Circuit mechanisms of persistent activity in the primate cortex during working memory
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Working memory is the cognitive ability to actively maintain and manipulate information on the timescale of seconds. Neurons in the prefrontal and posterior parietal cortices of the primate brain remain active in absence of sensory input and appear to correlate with working memory. In this thesis, I investigate the mechanisms of persistent activity during working memory in the frontoparietal network of the macaque. By conducting simultaneous electrophysiological recordings in two of the key regions of this network, the lateral intraparietal area (LIP) and the frontal eye fields (FEF), and employing statistical models of the neural population activity, I characterized the interactions between neurons locally in each area and between these two distant brain regions. In a visuospatial working memory task, during which the subject must remember the spatial location of a target, I found strong recurrent activity on single trials both within and between these areas that was not due to the visual stimulus or the motor response. The strength and timescale of functional interactions between LIP and FEF were highly reciprocal and symmetrical, providing evidence for the theory that reverberatory activity in this circuit does, in fact, support working memory. However, contrary to current models of the frontoparietal network, area LIP exhibited greater local recurrent excitatory activity than FEF, and many individual neurons in LIP displayed activity on longer timescales. In addition, the concurrent population activity had a greater impact on the spiking activity of most neurons than each individual neuron’s own intrinsic drive, especially in LIP. This result further emphasizes the role of network mechanisms in generating and maintaining persistent activity. Taken together, these findings suggest revisions to the current models of working memory, and highlight the importance of studying population activity on single trials.