Computer simulations of a human brain under the influence of LSD show that entropy increases the most in regions responsible for processing vision and integrating sensory information.
Taking psychedelics increases the brain’s overall entropy but the effect is most prominent in regions responsible for visual perception and integration of inputs from all senses, possibly reflecting how the drugs can induce sensations of expanded awareness.
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| Illustration of brain cells in the temporal cortex of the human brain Shutterstock/Kateryna Kon |
Entropy measures how rich and disordered a system is. As an example, think of your kitchen sink. Entropy is high when the system has many parts that are not ordered – like plates, forks and glasses scattered all around – and low when the system is very uniform and organised, like a single stack of perfectly matching plates in one corner.
Researchers can calculate the entropy of the brain because it is a system full of various electrical signals so it can have differently ordered states depending on when and where those signals fire. Studies of those signals have shown some correlation between entropy and consciousness. For instance, our brains seem to have more entropy when we are awake than when we are asleep.
To understand the brain’s entropy under the influence of consciousness-altering psychedelic substances, Rubén Herzog at Sorbonne University in France and his colleagues built a simulation of electrical signals in the brain based on MRI scans of people using LSD. The simulations also accounted for known characteristics of the brain, such as the strength of anatomical connections between its different regions and the distribution of a receptor activated by psychedelics called 5HT2A.
In their simulations, the activation of this receptor acted as the equivalent of giving LSD to a simulated brain. The researchers then used the electrical signals the computer produced to calculate entropy. As expected, they found an overall increase in the brain’s entropy. But they also found that this increase was largest in the region of the brain responsible for processing vision and combining information from different senses.
Herzog says that at first, they thought the difference in entropy increase may be related to locations with more or fewer 5HT2A receptors. However, further mathematical analysis revealed that in many regions of the brain, the best predictor of increased entropy was how strongly connected the area was to other parts of the brain.
“If you are experiencing a lot, like having visual experiences, it makes sense that there is more entropy in that part of the brain locally, but because we found that connection strength is the main determinant of entropy increase, I think we can’t conceive of changes in consciousness only happening in one brain region. I think connectivity could be like a fingerprint for how everyone’s consciousness reacts to psychedelics differently,” says Herzog.
Herzog says studying brain connectivity could eventually be part of psychedelic-assisted therapy for people with depression or post-traumatic stress disorder. Researchers and medical professionals could predict a person’s reaction to the drugs by looking at the so-called “connectome” of their brain first, he says.
David Papo at the Italian Institute of Technology says the new study replicates other researchers’ past observations and insights, so the team’s model of which parts of the brain respond to psychedelics may be on the right track. However, he cautions that entropy alone may not be sufficient to convey all the complexities of how a person’s consciousness emerges or changes. “There are contradictory results when it comes to brain entropy and pathologies. It may be only a partial way to understand consciousness in different brains,” he says.
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