default mode network #1

Is there a “day dreaming network” in our brain?

The answer is YES, with a caveat.

Scientific term for this is “Default Mode Network”.

The discovery was an unexpected consequence of brain imaging by positron emission tomography or PET (Vijay can enlighten us on this). Some basics: Human brain relies almost entirely on glucose to function. Our brain weighs 2% of body weight, but uses 25% of total body glucose, 20% of total body oxygen, and receives 15% of cardiac output. The brain extracts about 50% of the oxygen and 10% of the glucose from arterial blood. Therefore, you can give positron emitting isotope (C11-Glucose – iv injection), O15-Oxygen (inhalation) and O15-H2O (iv injection) and scan the brain to measure Cerebral Metabolic Rate(CMR) for glucose and oxygen and cerebral blood flow (water). You need a cyclotron to generate positron isotopes, so very specialized facilities can do it. Vijay has one. Raju got one recently, if I’m not mistaken. At NIH, it is popular among summer students (gets paid $300 to 600), I had volunteered once in my late 30s when I was at NIH. They scan the brain with eyes closed, ears plugged to get the baseline. The intensity of the image reflects the brain usage of glucose, oxygen and blood flow. Then, you are asked to do a specific task (multiplication, memory recall, image identification, sound recognition, etc) depending on the investigators interest. The brain is scanned the whole time. The investigator would then calculate the increase in CMR glucose, oxygen and water from the baseline scan. They will then superimpose the increase in activity on the anatomical atlas to derive a functional map of the brain. They can then say which brain region respond to the specific task. An example is shown here (red arrow marks the regions that are activated by a given task).

Some interesting observations: When I was asked to do multiplication tables, in addition to the expected region of increase in blood flow, my language area was also activated suggesting that my brain converts the numbers in to Tamil and then responds as I have learnt multiplication tables in Tamil. Bilingual French Canadians show no difference in pattern. Interestingly, some native Pondicherians can do 3 languages (French, English and Tamil). More than 3, the brain converts to a dominant language, that is my understanding. It is worth testing multilingual Indians to know the limit of the brain.

Coming back to the subject, this was the situation from 1980 to 2000. In 2001, a pioneer in brain PET imaging, Dr. Marcus E. Raichle @ Washington University, St Louis, noticed that there were some brain regions that showed a decrease during the task, it was ignored before then. That meant, to start with at resting state they had increased blood flow and the task decreased the flow and increased the flow to the region of interest. It was not one area but a network of brain areas or regions. He went ahead and tested all the PET images and to his surprise he found that irrespective of the task the same areas showed a decrease. He did a meta-analysis of all the PET studies done by different investigators, this is when he came up with:

This is the map of “Day Dreaming” or “Default Mode Network or DMN”.

What are these brain regions?  They are in front and center of the brain. They belong to what is called “Association neocortices”.

Do these regions exist in mammals? The answer is yes but these regions have rapidly expanded in human brain evolution.

                          Rat                                                  Monkey                                 Human

Does it represent consciousness? NO, the patterns of resting-state functional connectivity appear to transcend levels of consciousness, it is present under anesthesia in humans, is present in monkeys and rats, also during the early stages of sleep in humans.

What does DMN regions represent?  These regions are related to emotion and memory. DMN is active when you are not attending to physical activity or engaging with the external environment or carrying on a conversation.

Caveat, the way I am using daydreaming is contemplating the future, reliving the past, or general rumination. But it could be argued that mind wandering, and daydreaming are conscious cognition. Therefore, it would be misnomer to call it daydreaming. I agree, is it natural-state brain or restless brain or simply DMN?

So, what, there are some regions that are active at rest, what is it implication?

DMN has important implications, consider what if DMN activity is low or high: depression (over regretted pasts, presents, or futures), anxiety (over potential pasts, presents, or futures). Small changes functional connectivity in younger subjects can push them to act violently to become incarcerated juveniles? Does the activity change in development or in aging or in diseases, I will come back to these topics later?

This brings us to the two points of view on brain organization. Since majority of the people respond to a situation in a similar manner, it must be reflexive, driven by momentary demands of the environment (Sherrington 1906). In contrast, his student (T, Graham Brown, 1914) posited the theory that the brain’s operations were mainly intrinsic, involving the acquisition and maintenance of information for interpreting, responding to, and even predicting environmental demands.

What I want to say is that default mode network plays critical role in the organization and expression of preplanned, reflexive behaviors that are critical to our existence in a complex world. You see some people respond in a cool collective manner, but others become impulsive and destructive. It’s all due to DMN.

Don’t lose hope, there are ancient Indian practices that is gaining popularity to address these issues.

But, before that I want to bring back to you the lectures of Profs. Ramakrishnan and Joseph on glucose metabolism and little bit of Prof. Prasanna (fatty acid metabolism). I bought the 29^th edition of Harpers Illustrated Biochemistry (for $2 from Amazon) to refresh my memory, I think we read 11^th or 12^th edition, the current edition is 30^th. Biochemistry of Brain Glucose Metabolism, that is my next posting.