The roles of prefrontal brain regions in components of working memory: Effects of memory load and individual differences

Rypma, Bart; Mark D’Esposito · 1999 · Crossref

DOI: 10.1073/pnas.96.11.6558

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Summary

This study investigates the specific roles of dorsal and ventral prefrontal cortex (PFC) regions in working memory (WM) components—encoding, delay, and response—under varying memory loads. Previous research using blocked designs could not distinguish temporal dynamics or account for individual differences in performance. The authors aimed to determine whether dorsal and ventral PFC support different WM processes and how memory load and individual retrieval rates influence activation patterns. Using an event-related functional MRI design, six right-handed subjects performed a WM task requiring them to encode two or six letters, maintain them during a 12-second delay, and respond to a probe. This design allowed isolation of neural activity during encoding, delay, and response periods. Data were analyzed using general linear models to assess activation extent and magnitude in dorsal (Brodmann’s areas 9 and 46) and ventral (areas 44, 45, and 47) PFC regions of interest. Behavioral performance showed significantly slower reaction times for the six-letter condition compared to the two-letter condition, with considerable variability in individual reaction time slopes. Results indicated that increased memory load significantly increased activation extent in dorsal PFC only during the encoding period, with right-hemisphere lateralization observed in the high-load condition. No significant load-dependent effects were found in dorsal or ventral PFC during the delay or response periods in the group analysis. Ventral PFC showed load-equivalent activation across conditions. Individual analyses revealed that dorsal PFC activation during the response period correlated strongly with subjects’ memory retrieval rates (reaction time slopes). Specifically, subjects with slower retrieval rates exhibited greater dorsal PFC activation during response, accounting for 76–84% of the variance. This relationship was not observed in ventral PFC or during other task periods. The findings suggest that dorsal and ventral PFC play distinct roles in WM. Dorsal PFC is recruited during the encoding of high-load information, potentially supporting strategic processes or monitoring required for complex maintenance. The lack of delay-period load effects in PFC suggests that maintenance of verbal information may rely more on posterior parietal regions. Furthermore, the correlation between slow retrieval rates and increased dorsal PFC activity during response implies that dorsal PFC involvement in retrieval increases for individuals with inefficient processing or degraded mnemonic representations. This supports a model where dorsal PFC facilitates efficient information processing, and its increased recruitment reflects compensatory mechanisms for slower or less efficient retrieval.

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• cognitive capacity variation