Cue-specific neuronal ensembles span intermittent rate coding of working memory

Persistent, memorandum-specific neuronal spiking activity has long been hypothesized to underlie working memory. However, emerging evidence suggests a possible role for ‘activity-silent’ synaptic mechanisms. This issue remains controversial because evidence for either view has largely depended on datasets that fail to capture single-trial population dynamics or on indirect measures of neuronal spiking. We addressed this by examining the dynamics of mnemonic information on single trials obtained from large, local populations of prefrontal neurons recorded simultaneously in monkeys performing a working memory task. We show that mnemonic information does not persist in the spiking activity of prefrontal neurons, but instead alternates between ‘On’ and ‘Off’ periods during memory delays. At the level of single neurons, Off periods are driven by a coordinated loss of selectivity for memoranda and a return of firing rates to baseline levels. Further exploiting the large-scale recordings, we asked whether the functional connectivity among large neuronal ensembles depended on information held in working memory. We show that mnemonic information is available in the pattern of ensemble connectivity during the memory delay in both On and Off periods of neuronal activity. Intermittent epochs of memoranda-specific spiking therefore coexist with activity-silent mechanisms to span memory delays.