The largest ever meta-analysis of brain imaging studies of patients under the influence of psychedelics has recently been published. The findings were published in Nature, bringing together over 500 brain scans of individuals under the influence of various psychedelic drugs.
Psychedelics have been understood to broadly increase connectivity between brain networks (or zones of the brain) while decreasing activity within them. The most prominent example in the literature is the default-mode network, through which we maintain a sense of “narrative self”, that is, the sense of an “I” that persists through time.
In laboratory studies conducted on Western people who’d been given psychedelics, the default mode network is downregulated to such an extent that many people experience a sense of “ego dissolution”, the dissolving of self into surroundings and sensation.
This is the story thus far. The meta-analysis found that while that story has held up in specific studies, it doesn’t hold up to the weight of all the neuroimaging data that has been collected.
Instead, the study found “a core signature of increased functional connectivity between transmodal (default, frontoparietal and limbic) and unimodal networks (visual and somatomotor), with subnetwork specificity”. In other words, cognitive centres of the brain became more co-active with the sensory and motor parts of the brain. The striatum, a deep brain structure involved in reward, habit and automatic processing of sensory information, showed reliably increased connectivity with sensory and cognitive zones of the brain.
The researchers also found that psilocybin and LSD were the most reliable in inducing these effects. DMT did so as well, but with more extreme results and with less statistical certainty (which researchers tied to fewer studies having been conducted).
There have been few neuroimaging studies of people under the influence of ayahuasca. What we have suggests that a downregulation of evolutionarily basic brain structures including the limbic system, which manages fear and threat perception, spatial navigation, and basic survival instincts like thirst and hunger. But like DMT, ayahuasca has been relatively little studied thus far.
We are set to keep learning more about how psychedelics work. On the US regulator’s registry, there are currently more than 400 active clinical trials using psychedelics.
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