Monday, January 22, 2007

Default Mode or Detritus?

Should we discard the concept of a "Resting State" of the brain?

NO: ["default mode" aka daydreaming]
Malia F. Mason, Michael I. Norton, John D. Van Horn, Daniel M. Wegner, Scott T. Grafton, C. Neil Macrae (2007). Wandering Minds: The Default Network and Stimulus-Independent Thought. Science 315: 393-395

Despite evidence pointing to a ubiquitous tendency of human minds to wander, little is known about the neural operations that support this core component of human cognition. Using both thought sampling and brain imaging, the current investigation demonstrated that mind-wandering is associated with activity in a default network of cortical regions that are active when the brain is "at rest." In addition, individuals' reports of the tendency of their minds to wander were correlated with activity in this network.
[NOTE: Oh, come on, is this news?? I guess so...]

Brain study finds the stuff of daydreams

Escape from the Insipid: Our Brains May Be Wired for Daydreaming

Default mode network illustrated below (in blue-green colors).

At rest, but active. fMRI images of a normal human brain at rest. The images reveal the highly organized nature of intrinsic brain activity, represented by correlated spontaneous fluctuations in the fMRI signal. Correlations are depicted by an arbitrary color scale. Positive correlations reside in areas known to increase activity during responses to controlled stimuli; negative correlations reside in areas that decrease activity under the same conditions. [from Raichle, 2006]

Raichle commentary from Nov 2006:
Marcus E. Raichle (2006). The Brain's Dark Energy. Science 314: 1249-1250.

. . .

The adult human brain represents about 2% of the body weight, yet accounts for about 20% of the body's total energy consumption, 10 times that predicted by its weight alone. What fraction of this energy is directly related to brain function? Depending on the approach used, it is estimated that 60 to 80% of the energy budget of the brain supports communication among neurons and their supporting cells (2). The additional energy burden associated with momentary demands of the environment may be as little as 0.5 to 1.0% of the total energy budget (2). This cost-based analysis implies that intrinsic activity may be far more significant than evoked activity in terms of overall brain function.

Consideration of brain energy may thus provide new insights into questions that have long puzzled neuroscientists. For example, researchers have sought to explain the relative disproportion of connections (i.e., synapses) among neurons that appear to perform functions intrinsically within the cerebral cortex. Take the visual cortex, whose primary function is to respond to external input to the retina. Less than 10% of all synapses carry incoming information from the external world (3)--a surprisingly small number. From a brain energy perspective, however, the cortex may simply be more involved in intrinsic activities.

What is this intrinsic activity? One possibility is that it simply represents unconstrained, spontaneous cognition--our daydreams or, more technically, stimulus-independent thoughts. But it is highly unlikely to account for more than that elicited by responding to controlled stimuli, which accounts for a very small fraction of total brain activity.
YES: [detritus]
Morcom AM, Fletcher PC. (2006). Does the brain have a baseline? Why we should be resisting a rest. NeuroImage Oct 16; [Epub ahead of print]

In the last few years, the notion that the brain has a default or intrinsic mode of functioning has received increasing attention. The idea derives from observations that a consistent network of brain regions shows high levels of activity when no explicit task is performed and participants are asked simply to rest. The importance of this putative "default mode" is asserted on the basis of the substantial energy demand associated with such a resting state and of the suggestion that rest entails a finely tuned balance between metabolic demand and regionally regulated blood supply. These observations, together with the fact that the default network is more active at rest than it is in a range of explicit tasks, have led some to suggest that it reflects an absolute baseline, one that must be understood and used if we are to develop a comprehensive picture of brain functioning. Here, we examine the assumptions that are generally made in accepting the importance of the "default mode". We question the value, and indeed the interpretability, of the study of the resting state and suggest that observations made under resting conditions have no privileged status as a fundamental metric of brain functioning. In doing so, we challenge the utility of studies of the resting state in a number of important domains of research.
More later...

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