When you're sedated, there are radical changes to the architecture of your brain...
The brain is the most complex organ in the body, and the organisation of its components has been largely elusive. An emerging field of study in neuroscience, though, is looking at the extensive communication pathways throughout the entire brain. These connections indicate a high level of organisation which is vital to cognition and consciousness.
But what happens to this organisation when you are at a different level of consciousness? Scientists at Cambridge University have investigated the impact of sedation on the brain using fMRI. They found that when you are sedated, there are radical changes in the connectivity of the brain, particularly in the regions involved in the senses and information processing.
The authors assessed the effects of a drug called propofol, which is often used as an anaesthetic in hospitals. They compared healthy patients given no propofol with those given a low dose, and with a moderate dose. When they had a low dose, the participants were slow to react in conversation. With a moderate dose of propofol, they were a lot less responsive to stimuli like noise.
The researchers found that the modular architecture was generally preserved when the participants were under sedation. In the brain, modules are regions which have lots of connections between them, but are less connected to outside regions. This organisation increases the efficiency and flexibility of information processing. An analogy could be the London Underground, and modules represent the different lines between stations, which are the distinct regions.
However, the team also found that propofol significantly altered the frequency of hubs in many brain regions. A hub is a region which is highly connected to other regions. To use the Tube analogy, a hub might be Baker Street or Waterloo stations, as they’re very well connected.
The number of hubs decreased in various sensory networks including the visual and auditory networks, suggesting that propofol has an effect on the way we process the world around us.
At the same time, the scientists found that propofol increased the number of hubs in other networks, indicating that these networks are becoming stronger and more efficient. The networks which saw an increase in “hubness” were the subcortical and salience networks, which are more involved in thought and emotion
Finally, administration of propofol significantly decreased the rich-club coefficient, the measure of connectivity between hubs, instead of between a hub and a less-connected node. So this would be the line between Baker Street or Waterloo, compared with, for example, another line between Waterloo and Clapham Common.
Importantly, those with more reduced rich-club organisation made more mistakes in a test involving sorting words into ‘living’ or ‘non-living’ categories, which tested their ability to attach meanings to terms. Those with a larger reduction in connectedness between hubs, therefore, tended to make more errors in understanding words.
Overall, the researchers found that there was widespread reorganisation of brain networks when the participants were sedated. The strength and number of connections in sensory networks were reduced but increased in networks involved in thought and emotion. There are still some questions that this study doesn’t address, such as the mechanisms of action, or how it goes back to normal after the sedative has worn off.
Interestingly, propofol is also known as “milk of amnesia” as it seems to affect memory, and studies have looked at its use as a treatment for PTSD. Perhaps propofol’s effect on connectivity in the brain could explain its impact on memory as well as consciousness.
The brain networks themselves demonstrate how the brain co-ordinates and processes information. In the past, scientists would often look at a region in isolation to analyse its function. But in reality, brain regions are interdependent and are affected by one another. From this study, we can see that the connections between regions change when consciousness is reduced. Could the secrets to consciousness therefore lie in the connections themselves? This possibility is being explored through the Human Connectome Project, so we may yet unravel the secrets held in our brains.
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