive and Neural Systems and Center for Adaptive Systems, Boston University, MA 02215, USA. okatan@cns.bu.edu
Grossberg
S
S
D000200:Action Potentials; D000465:Algorithms; D000818:Animals; D002540:Cerebral Cortex / Q000502:physiology* / Q000648:ultrastructure; D019706:Excitatory Postsynaptic Potentials; D008959:Models, Neurological; D017966:Pyramidal Cells / Q000502:physiology*; D051381:Rats; D013569:Synapses / Q000502:physiology*; D009435:Synaptic Transmission