Synaptic mechanisms and network dynamics underlying spatial working memoryin a cortical network model

Single-neuron recordings from behaving primates have established a link bet ween working memory processes and information-specific neuronal persistent activity in the prefrontal cortex. Using a network model endowed with a col umnar architecture and based on the physiological properties of cortical ne urons and synapses, we have examined the synaptic mechanisms of selective p ersistent activity underlying spatial working memory in the prefrontal cort ex. Our model reproduces the phenomenology of the oculomotor delayed-respon se experiment of Funahashi et al. (S. Funahashi, C.J. Bruce and P.S. Goldma n-Rakic, Mnemonic coding of visual space in the monkey's dorsolateral prefr ontal cortex. J Neurophysiol 61:331-349, 1989). To observe stable spontaneo us and persistent activity, we find that recurrent synaptic excitation shou ld be primarily mediated by NMDA receptors, and that overall recurrent syna ptic interactions should be dominated by inhibition. Isodirectional tuning of adjacent pyramidal cells and interneurons can be accounted for by a stru ctured pyramid-to-interneuron connectivity. Robust memory storage against r andom drift of the tuned persistent activity and against distracters (inter vening stimuli during the delay period) may be enhanced by neuromodulation of recurrent synapses. Experimentally testable predictions concerning the n eural basis of working memory are discussed.