The origins of subjective consciousness probably lie in an introspective brain network common to most mammals, says a new study.
When we “zone out” and let our minds wander, a functional (as opposed to structural) brain network known as the default mode network (DMN) becomes active. The DMN links our frontal lobe – an area associated with planning and abstract thought – with areas of the temporal and parietal lobes that help us associate memories with ideas and emotions. In short, this network allows us to become “lost” in thought, rather than occupied with our environment, or with a specific goal.
Since goal-directed behavior – say, hunting for food or a mate – seems to be more crucial for a species’s survival than mind-wandering is, the discovery of the DMN (which, like most new discoveries in neuroscience, has seen its share of controversy) prompted scientists to ask what purpose the DMN’s ancestors might have originally served.
Now, researchers are zoning in on the origins of zoning out, by mapping what they think is a primitive version of the DMN in rat brains. By comparing fMRI scans of rats’ brains when the animals were at rest with scans of those same rats’ brains when the animals received a mild electric shock, a team led by Yihong Yang at the US National Institute on Drug Abuse has identified a rat brain network that corresponds to non-goal-directed behavior – in short, a proto-DMN.
Though rats don’t seem to have much capacity for abstract thought, it’s likely that this network allows them to review their memories:
“[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,” Yang says.
Whether rats have what we’d consider a sense of self is a more complicated question. The rat brain does include a more primitive version of our prefrontal cortex (PFC), but exactly what this region does for the rat remains an open question:
“The activity in frontal areas [could suggest] the notion of a sense of self in the rat,” says Michael Greicius of the Stanford University School of Medicine. “I’ve got to believe it’s different from humans, but it’s certainly provocative.”
Some other new findings make this question even more intriguing: a recent paper described a close analog of the DMN in monkey brains, and a 2009 study found that while the DMN is active in human patients suffering from locked-in syndrome, it seems to be disrupted in vegetative patients. But, since recent research demonstrates that vegetative patients can respond to questions by thinking certain thoughts for “yes” and others for “no,” it seems that what we call “consciousness” may be much more multi-layered than we think.
If so, it may be that rats possess some of the abilities we associate with consciousness – such as mind-wandering and memories – but that they still lack a true concept of “I.” It may be that their minds lack abstract concepts altogether, or that “abstract concepts” are a more complex phenomenon than we’re assuming. That’s the tough thing about analyzing consciousness: nothing else remotely like it seems to exist in nature, and our minds seem to be poorly adapted for understanding what exactly it is.
Still, discoveries like these are helping us inch closer to that understanding – even if it’s in tiny little mouse steps.