Neural Correlate of Visual Working Memory in the Macaque Monkey
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Working memory is the ability to store relevant information temporarily to guide thoughts and behavior. As a basic executive function that is required for everyday tasks, it is essential to understand working memory to fully understand cognition. A neural correlate of working memory has been suggested in the persistent activity displayed by single neurons during the retention interval of working memory tasks performed by monkeys. Physiological and computational studies have proposed that the maintenance of this activity depends on NMDA receptor activation. Human homologues of persistent activity have been suggested using neuroimaging methods and electroencephalogram (EEG) recordings. To help bridge the gap between these studies within and between species, EEG signals in the form of event-related potentials (ERPs) were recorded while one female rhesus macaque monkey (Macaca mulatta) performed a series of tasks that involve visual working memory, in which a visual array presented for 500 ms must be maintained in working memory for a retention interval of 1 s to guide a subsequent saccade to a stimulus. In addition, the effect of NMDA blockade on ERPs was investigated by administering a sub-anesthetic dose of the NMDA antagonist ketamine. A neural correlate of visual working memory was identified in the ERP that was contralaterally-specific to the to-be-remembered target during the retention interval of the tasks. For the first time in the monkey, it was shown that the amplitude and polarity of this activity reflects the spatial location of the target stimulus, scales with the number of items that had to be remembered, and is predictive of trial outcome. The activity was less positive for contralaterally presented stimuli, greater memory loads, and correct trials. Furthermore, this contralateral activity was modulated by ketamine, primarily during the retention interval but also during memory item presentation at P100 and P300. Taken together, these properties support this contralateral activity as a candidate for an electrophysiological index of working memory that is similar to that of humans. These findings also provide a link to the single-neuron mechanisms of working memory in monkeys and further validate the monkey as a model of human visual working memory.