A neural system for mindlessness

If you are like me, you spend a lot of time not thinking about anything in particular. You read a couple papers, get a little work done, and then you stare off into space for a period of pleasant mindlessness.

From a neuroscientist's perspective, we spend a lot of time determining how we react to particular stimuli or how we accomplish certain cognitive tasks. We tend to view the brain as a little black box that coordinates responses to particular stimuli -- admittedly elaborate responses, but responses nonetheless.

What we don't spend much time doing is figuring out what we are doing when we are not doing any of those things -- such as when I am staring into space.

Your intuition is probably that when your brain isn't doing anything, then the activation in the brain would be less all over -- no parts are particularly required. However, it could also be that there is a system that comes online when everything else is offline -- sort of a master controller.

An interesting fMRI study in Science addressed this issue of stimulus independent thought (SIT), particularly whether there is a particular part of the brain that is associated with it.

The researchers got subjects to lapse into SIT by training them to do a verbal or visuospatial learning task repeatedly. They trained them so much that the subjects got really good at it, and this allowed their minds to wander because the task no longer required their complete attention.

The researchers then placed the subjects doing the task into an fMRI scanner and looked for those parts of the brain that were associated with mind wandering as opposed to when they had to perform a novel task which required them to actually pay attention. They also asked the patients to report the degree to which their minds had been wandering.

What they found was a consistent set of activation associated with SIT. Here is an image of the results:

Areas of activation during SIT included:

Regions of the default network that exhibited greater activity during these periods included bilateral aspects of the mPFC (BAs 6, 8, 9, and 10); bilateral superior frontal gyri (SFG; BAs 8 and 9); the anterior cingulate (BA 10); bilateral aspects of the posterior cingulate (BAs 29 and 30) and precuneus (BAs 7 and 31); the left angular gyrus (BA 39); bilateral aspects of the insula (BA 13); the left superior temporal (BA 22), the right superior temporal (BA 41) and the left middle temporal gyri (BA 19)

The fact that there is a cognitive system that comes online when the mind is wandering raises several interesting questions. What is the purpose of this system? Do we always go back to this system when we aren't thinking about anything?

The researchers speculate about the meaning of their results:

We proposed that mind-wandering constitutes a psychological baseline that emerges when the brain is otherwise unoccupied, supported by activity in a default network of cortical regions. Results demonstrated that reductions in processing demands, that is, performing practiced versus novel sequences of otherwise identical tasks, were accompanied by increases in both the generation of SIT and activity in the default network. Furthermore, the magnitude of BOLD [fMRI] increases that participants exhibited as they were able to generate increasing levels of SIT was positively correlated with their self-reported daydreaming propensities. Other research provides further evidence for default network involvement in the production of SIT. First, damage to parts of the network (e.g., mPFC) is associated with "mental emptiness" and an absence of spontaneous speech and thought. Second, aging is associated with the development of plaques in default network regions and a corresponding reduction in SIT. Taken together, these findings provide converging evidence for the role of the default network in mind-wandering.

...

The purpose of the current inquiry was to explore how and when the mind generates SIT. A more intractable question, however, is why these thoughts emerge at all. What is the functional significance of a system that wanders from its current goals? One possibility is that SIT enables individuals to maintain an optimal level of arousal, thereby facilitating performance on mundane tasks. A second possibility is that SIT -- as a kind of spontaneous mental time travel -- lends a sense of coherence to one's past, present, and future experiences. Finally, the mind may generate SIT not to attain some extrinsic goal (e.g., staying alert) but simply because it evolved a general ability to divide attention and to manage concurrent mental tasks. Although the thoughts the mind produces when wandering are at times useful, such instances do not prove that the mind wanders because these thoughts are adaptive; on the contrary the mind may wander simply because it can. (Citations removed.)

I am intrigued by the possibility that this subsystem is necessary for the mental continuity of the individual, although I suspect that it might also be involved in dynamic allocation of attention.

Hat-tip: Faculty of 1000.