Functional Significance of Synaptic Depression between Cortical Neurons

Abstract

Intracortical synapses exhibit several forms of short-term plasticity that cause synaptic efficacy at any given time to depend on the previous history of presynaptic activity. We have measured synaptic transmission between layer 4 and layer 2/3 in slices of rat visual cortex and used the data to construct an accurate mathematical description of intracortical short-term synaptic plasticity. These data show rapid synaptic facilitation and three forms of synaptic depression differing in their rates of onset and recovery. The dominant effect seen is overall synaptic depression that causes steady-state synaptic efficacy to decrease as a function of presynaptic firing rate. At high rates, the steady-state efficacy is inversely proportional to firing rate which implies that cortical synapses do not convey information about the magnitude of sustained high firing rates. However, this same dependence means that, for transient signals, synapses convey information about fractional rather than absolute changes in presynaptic firing rates. We explore the functional significance of this result including its implications for spike-rate adaptation and mechanisms that produce directional selectivity in visually responsive neurons.