We have adapted Sternberg’s context-recall task to investigate the neural mechanisms of encoding serial order information in working memory, in 2 male rhesus monkeys. We recorded from primary motor, premotor, and dorsolateral prefrontal cortex while the monkeys performed the task. In each cortical area, most neurons displayed marked modulation of activity during the list presentation period of the task, whereas the serial order of the stimuli needed to be encoded in working memory. The activity of many neurons changed in a consistent manner over the course of the list presentation period, without regard to the location of the stimuli presented. Remarkably, these neurons encoded serial position information in a relative (rather than absolute) manner across different list lengths. In addition, many neurons showed activity related to both location and serial position, in the form of an interaction effect. Surprisingly, the activity of these neurons was often modulated by the location of stimuli presented before the epoch in which the activity changes occurred. In motor and premotor areas, a large proportion of neurons with list presentation activity also showed direction-related activity during the response phase, whereas in prefrontal cortex most cells showed only list presentation effects. These results show that many neurons had a heterogeneous functionality by representing distinct task variables at different periods of the task. Finally, potential confounds could not account for the effects observed. For these reasons, we conclude that these neurons were indeed participating in sequence encoding in working memory.
Bibliographical noteFunding Information:
Received Nov. 20, 2017; revised March 29, 2018; accepted April 23, 2018. Authorcontributions:A.F.C.andG.P.wrotethefirstdraftofthepaper;A.F.C.,G.B.-B.,A.P.G.,andG.P.editedthe paper;A.F.C.,A.P.G.,andG.P.designedresearch;A.F.C.andG.P.performedresearch;A.F.C.,G.B.-B.,A.P.G.,andG.P. analyzed data; A.F.C. and G.P. wrote the paper. ThisworkwassupportedbyDepartmentofVeteransAffairsMeritReviewAwardCX-0004-37toG.P.andNational Institutes of Health Grant NS17413 to A.P.G. The authors declare no competing financial interests. Correspondence should be addressed to either Dr. Giuseppe Pellizzer or Dr. Adam F. Carpenter, Brain Sciences Center (11B), 1 Veterans Drive, Minneapolis, MN 55417, E-mail: email@example.com or firstname.lastname@example.org.
- Frontal cortex
- Memory scanning
- Serial order
- Single-unit activity
- Working memory