The New Scientist site had an article by Douglas Fox, The Secret life of the Brain,
“The brain areas in the network were known and previously studied by researchers. What they hadn’t known before was that they chattered non-stop to one another when the person was unoccupied but quietened down as soon as a task requiring focused attention came along. Measurements of metabolic activity showed that some parts of this network devoured 30 per cent more calories, gram for gram, than nearly any other area of the brain.
All of this poses the question – what exactly is the brain up to when we are not doing anything? When Raichle and Shulman outlined the default network, they saw clues to its purpose based on what was already known about the brain areas concerned.
One of the core components is the medial prefrontal cortex, which is known to evaluate things from a highly self-centred perspective of whether they’re likely to be good, bad, or indifferent. Parts of this region also light up when people are asked to study lists of adjectives and choose ones that apply to themselves but not to, say, Britney Spears. People who suffer damage to their medial prefrontal cortex become listless and uncommunicative. One woman who recovered from a stroke in that area recalled inhabiting an empty mind, devoid of the wandering, stream-of-consciousness thoughts that most of us take for granted.
Parts of the default network also have strong connections to the hippocampus, which records and recalls autobiographical memories such as yesterday’s breakfast or your first day of kindergarten.
To Raichle and his colleague Debra Gusnard, this all pointed to one thing: daydreaming. Through the hippocampus, the default network could tap into memories – the raw material of daydreams. The medial prefrontal cortex could then evaluate those memories from an introspective viewpoint. Raichle and Gusnard speculated that the default network might provide the brain with an “inner rehearsal” for considering future actions and choices.
…. Daydreaming may sound like a mental luxury, but its purpose is deadly serious: Buckner and his Harvard colleague Daniel Gilbert see it as the ultimate tool for incorporating lessons learned in the past into our plans for the future. So important is this exercise, it seems, that the brain engages in it whenever possible, breaking off only when it has to divert its limited supply of blood, oxygen and glucose to a more urgent task.
…. In support of this idea, Raichle points out that the default network constantly chatters with the hippocampus. It also devours huge amounts of glucose, way out of proportion to the amount of oxygen it uses. Raichle believes that rather than burning this extra glucose for energy it uses it as a raw material for making the amino acids and neurotransmitters it needs to build and maintain synapses, the very stuff of memory. “It’s in those connections where most of the cost of running the brain is,” says Raichle.”
This medial pre-frontal cortex involvement is interesting. I can remember a time when I was very ill with radiation sickness; I sat all day and stared at a blank wall with practically no thoughts. I can understand the lady with the ‘blank mind’. What an apt expression. My default network must have shut down temporarily.