Regions of the prefrontal cortex (PFC) are typically activated in many different cognitive functions. In most studies, the focus has been on the role of specific PFC regions in specific cognitive domains, but more recently similarities in PFC activations across cognitive domains have been stressed. Such similarities may suggest that a region mediates a common function across a variety of cognitive tasks. In this study, we compared the activation patterns associated with tests of working memory, semantic memory and episodic memory. The results converged on a general involvement of four regions across memory tests. These were located in left frontopolar cortex, left mid-ventrolateral PFC, left mid-dorsolateral PFC and dorsal anterior cingulate cortex. These findings provide evidence that some PFC regions are engaged during many different memory tests. The findings are discussed in relation to theories about the functional contribition of the PFC regions and the architecture of memory.
Functional brain imaging with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have revealed a strong association between cognitive operations and activity in regions of the prefrontal cortex (PFC). For example, in a review of PET and fMRI studies [6], PFC regions were found to be part of the typical activation pattern for many different cognitive functions, including sustained attention, smell perception, written word recognition, verbal and spatial working memory, semantic memory, episodic memory, and conceptual priming.
In most prior studies, the focus has been on the role of specific PFC regions in specific cognitive domains. However, more recently it has been noted that regionally specific PFC activations show substantial similarities across cognitive domains [8] and [26]. In one recent analysis, Duncan and Owen [17] focused on five cognitive demands: response conflict, task novelty, number of elements in working memory, working-memory delay, and perceptual difficulty. They found that there was joint recruitment of three PFC regions for all five cognitive demands: mid-dorsolateral PFC, mid-ventrolateral PFC, and a dorsal part of the anterior cingulate cortex. These regions were seen as forming a common network recruited by as diverse challenges as response selection, working memory maintenance and stimulus recognition. It was furthermore noted that retrieval from episodic memory also tends to engage the same regions, and, in addition, that episodic retrieval showed a higher proportion of activations close to the frontal pole. These observations indicated that the common network is operating during episodic retrieval, but that additional processing associated with more anterior PFC regions also come into play.
The results from a recent fMRI study that directly contrasted working memory and episodic memory provide support for specific activation of frontopolar regions during episodic retrieval [7]. In contrast, another recent within-study fMRI comparison of working memory and episodic memory suggested that frontopolar activation was greater for working memory than for episodic memory [2], and frontopolar activation has also been associated with semantic monitoring [23]. Furthermore, both fMRI-studies [2] and [7] found evidence that dorsolateral PFC activation was stronger for working memory than for episodic memory. Thus, it is unclear whether dorsolateral PFC is engaged to a similar extent for different memory systems, and whether frontopolar activation is especially salient for episodic memory retrieval. At least in part, the unclear issues may have to do with different strategies for data analysis (between- versus within-study comparisons) and with factors that are task specific.
The aim of the present study was to further explore similarities in regionally specific activations associated with different memory systems. This was accomplished by analysis of data from two PET experiments [27]. Across the two experiments, three tests were included for each of three memory systems: episodic memory, working memory, and semantic memory. This design allowed analysis of regional activations that are common for a wide range of memory tasks.
