Differential activation of ventrolateral prefrontal cortex during working memory retrieval.

Brain imaging studies have suggested a predominant involvement of prefrontal areas during retrieval of information from working memory (WM). This study used event-related functional magnetic resonance imaging to assess the gradual recruitment of brain areas during verbal WM-retrieval with a parametrically varied modified version of the Sternberg Item Recognition Paradigm. In particular, we were interested in activation differences during retrieval of negative and positive probes. Fifteen subjects performed a WM-task which required the retrieval of a probe letter from a set of a maximum of three letters. The analysis of the retrieval period regardless of probe type revealed bilateral VLPFC activation during retrieval from a single remembered item. These initially activated regions showed a gradual activation increase of left VLPFC (BA 47) and anterior PFC (BA 10) as well as and bilateral DLPFC (BA 9) with increasing retrieval demand, i.e. during retrieval of two and three previously remembered letters. The comparison of negative and positive probes (non-targets versus targets) revealed greater activity in VLPFC (BA 47) in response to negative than to positive probes. These findings demonstrate that ventral areas of prefrontal cortex seem to be differentially engaged during the discrimination of a non-target from a previously manipulated set.