Tuning in to the temporal dynamics of brain activation using functional magnetic resonance imaging (fMRI)

Abstract

B addeley’ introduced the term ‘working memory’ into the experimental psychology literature to replace the existing concept of a passive shortterm memory store and to emphasize, within a single theoretical framework, both the temporary storage and the ‘on-line’ manipulation of information that occurs during a wide variety of cognitive activities. Since then, considerable evidence has accumulated to suggest that the lateral frontal cortex plays a critical role in certain aspects of working memory for both spatial and non-spatial material. This evidence comes from the study of patients with excisions of the frontal cortex2+, from lesion and electrophysiological recording work in nonhuman primate+‘, and more recently from functional neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI; see Ref. 8 for review). One particular focus of many functional imaging studies has been to investigate whether there are functionally distinct subdivisions of the lateral frontal cortex that subserve different aspects of working memory and, if so, how the functions of these regions might best be described. By and large, no consensus has been reached yet; spatial and non-spatial working memory studies using a cornucopia of different tasks have produced a widely distributed pattern of overlapping activation foci within dorsolateral and ventrolateral frontal cortical regions8 (see Fig. 1). Here, one contributory factor is undoubtedly that the tasks used in different studies vary widely in terms of their specific mnemonic (e.g. processing) requirements and the nature (e.g. modality) of the material to be remembered. Moreover, few of these imaging studies have had sufficient temporal resolution to isolate the specific neural events that are related to any particular aspect of task performance, such as the sustained localized activity that is widely assumed to underlie the rehearsal processes that keep a stimulus ‘in mind’ after it is removed from view. Two recent studies published in Nature have moved closer to addressing this issue using novel and innovative statistical techniques to demonstrate that high-speed fMRl can be used to accurately capture the time course of working memory processes within the human brain9~‘0. In general, fMRl has been used to make functional maps of changes in cerebral venous oxygen concentration that correlate with neuronal activity. Typically, the subject performs the task of interest (e.g. a working memory task), followed by a ‘control’ task requiring many, but not all, of the same motoric, perceptual and cognitive components as the experimental task. The imaging data