Irregular persistent activity induced by synaptic excitatory feedback

Abstract

Neuro-physiological experiments on monkeys [1] have reported highly irregular persistent activity during the per-formance of an oculomotor delayed-response task. These experiments show that during the delay period the ISI's coefficient of variation (CV) of prefrontal neurons is above 1, on average, and larger than during the fixation period, regardless of whether the cue is preferred or non-preferred. Previous models [2,3] of spontaneous and selective per-sistent activity in the cortex based on excitatory synaptic feedback do not reproduce this feature because the excita-tory feedback during persistent activity brings neurons in a region of the f-I curve in which the firing is relatively independent from fluctuations and hence the CV is small. To overcome this problem, we introduced two ingredi-ents: (1) a high post-spike reset potential (close to thresh-old), (2) a non-linear relationship between synaptic efficacy and pre-synaptic firing rate via a short-term depression (STD) mechanism. We show that when the reset potential is close enough to the threshold, the CV-I curve has a maximum above 1 for a sub-threshold mean current. The range of the mean syn-aptic input values for which the CV is greater than 1 is always in the sub-threshold regime in which firing is dom-inated by fluctuations of the mean synaptic input. With short-term depression, synaptic efficacies saturate at a cer-tain limiting value of the presynaptic frequency; this in turn provokes a saturation of the mean synaptic current to a neuron at the same limiting presynaptic frequency. This