Plasticity of Neocortical Synapses Enables Transitions between Rate and Temporal Coding

The transmission across neocortical synapses changes dynamically as a function of presynaptic activity [1]. A switch in the manner in which a complex signal, such as a burst of presynaptic action potentials, is transmitted between two neocortical layer 5 pyramidal neurons was observed after coactivation of both neurons. The switch involved a redistribution of synaptic efficacy during the burst such that the synapses transmitted more effectively only the first action potential in the burst. A computational analysis reveals that this modification in dynamically changing transmission enables pyramidal neurons to extract a rich array of dynamic features of ongoing activity in network of pyramidal neurons, such as the onset and amplitude of abrupt synchronized frequency transitions in groups of presynaptic neurons, the size of the group of neurons involved and the degree of synchrony. These synapses transmit information about dynamic features by causing transient increases of postsynaptic current which have characteristic amplitudes and durations. At the same time, the ability of synapses to signal the sustained level of presynaptic activity is limited to a narrow range of low frequencies, which become even narrower after synaptic modification.