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.