Differential short-term synaptic plasticity and transmission of complex spike trains: to depress or to facilitate?

Experimental studies have revealed conspicuous short-term facilitation and depression that are expressed differentially at distinct classes of cortica l synapses. To explore computational implications of synaptic dynamics, we investigated transmission of complex spike trains through a stochastic mode l of cortical synapse endowed with short-term facilitation and vesicle depl etion. Inputs to the synapse model were either real spike train data record ed from the visual and prefrontal cortices of behaving monkeys, or were gen erated numerically with prescribed temporal statistics. We tested the hypot hesis that short-term facilitation could enable synapses to filter out sing le spikes and favor bursts of action potentials. We found that the ratio be tween release probabilities for a burst spike and an isolated spike grows m onotonically with increasing number of spikes per burst, and with increasin g interval between isolated spikes. Burst detection is optimal when the fac ilitation time constant matches the average burst duration. Using fractal-l ike spike patterns characterized by long-term power-law temporal correlatio ns and similar to those seen in sensory neurons, we found that facilitation increases correlation at short time scales. In contrast, depression leads to a dramatic reduction in temporal correlations at all time scales, and to a flat ('whitened') power spectrum, thereby decorrelating natural input si gnals.