Domain-related differentiation of working memory in the Japanese macaque (Macaca fuscata) frontal cortex: a positron emission tomography study

Are there internal thought processes in the monkey?--default brain activity in humans and nonhuman primates

Human neuroimaging studies have well demonstrated the presence of a "default system" in the brain, which shows a "default mode of brain activity", i.e., greater activity during the resting state than during an attention-demanding cognitive task. The default system consists mainly of the medial prefrontal and medial parietal areas. It has been proposed that this default activity is concerned with internal thought processes. Here, I first describe activities observed in the human default system measured by several methods, in relation task performance, development, aging and psychiatric disorder. I will then introduce recent nonhuman primate studies that indicate correlated low-frequency spontaneous brain activity within the default system, high metabolic levels in these medial brain areas during rest and task-induced suppression of neuronal activity in the medial parietal area. Furthermore, I will present our data in which we found task-induced deactivation in the monkey default system, and will examine similarities and differences in default activity between the human and nonhuman primate. Finally, I will discuss the functional significance of the default system and consider the possibility of internal thought processes in the monkey.

Default mode of brain activity demonstrated by positron emission tomography imaging in awake monkeys: higher rest-related than working memory-related activity in medial cortical areas

Human neuroimaging studies have demonstrated the presence of a "default system" in the brain, which shows a "default mode of brain activity," i.e., greater activity during the resting state than during an attention-demanding cognitive task. The default system mainly involves the medial prefrontal and medial parietal areas, including the anterior and posterior cingulate cortex. It has been proposed that this default activity is concerned with internal thought processes. Recently, it has been indicated that chimpanzees show high metabolic levels in these medial brain areas during rest. Correlated low-frequency spontaneous activity as measured by functional magnetic resonance imaging was observed between the medial parietal and medial prefrontal areas in the anesthetized monkey. However, there have been few attempts to demonstrate a default system that shows task-induced deactivation in nonhuman primates. We conducted a positron emission tomography study with [(15)O]H(2)O to demonstrate a default mode of brain activity in the awake monkey sitting on a primate chair. Macaque monkeys showed higher level of regional blood flow in these medial brain areas as well as lateral and orbital prefrontal areas during rest compared with that under a working memory task, suggesting the existence of internal thought processes in the monkey. However, during rest in the monkey, the highest level of blood flow relative to that in other brain regions was observed not in the default system but in the dorsal striatum, suggesting that regions with the highest cerebral blood flow during rest may differ depending on the resting condition and/or species.