Models of Functional Organization of the Lateral Prefrontal Cortex in Verbal Working Memory: Evidence in Favor of the Process Model

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Abstract
Research on the functional organization of the lateral prefrontal cortex (PFC) in working memory continues to be fairly equivocal between two major frameworks: organization-by-process or organization-by-material. Although there is fairly strong evidence for organization-by-process models from event-related fMRI studies, some investigators argue that the nature of the stimulus material better defines the functional organization of the lateral PFC, particularly in more ventral regions (BA 47/45/44). Specifically, the anterior region of the ventrolateral PFC (BA 47/45) is hypothesized to subserve semantic processing while the posterior region (BA 44) may subserve phonological processing. In the current event-related fMRI study, we directly compared process-related versus material-related organizational principles in a verbal working memory task. Subjects performed a modified delayed response task in which they (1) retained a list of five words or five nonwords during the delay period (“maintenance”), or (2) performed a semantic (size reordering) or phonological (alphabetical reordering) task on the word or nonword lists, respectively (“manipulation”). We did not find evidence during the delay period of our task to support claims of anterior-posterior specializations in the ventrolateral PFC for semantic versus phonological processing. Subjects did, however, display greater neuronal activity during the delay period of manipulation trials than maintenance trials in both the dorsolateral PFC and posterior ventrolateral regions. These data are more consistent with the process model of the organization of lateral PFC in verbal working memory.