Even when you are fast asleep distinct and widely dispersed groups of brain areas, which support memory retrieval, continue to act in concert. Neuroscientists at the Stanford University School of Medicine have shown this by directly recording electrical activity from the human brain.
Published in Neuron on April 8, this is the first scientific confirmation that specific electrical patterns of coordinated neural activity persist round the clock across widely separated human brain structures during memory retrieval, regardless of whether you are awake or sleeping. This is probably why the brain paradoxically spends so much of the body’s energy even when you are sleeping (almost like an idling car engine burning fuel).
The human brain is an energy guzzler: consuming 20 per cent of the body’s energy despite accounting for only 2 percent of its weight. Yet the rate at which the brain devours glucose is more or less the same regardless of whether we are active or snoozing. This is because even at rest, the brain is engaged in a blizzard of electrical activity, which neuroscientists have historically viewed as useless “noise.”
Says Josef Parvizi, senior author of the Neuron study: “Increased brain activity at times of conscious thought and actions is only the tip of the iceberg. The brain consumes a vast amount of energy due to its spontaneous activity at all times when we may or may be consciously involved in a specific task.”
The Neuron study substantiates similar inferences drawn from studies involving imaging (fMRI) techniques. However, so far the issue has remained inconclusive because brain imaging provides no more than indirect assessments of electrical activity in different brain regions. Also, while fMRI scans can map activity in the brain panoramically, their temporal resolution is imperfect. Parvizi and his colleagues got around these issues by eavesdropping on the activity of distinct populations of nerve cells in the human brain using a technique called intracranial electrophysiology. The method provides resolution at a scale of milliseconds and millimeters, letting researchers obtain meaningful results from inspecting a single individual’s brain.
Using the technique researchers found that electrical activity in two distant regions of the brain responded with surprising simultaneity when subjects were made to contemplate on several statements such as “I ate a banana for breakfast this morning. ” “There was effectively zero time lag” in the response of these regions, said the study’s lead author, Brett Foster, Ph.D., a postdoctoral scholar in Parvizi’s lab.
Thanks to the technique’s temporal the researchers were able to plot the sequence of a subject’s responses to autobiographical-memory queries. The electrical activity started in the brain’s vision centers and traveled through the angular gyrus, the posterior cingulate cortex, the brain’s decision centers and finally, as subjects pushed a “True” or “False” key on the computer before them, to the motor area. This pattern remained unchanged whether the individuals were awake, at rest or asleep.
The findings have put one debate to bed: the coordinated resting-state network activity observed in neuroimaging is real. However, it raises another question: What advantage might an organism derive from the immense energy expenditure needed to keep all that activity going even during sleep?