Working memory is an evolving concept that refers to the short-term storage of information that is not accessible in the environment and to the set of processes that keep this information active for later use. From a psychological perspective, working memory has been conceptualized as comprising multiple components, such as executive control and active maintenance processes. This chapter will review functional neuroimaging studies (PET and fMRI) which have provided evidence that prefrontal cortex is a critical node in the functional neural network supporting working memory. Also, recent data will be reviewed regarding the functional organization of prefrontal cortex.

Introduction

Working memory refers to the temporary representation of information that was just experienced or just retrieved from long-term memory but no longer exists in the external environment. These internal representations are short-lived, but can be maintained for longer periods of time through active rehearsal strategies, and can be subjected to operations that manipulate the information in such a way that it becomes useful for goal-directed behavior. Working memory is a system that is critically important in cognition and seems necessary in the course of performing many other cognitive functions, such as reasoning, language comprehension, planning, and spatial processing. Although working memory is an evolving construct, most often its definitions include both storage and (executive) control components (Miyake & Shah, 1999). Cognitive neuroscientists are searching for ways to dissociate the separable components of working memory in such a way that these separate functions can be localized within separate brain regions (i.e., brain mapping). Equally important is the goal of engineering models of the mechanisms by which the brain supports high-level cognitive processes such as working memory.

The prefrontal cortex (PFC) appears to be the most important brain region necessary for working memory (figure 9.1). Two consistent findings from studies of monkeys performing delayed response tasks suggest a critical role for the PFC in working memory. First, experimental lesions of the principal sulcus in the dorsolateral prefrontal cortex (DLPFC) impair performance on working memory tasks (Jacobsen, 1936; Fuster, 1997; Curtis & D’Esposito, 2004) and exacerbate with increasing memory retention intervals (Miller & Orbach, 1972; Bauer & Fuster, 1976; Funahashi et al., 1993). That is, forgetting increases not only when a delay is imposed but also increases with the length of the delay. Second, electophysiological single-unit recordings from the DLPFC often show persistent, sustained levels of neuronal firing during the retention interval of delayed-response tasks (Fuster & Alexander, 1971; Kubota & Niki, 1971; Funahashi et al., 1989). This sustained activity is thought to provide a bridge between the stimulus cue—for instance, the location of a flash of light—and its contingent response—for instance—a saccade to the remembered location. Persistent activity during blank memory intervals is a very powerful observation and has established a strong link implicating the DLPFC as a critical node supporting working memory.

Figure 9.1.  

Schematic parcellation of the human brain based on cytoarchitecture divisions of Brodmann (1909). In the frontal cortex, premotor areas (6 and 44) include the frontal eye fields bilaterally and Broca's area in the left hemisphere. Multimodal association frontal cortex includes dorsolateral prefrontal cortex (DLPFC), which refers to areas 46 and 9, ventrolateral prefrontal cortex (VLPFC), which refers to areas 45 and 47 and frontopolar or anterior PFC, which refers to area 10.