Since Albert Hoffman discovered LSD (lysergic acid diethylamide) in 1938 and hippie culture made it a popular social drug in the 1960s, its psychedelic effects on the brain have been a source of scientific investigation. Anecdotally it’s known to cause visual and auditory hallucinations as well as reports of an individual’s sense of self dissolving and merging with a larger consciousness.

Recent research done at Imperial College London discovered through brain imaging that there is indeed a great deal of activity in the visual cortex of the brain when on LSD, and that it seems to interrupt connections between other brain networks, leading to that ephemeral state of oneness reported by users.

Yet these overlapping sensory experiences also share a lot in common with the brain condition synesthesia, in which an individual’s senses overlap or trigger each other in a way that is atypical. The disorder is rare. Estimates range, but most agree on approximately 1 percent of the population.

A group of researchers at the University of London (UoL) recently set out to study whether the effects of LSD qualified as true synesthesia. Their results, published in Neuropsychologia, suggest that what LSD-users are experiencing is not genuine synesthesia. The researchers say their findings open the door to a better understanding of how we process sensory perceptions.

“Synesthesia is basically understood as … a kind of condition in which a stimulant, known as an inducer, will consistently elicit a secondary experience that is atypical—not something generally experienced in the general population,” Devin Terhune, cognitive neuroscientist at UoL, tells mental_floss.

While every synesthete’s experience is unique, Terhune says there are some commonalities. For instance, many report seeing the letter B as blue, zero as white, and one as black. The most common instances of synesthesia are sound/color pairings (where the sound of a doorbell may evoke a green aura, for example) and color/grapheme pairings (where a particular letter or part of a word may appear in a specific color and even shape, like a blob or spiny edges).

To be considered congenital synesthesia, however, the response must be confirmed by consistency and specificity—that is, the same inducer must produce the same reaction every time.

For the placebo-controlled study, 10 physically and psychologically healthy participants were injected with saline solution for their first session, then they completed psychological tests to measure synesthesia-like experiences: a grapheme-color association test and a sound-color association test. After five to seven days, they were injected with 40-80 micrograms of LSD, and the tests were repeated.

While the participants said they had spontaneous synesthesia-like experiences while on LSD, they didn't report specific color experiences with graphemes and sounds, and sounds and colors were no more consistent on LSD than with the placebos. These results suggest that whatever is happening while under the influence of LSD, it isn't “true” synesthesia.

Given such anecdotal associations of color hallucinations in the existing literature about LSD, Terhune says he was surprised to find that “the color experiences effect was not even statistically significant.”

Terhune says the small sample size of 10 participants may have something to do with the weakness of the results. Another factor may have been the laboratory setting itself. Most people who take LSD aren't experiencing the drug's effects in a lab environment. “Factors like novelty and exposure to stimuli may be more critical,” he says. “Congenital synesthesia is really known as a stimulant-specific phenomenon—that something in your environment triggers your experience, reliably and automatically.”

He suggests future studies could be designed that would follow people taking LSD “out in the field” and ask them at various times, using an app, to report what they’ve been experiencing. This could yield a wider range of data.

Another question for future researchers is whether there is a “fundamental distinction between spontaneous forms of synesthesia and the inducer-specific experiences that congenital synesthetes’ experience,” Terhune says.

There may be genetic underpinnings to the disorder, which seems to be inherited in families. There are several working theories on its origins. One is the immune hypothesis, which considers that the genes responsible for normal cortical development are also involved in the development of synesthesia. The hyperconnectivity theory suggests that synesthetes, whose brains have been shown to have extra-developed myelin along sensory pathways, may experience a collaboration of senses resulting in synesthesia. Other theories consider the influence of the childhood environment or potentially higher levels of serotonin in the brains of synesthetes.

While the results of this study may not appear to have immediate implications—and no researcher is out to “cure” synesthesia—Terhune says that one motivation for his work is to understand the neurochemicals involved in the phenomenon. Plus, there is research to suggest that synesthetes with grapheme-color synesthesia have enhanced recognition memory compared to the average person, which could be of benefit to cognitive research.

“I don’t think synesthesia is going to reveal the really deep insights into different psychological phenomena,” Terhune concludes, “but it can provide us with some useful insights and potentially interesting models for things such as memory, imagery, and other cognitive functions.”