Acid, doctor

Brains have critical periods, and apparently trace amounts of chemicals can retrigger them.

Animals (people) have critical periods. e.g. a baby chick will imprint whatever is moving around in its environment after hatching as their mother.

It is not as drastic in people, but it does happen. Some examples:

• Vocal accents are imprinted in the formative years of a child.

• Children with cataracts, if left untreated in their formative years, remain blind even if the cataracts are removed later (the eyes can see, but the brain can't).

Critical periods have measurable neurobiological markers.

The psychological impact is that the brain is more "open" for some modality during its corresponding critical period. Instead of encoding a behaviour in the gene itself, what gets encoded is the ability to learn that behaviour from the environment at particular times.

For example, instead of encoding a particular language and vocal accent in the genes, the brain learns these from its environment during a critical period.

Not only does this save on genetic space, this is also arguably more adaptive to changes in the environment. For example, an immigrant's child has to live in their new environment, so it is better they pick up their new environment's language and accent instead of the ones they might've (hypothetically) gotten genetically from their parents.

When the critical period passes, the brain stops this imprinting. Critical periods close because extreme impressibility is neither desirable nor cheap.

Critical periods happen after childhood too. Teenage years are a critical period for peer social reward learning.

Critical periods can happen in adulthood too. The aftermath of trauma is a critical period with strong memory imprints.

Harmful imprinting during any critical period cannot be "unlearned" easily because the brain is neurobiologically not open anymore to overwrite what got imprinted. PTSD is a canonical example, but it's not the only one, other psychiatric ailments can also be explained in terms of problematic imprints.

So that's about critical periods. Now the psychedelics.

The new claim I heard is that psychedelics reopen critical periods. This allows a therapist (or the person themselves introspectively) to "overwrite" what got imprinted.

The psychedelic doesn't fix, it just biologically reopens the critical period, allowing a window of intervention. The context determines which critical period gets reopened.