UC Berkeley

This work examines two questions in the working memory field: the organization of working memory function within the prefrontal cortex (PFC) and the nature of the limited capacity of visual working memory. To study the organization of working memory function within the PFC, we recorded the activity of single neurons from different areas within the PFC and from the gustatory cortex (GUS) of two subjects while they performed a gustatory delayed-match-to-sample task with intervening gustatory distraction. Neurons that encoded the gustatory stimulus across the delay, consistent with a role in gustatory working memory, were most prevalent in the orbitofrontal cortex (OFC) (the main recipient of gustatory inputs within the PFC) and GUS compared with dorsolateral PFC and ventrolateral PFC. Gustatory information in OFC was more resilient to intervening distraction, paralleling previous findings regarding visual working memory processes in PFC and posterior sensory cortex. Our findings are consistent with a model of working memory organization in which different PFC areas encode different types of information in working memory depending on their afferent connections with different sensory brain areas.

In the second part of this work, we investigate the nature of the limited capacity in visual working memory by training two subjects on a multiple-item color change detection task. Our results show that when subjects have to store multiple items in visual working memory, the fidelity of the memory traces decreases as more items are loaded into memory. Thus, visual working memory can be described as a limited resource that must be shared in the representation of multiple items. By recording activity from single neurons in the ventral PFC while subjects held multiple colors in memory, we show that for a subset of color-tuned neurons, the sharpness of tuning decreases when two items are being held in working memory. Thus the amount of information encoded by the neurons decreases as more items are loaded in memory, analogous to the loss in precision of the internal memory representations. This finding, however, only applies when we take into account which color the subjects were covertly attending to, suggesting that attention plays a significant role in the allocation of the limited memory resources.