Reading 7: Brain Networks and Spontaneous Brain Activity

Question 1

The human brain

Answer 1

• In one sense, your brain is remarkably energy efficient. It can do things the worlds most powerful computers cannot, and yet, under normal circumstances, it never gets much hotter than 37 degrees centigrade, and it consumes energy at the rate of a 40-watt lightbulb.
• On the other hand, compared to the rest of the body, it’s an energy hog. The human brain is about 2% of total body weight, but it accounts for 20% of the body’s energy consumption, reflecting the energy demands of electrical and synaptic activity. Interestingly, most of this energy is consumed by the brain’s intrinsic “resting state” activity, the ongoing activity that’s present even when you are just sitting around staring out the window and daydreaming.
• Your brain is always idling, like a car sitting at a stoplight. Compared to this resting state activity, execution of specific perceptual or cognitive tasks, like those typical of fMRI studies, increases brain energy consumption by only a few percent.

Question 2

What is fMRI ?

Answer 2

• An important insight into this problem came from work in the 1990s and early 2000s, using first PET (positron emission tomography) and then fMRI (functional magnetic resonance imaging).
• fMRI is the main technique used to generate images that show a specific brain region “lighting up” during some perceptual, motor, or cognitive task; however, fMRI does not directly measure neuronal activity.Instead, it detects changes in blood flow and oxygen consumption that occur in activated brain areas.
• This signal, called the blood oxygen level dependent (BOLD) signal, is generally thought to reflect increased activity of neurons. The BOLD signal is noisy, and the small changes in the signal that occur when subjects perform a task are detected by taking the difference between an image obtained in experimental conditions and a control image.
• For example, suppose that the experimenters want to identify the brain regions involved in reading words. In the experimental condition, subjects might stare at a computer screen while words appear on the screen. The control condition would be the same, except that the words would be replaced by random strings of letters. Taking the difference between the experimental and control scans shows the brain areas that are activated specifically by reading words.

Question 3

Default mode network

Answer 3

• In addition to data collected during these types of tightly controlled experiments, data can also be obtained from scans before the experiments began, while the subjects are quietly in the scanner, doing nothing in particular, waiting for the experiment to begin.
• Taking the difference between these resting state scans and experimental scans reveals something interesting and surprising: regardless of the task, a set of brain regions, mainly along the midline in the frontal and parietal lobes, consistently decreases in activity in response to the task (Fig. 1).
• In other words, there is a network of functionally interconnected brain regions that are active when subjects are quietly thinking their own thoughts, but decreases in activity when subjects have to direct their attention to an external task.
• This network of brain regions has come to be known as the default mode network (DMN).

Question 4

Additional evidence in support of the idea of functionally
connected cortical regions forming a default mode network
has come from careful analysis of seemingly random
fluctuations in the BOLD signal.

Answer 4

• As mentioned above, the BOLD signal is noisy, and imaging studies are designed to subtract off this noise.
• However, it was noticed, first in somatic sensory regions and subsequently in the default mode network, that these seemingly random fluctuation in different parts of the network were highly correlated (Fig. 2).
• It is now apparent that this noise reflects ongoing brain activity, and that correlations in the noise indicates brain regions that are connected and are talking to each other
• This approach has been used to identify numerous distinct brain networks.

Question 5

What is the function of the default mode network?

Answer 5

• The default mode network consists of the medial surface of the frontal lobes, medial regions of the parietal lobes, and parts of the lateral parietal lobes and the medial temporal lobes.
• As we’ve discussed in class, the ventral medial prefrontal cortex connects cognition to signals from the body, autonomic responses, and emotions. This region has been linked to social behavior, mood control and motivation.
• More dorsal regions of medial prefrontal cortex are associated with self-monitoring.
• The medial parietal areas are interconnected with regions of the medial temporal lobe that are involved in episodic memory, the memory of our past that enables us to form a coherent story about our lives.
• Taken together, these observations suggest that the default mode network may be especially involved when our thoughts are turned inward, reflecting on memories from the past and monitoring our inner thoughts and feelings.

Question 6

What are the brain networks Involved in executive control?

Answer 6

• The default mode network illustrates an important emerging insight in neuroscience: the cerebral cortex is not a collection of separate regions, each working in isolation to carry out a specific function, but rather a highly integrated set of networks, each comprising multiple interconnected cortical regions.
• Through correlations in fluctuations in the BOLD signal, numerous networks have been identified. Fig. 3 shows four of the networks involved in executive control.
• In addition to the default mode network these include:
  
  • an executive control network, involved in planning, execution, and control over goal-directed behaviors,
  • a dorsal attention network, which controls top-down attention, and a salience network, which detects important stimuli from the external environment and from the body and initiates appropriate responses.

Question 7

What initiates the switch between these networks?

Answer 7

• When subjects switch from daydreaming in the scanner to performing a task, activity in the default mode network decreases.
• At the same time, the activity of the executive control and attention networks increases.
• One idea is that this switch is initiated by the salience network. According to this hypothesis, the salience network detects a novel situation that requires concerted attention and effort and says, “OK, time to stop daydreaming and take on this new problem. Turn off the default network and turn on the executive control and attention networks.”