HUMAN minds wander when they have nothing else to do. This is when people start to introspect, using a specific network of brain structures. The same network has now been identified in monkeys and rats, suggesting that 鈥渮oning out鈥� might serve a key function in our survival. The findings raise questions over whether lower animals might also be capable of something akin to introspection.
The default mode network (DMN) is one of about 10 networks of brain regions that are active when a person is at rest. What makes the DMN interesting is that it becomes active when a person is asked to let their mind wander, but the network鈥檚 activity drops away completely as soon as that person is given an external task. This suggests that, in humans at least, the DMN is involved in self-reflection and introspective thought processing.
Building on recent evidence that anaesthetised , and his colleagues at the Catholic University of Leuven (KUL) in Belgium collected a host of data from 15 studies which imaged the brain activity of 10 awake monkeys. By looking at the baseline brain activity measured in each project, the group was able to spot a network of brain structures that were active when the monkeys were not engaged in a task. This network looked strikingly similar to the human DMN (Journal of Neuroscience, ).
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Perhaps more surprisingly, the latest evidence from and his colleagues at the US National Institute on Drug Abuse in Bethesda, Maryland, suggests that a similar network also exists in resting rats. Team member minimally sedated 16 rats before assessing their brain activity using functional MRI. By comparing the brain activity of each rat while they were resting or stimulated with a gentle shock to the paw, the group was able to identify a DMN in the rats. The results were presented at the earlier this year.
The big question raised by the results is whether the monkey or rat DMN might perform a similar function to the human network. Can such animals really share our self-reflecting ability or does the presence of the DMN in monkey and rat brains suggest another, more basic function?
The network seems to play a key role in consciousness, says at the University of Liege in Belgium. In locked-in syndrome, in which a person is conscious but unable to move, the DMN looks exactly the same as normal, Soddu says, 鈥渂ut in the vegetative state the network is really disrupted 鈥� you can鈥檛 even recognise it鈥�. That monkeys and rats have a DMN might not mean that they are conscious in the same way that humans are. 鈥淲e know the network is involved in consciousness,鈥� says Soddu, 鈥渂ut we can鈥檛 say that it is responsible for consciousness.鈥�
It is more likely that the DMN鈥檚 role in self-reflection makes it useful in consolidating memories, says of Stanford University School of Medicine in California. 鈥淭he DMN draws heavily on the medial temporal lobe and memory areas, and we recently found we can increase DMN connectivity with self-driven memory retrieval tasks,鈥� he says.
Yang agrees that the DMN could be useful in rat memory. 鈥淸The rats could be] thinking about their past, mind wandering, and this kind of passive brain activity might be important for memory in the rat,鈥� he says. Whether the animals are doing something like self-reflection is a much more complicated matter. The original study that identified a DMN in anaesthetised monkeys found the network surprisingly similar to the human version with one key exception 鈥� the monkey DMN showed no activation in the prefrontal cortex (PFC), the part of the network tied most strongly to self-reflection in humans.
The latest study by Vanduffel鈥檚 team, however, found that the DMN of awake monkeys does include the PFC, suggesting that the original findings may have been affected by sedation instead of representing a species difference. 鈥淭he leap you now have to make,鈥� says Greicius, 鈥渋s this: is the monkey PFC doing something like self-reflection?鈥�
Intriguingly, the results from Yang鈥檚 team show that the DMN in the minimally anaesthetised rat also includes prefrontal regions. 鈥淭he activity in frontal areas [could suggest] the notion of a sense of self in the rat,鈥� says Greicius. 鈥淚鈥檝e got to believe it鈥檚 different from humans but it鈥檚 certainly provocative鈥�.
聯The activity in frontal brain areas could suggest the notion of a sense of self in the rat聰
