Academic journal article Cognitive, Affective and Behavioral Neuroscience

Neural Mechanisms of Spatial Working Memory: Contributions of the Dorsolateral Prefrontal Cortex and the Thalamic Mediodorsal Nucleus

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Neural Mechanisms of Spatial Working Memory: Contributions of the Dorsolateral Prefrontal Cortex and the Thalamic Mediodorsal Nucleus

Article excerpt

The dorsolateral prefrontal cortex (DLPFC) has been known to play an important role in working memory. Neurophysiological studies have revealed that delay period activity observed in the DLPFC is a neural correlate of the temporary storage mechanism for information and that this activity represents either retrospective or prospective information, although the majority represents retrospective information. However, the DLPFC is not the only brain area related to working memory. The analysis of neural activity in the thalamic mediodorsal (MD) nucleus reveals that the MD also participates in working memory. Although similar task-related activities were observed in the MD, the directional bias of these activities and the proportion of presaccadic activity are different between the MD and the DLPFC. These results indicate that, although the MD participates in working memory, the way it participates in this process is different between these two areas, in that the MD participates more in motor control aspects than the DLPFC does.

Working memory is a mechanism for short-term active storage of information, as well as for processing stored information (Baddeley, 1986). Working memory is a basic mechanism for many higher cognitive functions, including thinking, reasoning, decision making, and language comprehension. Therefore, understanding the neural mechanisms of working memory is crucial for understanding the neural mechanisms of these cognitive functions. As Goldman-Rakic (1987) initially proposed, the dorsolateral prefrontal cortex (DLPFC) has been known to play an important role in working memory. Neurophysiological studies in which nonhuman primates have been used have revealed that tonic sustained delay period activity is a neural correlate of the temporary storage mechanism for information (Funahashi & Kubota, 1994; Fuster, 1997; Goldman-Rakic, 1987, 1996a, 1996b). Most delay period activity has been shown to have directional selectivity, so that delay period activity has been observed only when the visual cue has been presented at a particular location in the visual field (Funahashi, Bruce, & Goldman-Rakic, 1989; Niki, 1974; Niki & Watanabe, 1976; Rainer, Asaad, & Miller, 1998; Rao, Rainer, & Miller, 1997; Wilson, Ó Scalaidhe, & Goldman-Rakic, 1993). In addition, delay period activity has been observed only when a monkey performed correct responses. When the monkey made an error, delay period activity was not observed or was observed but truncated (Funahashi et al., 1989; Funahashi, Inoue, & Kubota, 1997; Fuster, 1973; Niki & Watanabe, 1976). In addition, delay period activity has been shown to represent either retrospective (e.g., sensory) or prospective (e.g., motor) information, although the majority of delay period activity represents retrospective information in the DLPFC (Funahashi, Chafee, & Goldman-Rakic, 1993; Niki & Watanabe, 1976; Takeda & Funahashi, 2002). On the basis of these observations, delay period activity has been considered a neural correlate of the mechanism for temporary active storage of information in working memory (Funahashi & Kubota, 1994; Fuster, 1997; Goldman-Rakic, 1987, 1996a, 1996b).

Although the DLPFC participates in working memory, the DLPFC is not the only brain structure related to working memory. Many other brain structures could also participate in working memory. For example, tonic sustained delay period activity similar to that observed in the DLPFC has been observed in the posterior parietal cortex (Chafee & Goldman-Rakic, 1998; Gnadt & Andersen, 1988), the inferior temporal cortex (Fuster, 1990; Miller, Lin, & Desimone, 1993; Miyashita & Chang, 1988), and the caudate nucleus (Hikosaka, Sakamoto, & Usui, 1989) while monkeys performed working memory tasks (e.g., delayed response tasks, delayed matching-to-sample tasks, and memory-guided saccade tasks). Therefore, these areas could also participate in working memory. …

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