In a January article in Science, Mason et al. reported that they had identified the brain regions responsible for “mind-wandering.” They scanned patients while they performed novel or overlearned tasks. During the tasks they had practiced, subjects reported thinking about things unrelated to the task at hand. These self-reported instances of mind-wandering correlated with activity in a default state network including the medial prefrontal cortex (mPFC).

In the current issue, a group of British neuroscientists including a favorite of mine, Chris D. Firth, argue that the activity of these brain regions is not necessarily stimulus-independent. Gilbert et al. propose that this activity actually reflects watchfulness, or stimulus-oriented thought. They point out that the self reports of daydreaming were not made during scanning, and therefore the correlation is tenuous. They also point out that increased reports of daydreaming correlate with increased reports of distractibility, a symptom of environmental awareness.

The British gang also cites data showing that increased activity in the mPFC improves reaction times in a button-pressing task. If activity in the default network improves task performance, then it may reflect preparedness rather than mind-wandering.

Science allows the original authors of the paper to rebut the criticisms in a companion commentary. Mason et al. challenge the performance-enhancing effects of default network activity reported by Gilbert et al. Mason et al. also defend their experimental design, arguing that asking for real-time self reports would have polluted fMRI data by encouraging subjects to engage in introspection rather than mind-wandering. Furthermore, these self reports unambiguously endorse the hypothesis that default activity is stimulus-independent:

One participant remarked, “I would try to focus on the `+’ sign in anticipation of the next task. After a few seconds, however, my mind would wander to other thoughts (e.g., I wondered how much longer the experiment would take, what I’d do tonight, and considered how much my head hurt). Also, I had a song in my head during the easier tasks; it was `Just What I Needed’ by the Cars.”

Yes. This is exactly what I sing to myself when I’m bored.


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