Persistent activity and the single-cell frequency-current curve in a cortical network model

Neurophysiological experiments indicate that working memory of an object is maintained by the persistent activity of cells in the prefrontal cortex an d infero-temporal cortex of the monkey. This paper considers a cortical net work model in which this persistent activity appears due to recurrent synap tic interactions. The conditions under which the magnitude of spontaneous a nd persistent activity are close to one another (as is found empirically) a re investigated using a simplified mean-field description in which firing r ates in these states are given by the intersections of a straight line with the f-1 curve of a single pyramidal cell. The present analysis relates a n etwork phenomenon-persistent activity in a 'working memory' state-to single -cell data which are accessible to experiment. It predicts that, in network s of the cerebral cortex in which persistent activity phenomena are observe d, average synaptic inputs in both spontaneous and persistent activity shou ld bring the cells close to firing threshold. Cells should be slightly sub- threshold in spontaneous activity, and slightly supra-threshold in persiste nt activity. The results are shown to be robust to the inclusion of inhomog eneities that produce wide distributions of firing rates, in both spontaneo us and working memory states.