Since Albert Hofmann first synthesized LSD in 1938, there have been anecdotal reports of the drug inducing synesthesia–a condition in which individuals experience an overlap in their senses, such as seeing sounds–in those who would not otherwise have such experiences. But until now there has been little rigorous research into the phenomenon. However, a new study by scientists at the University of London has found the first evidence not of LSD-induced synesthesia itself, but of a peculiar, similar phenomenon.
"Genuine" synesthesia is involuntarily encountered by around one in 27 individuals. The neurological phenomenon creates a situation in which one perception, such as color, sound, numbers, or taste (the inducer) triggers a secondary perception (the concurrent). Two of the most common, well-studied forms of synesthesia are grapheme-color synesthesia (in which numbers or characters trigger colors), and sound-color synesthesia.
As for how this occurs naturally, NPR reports:
[T]he brains of synesthetes do appear to be anatomically different... In particular, it seems that the neural connections between different sensory parts of the brain are more myelinated in people with synesthesia. Myelin is a fatty sheath that surrounds neurons and enables neural signals to travel more quickly.
So the University of London team, led by Devin B. Terhune, gathered a small–10 person–pilot test group of non-synesthetes, all of whom were in good mental and physical health, and who had prior experience of psychedelic drugs. In the first stage of the trial, the subjects were injected with a placebo saline solution. Around a week later, the courageous volunteers were injected with between 40 and 80 micrograms of LSD.
Under both the placebo and the influence of LSD, the subjects participated in well-established tests for synesthesia, and the results were quite curious. LSD did not create experiences that meet two major criteria for "genuine" synesthesia, as the results lacked consistency and specificity.
Terhume explained to PsyPost:
The fact that we didn’t observe increased stimulus-color consistency under LSD is an important issue because consistency is the most widely-used criterion for congenital synesthesia. I tend to believe this data is reliable as it was in the opposite direction of what was predicted if LSD was producing genuine synesthesia... The inducer specificity data is harder to interpret because it was in the predicted direction but still clearly non-significant. This places it more in ambiguous territory.
Which is to say that the synesthesia experienced under the influence of LSD is qualitatively different to that experienced by those with the genuine neurological condition. More study is needed, with larger test groups, in order to gain a greater understanding of what this synesthesia-like experience actually is, but that in itself poses its own issues.
Studying drug-induced synesthesia-like experiences may prove very challenging if the experiences are completely transient and not very reliable. It may be conceptually similar to studying hallucinations, which are difficult to reliably induce in a laboratory setting
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