Abstract
Neurons in a distributed network of cortical and subcortical areas continue to discharge after the presentation and disappearance of stimuli, providing a neural correlate for working memory. While it is thought that the prefrontal cortex plays a central role in this network, the relative contributions of other brain areas are not as well understood. In order to compare the contributions of the dorsolateral prefrontal and posterior parietal cortex, we recorded neurophysiological activity in monkeys trained to perform two different visuo-spatial working memory tasks: a Match/Nonmatch task, and a Spatial Delayed-Match-to-Sample Task. Neurons in both areas exhibited discharges in the delay periods of the tasks that could be classified in two forms. Sustained discharges persisted after the presentation of a stimulus in the receptive field with a constant or declining rate. Anticipatory responses increased in rate during the delay period, often appearing after presentation of a stimulus out of the receptive field. Despite similarities, we uncovered distinct differences between patterns of delay period in each brain area. Only in the prefrontal cortex sustained responses related to the original stimulus survived presentation of a second stimulus, in the context of the Match/Nonmatch task. Our results provide insights on the nature of processing in two areas active during working memory, and on the unique role of the prefrontal cortex in memory maintenance.
Highlights
Working memory refers to the ability to maintain and manipulate information in memory over a time interval of seconds (Baddeley, 1992)
DATABASE Neuronal activity was recorded from area 7a of the posterior parietal cortex and areas 8 and 46 of the dorsolateral prefrontal cortex of four monkeys (Figure 1)
OVERVIEW OF THE FINDINGS Our study revealed differences between the patterns of delay period activity among prefrontal and parietal neurons in a spatial working memory task
Summary
Working memory refers to the ability to maintain and manipulate information in memory over a time interval of seconds (Baddeley, 1992). Two other studies examining responses of neurons during spatial working memory reached equivalent conclusions for the prefrontal and posterior parietal cortex: Activity of Prefrontal neurons appeared to survive distracting stimulation (di Pellegrino and Wise, 1993) whereas, activity of posterior parietal neurons only tracked the most recent stimulus (Constantinidis and Steinmetz, 1996). Based on these results, it was concluded that prefrontal cortex was unique in its ability to maintain information in memory in the face of distraction. The only direct comparison of prefrontal and posterior parietal neurons in the same animals performing the same task relied on an oculomotor delayed response task, which did not involve a distractor, and revealed little difference between prefrontal and parietal areas in terms of delay period activity (Chafee and Goldman-Rakic, 1998)
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