Neuroscience has shown that human beings’ ability to let their thoughts wander when they’re not doing anything is closely linked to a brain activity called “default mode networking,” abbreviated as MPD. Such functionality notably allows for introspection, which we imagine is unique to humans. But is this MPD also present in other primates and does it perform the same function? A team of CNRS researchers published a study on April 12, 2022 in Cell Reports, where they question the development of a species of standard-mode network in primates, comparing the brain activity of four species: a hominoid, the human, and the three other non-hominoids, the rhesus monkey, the common marmoset and the tiny mouse lemur, only 10 cm tall and also one of the most distant primates from humans from a genetic point of view.
They were able to conclude that the MPD observed in humans, as well as the unique cognitive abilities it enables, does not exist identically in the other species studied. However, they noted that other areas of the brain may be associated with MPD-like functioning.
The brains of four species scanned
The aim was to use the same method to characterize the brain connectivity patterns of the four primates. To do this, the brains of 17 monkeys, 4 marmosets and 13 mouse lemurs were scanned. The animals were anesthetized but awake enough to maintain good brain activity. The brains of 40 human volunteers were scanned by MRI. The researchers emphasize, however, that isoflurane (anesthetic agent used for the experiment) can influence, in high doses, the functioning of the brain and that this data must be taken into account in the results.
A total of seven brain networks were analyzed for each individual to understand whether human MPD has a primate equivalent. Each brain image was transferred to a 3D atlas, established for each of the four species. To test the consistency of their results, the team compared these 3D atlases they had just established with anatomical atlases and concluded that the results matched their observations.
A difference in the brain network of non-hominoid apes
In humans, the default mode network has been shown to be activated thanks to the simultaneous connection of two areas of the brain: the posterior cingulate cortex and the medial prefrontal cortex. In contrast, in the same situation, the connections in the brains of the other three non-hominoids differ, although they have some similarities. This simulacrum of MPD in these primates has been termed a “fronto-parietal network”.
When comparing the 3D atlas produced at the time, the researchers noticed that the same area of the brain came into action during inactivity in the three non-humanoid primates: this is the 8Ad area, located in the frontal lobe. It should be noted that this brain area was not detected in the mouse lemur, given the small brain size of this species. In monkeys and marmosets, the 8Ad area becomes active at the same time as the posterior cingulate cortex, but unlike in humans, connections with the medial prefrontal cortex are very weak. Generally speaking, researchers claim that connections in the posterior cingulate cortex, while they exist, are “systematically weaker in primates”.
An “unexpected” result
In the three monkeys studied, the 8Ad area of the brain is known for its role in exogenous (auditory and visual) guidance. Such a function is therefore in complete opposition to the researchers’ postulate that MPD is geared towards self-directed thinking and introspection. The default mode of non-hominoid apes, if it exists, would therefore be associated with changes in attention. Such a network that mobilizes different temporal areas of the brain, without, however, mobilizing the posterior cingulate cortex as in humans, was an “unexpected” result according to the researchers.
Of the three monkeys, the rhesus monkey appears to have the most similarities to the MPD, without having the equivalent. These comparisons of the fronto-parietal network revealed an architecture similar to the human MPD, but which does not seem to have the same role from a cognitive point of view. In the end, although it presents surprising connections, the researchers are not sure that such a network is a homologous candidate for MPD.
The default mode network at the source of human capabilities
This difference in brain connectivity between humans and other apes during periods of inactivity could explain the cognitive gap that characterizes us. In 2019, Alizée Lopez-Persem and her team at Inserm hypothesized that variations in the morphology of the cerebral grooves would participate in the reorganization of the network in a pattern mode in primates. By making it possible to increase the volume of gray and white matter in the human brain, this evolutionary trait would have strengthened the medial prefrontal cortex, essential in the activation of MPD. Humans, for example, have 1.9 times more gray matter and 2.4 times more white matter than the rhesus monkey.
This evolution would have allowed the immediate stop of the MPD when the brain is mobilized in cognitive tasks. A stop that is all the more effective as the cingulate and prefrontal cortices are developed. In humans, this function would have been necessary to effectively disengage from external distractions and allow for far more exceptional cognitive benefits than in other apes.