195 Traditional theories of working memory and executive function, when mapped in straightforward ways into the neural domain, yield predictions that are only partly supported by the recent neuroimaging studies. Neuroimaging studies suggest that some constituent functions, such as maintaining information in active form and manipulating it, are not discretely localized in prefrontal regions. Some hypothesized executive processes, such as goal management, have effects in several cortical regions, including posterior regions. Such results suggest a more dynamic and distributed view of the cortical organization of working memory and executive functions. Addresses Center for Cognitive Brain Imaging, Department of Psychology, Carnegie Mellon University, Pittsburgh, PA 15213, USA Correspondence: Patricia A Carpenter; e-mail: carpenter+@cmu.edu Current Opinion in Neurobiology 2000, 10:195–199 0959-4388/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved. Abbreviations BA Brodmann’s area DLPFC dorsolateral PFC fMRI functional magnetic resonance imaging PET positron emission tomography PFC prefrontal cortex SPECT single photon emission computed tomography Introduction Executive processes and working memory have proven to be a fruitful test bed for neuroimaging studies of cognition. Executive processes are implicated in complex cognition, such as novel problem solving, which entails identifying and coordinating the steps to a new goal, evaluating the intermediate outcome, and modifying the plan as needed. Executive processes are also associated with task-set con- trol, modifying behavior as appropriate in light of changes in the environment, such as inhibiting prepotent or previ- ous responses. Executive processes are also associated with the functions that are impaired as a consequence of frontal lesions, particularly those related to the implementation of schemas that organize behavior over time [1,2]. Working memory has been operationalized primarily as the process- es and structures that keep information available over a relatively short time, such as postulated verbal and spatial peripheral buffers [3,4 • ]. In this standard perspective, executive processes manipulate the contents of the work- ing memory buffers. In the first part of this article, we describe how recent advances in functional neuroimaging have been framed in light of some classic questions about the organization of working memory and executive processes. In the second part, we examine how these same neuroimaging results suggest that the classic questions might be reconstrued, and then suggest some possible implications of these alter- native hypotheses. Classic issues Perhaps the major theoretical issue concerning executive processes is whether discrete regions (modules) of the pre- frontal cortex (PFC) are dedicated to particular operations and, if so, the characterization of those processes, including their domain specificity [5 •• ]. For example, Owen, Petrides and their colleagues [6,7] proposed that the mid- ventrolateral region (Brodmann’s area [BA] 45/47) supports the organization of response sequences based on informa- tion retrieved from posterior areas, whereas the mid-dorsolateral region (BA 9/46) supports the active manipulation or monitoring of information within working memory. Another proposal by Goldman-Rakic [8] is that PFC is organized by processing domain into object, spatial position, and verbal PFC regions. Another issue concerns the relation between the computa- tions of the prefrontal regions and those of more posterior regions in working-memory functions. For example, Goldman-Rakic [8] proposed that, in working memory tasks, the PFC regions operate conjointly with posterior regions as multi-modal domain-specific networks. The hypothesis that working-memory functions are accom- plished by large-scale networks overlaps with other theoretical proposals, including Mesulam’s framework [9]. Although the temporal relations among these various com- ponents may be critically important [10], they are only beginning to be addressed by neuroimaging studies [11]. A third issue is how to conceptualize the capacity con- straints of working memory — a topic that was framed by Miller’s classic chunking hypothesis [12]. When working memory is equated with information maintenance, con- straints can be conceptualized as temporal. When the task is more complex, such as problem solving, these con- straints may be conceptualized as limits in the complexity of the computations or representations [13,14]. For exam- ple, one proposal is that immediate thought varies in the amount of concurrent processing demand relative to a sys- tem’s resources, and that constraints emerge from system throughput [15]. Neuroimaging results The localization of working memory versus executive pro- cessing [6,7] was addressed in a meta-analysis of several positron emission tomography (PET) and functional mag- netic resonance imaging (fMRI) studies of verbal and spatial n-back tasks [16 • ,17 • ]. In an n-back task, sequen- tially presented items (letters, spatial positions, or Working memory and executive function: evidence from neuroimaging Patricia A Carpenter, Marcel Adam Just and Erik D Reichle