Balance of excitation and inhibition and short term plasticity: a new paradigm for working memory

DAVID HANSEL, GERMAN MATO

Washington D.C., USA

Conferencia; 38 Society for Neuroscience Meeting; 2008

Society for Neuroscience

Working Memory (WM) is the ability to hold information for processing purposes. Persistent activity observed during delayed-response experiments is a neural correlate of WM that encodes information about the preceding stimulus or impending response. It consists of enhanced emission rates within neural populations in the interval between cue and response. Patterns of neuronal discharge in persistent activity are highly irregular, even more so than at baseline. This apparently innocuous feature highlights a fundamental problem in neuronal and synaptic mechanisms of WM. In the most universal mechanism for irregular firing, the balance of excitation and inhibition, the non-linearities of the neuronal transduction function are washed out and are irrelevant in determining network activity (van Vreeswijk & Sompolinsky, 1996, 1998). Thus current WM network models that require nonlinear input-output neuronal transduction functions fail to account for the irregular patterns of persistent activity. Here we argue that the key nonlinearities in persistent activity are synaptic. We tested a spiking neuronal network model in prefrontal cortex on a spatial WM task where the direction of a visual cue must be memorized. We assume that the recurrent excitatory synapses have short term plasticity (STP), facilitating (resp. depressing) at low (resp. large) presynaptic firing rates which endows the network with the appropriate nonlinearities to display multistability between balanced states. A baseline balanced state coexists with balanced states of persistent activity which encode the cue direction. The coefficient of variation of the interspike interval has a distribution peaked near 1 at baseline and larger than 1 during the delay period, consistent with experimental data. Single neuron activity during delay is selective to cue direction, the diverse shapes of the tuning curves and dynamical behavior during the delay are reminiscent of findings for prefrontal cortex in spatial WM experiments.