A new placebo-controlled study has confirmed that the psychedelic drug LSD can induce synesthesia-like experiences. But the preliminary research, published in the journal Neuropsychologia, raises questions about whether LSD produces “genuine” synesthesia.

Synesthesia occurs when a person experiences an overlap in their senses. A person experiencing synesthesia might be able to taste colors or see sounds.

“Genuine” synesthesia occurs when a specific perception (called the “inducer”) automatically provokes another specific perception (called the “concurrent”). In genuine synesthetes, this pairing between the inducer and the concurrent remains fairly consistent over time. People with grapheme-color synesthesia, for example, might see the color green whenever they see the number 6.

There have been anecdotal reports about LSD causing synesthesia ever since the psychedelic drug was first discovered by Albert Hofmann in the 1930s. Research from the 1950s and 1960s also suggested that LSD could induce synesthesia. But these initial studies had a number of methodological problems, causing researchers to remain uncertain about LSD’s potential to induce genuine synesthesia.

To start to reduce this uncertainty, a group of scientists from the United Kingdom, led by Devin B. Terhune of Goldsmiths, University of London (previously University of Oxford), recruited nine men and one woman for study on LSD and synesthesia.

The ten participants were all deemed to be physically and psychologically healthy, and had prior experience with psychedelic drugs. During an initial testing session, they were injected with a saline solution, which acted as a placebo, before completing psychological tests to measure synesthesia-like experiences. After five to seven days, the participants were invited back for a second session. But this time, they were injected with 40-80 micrograms of LSD.

Terhune and his colleagues employed two verified tests of synesthesia — a grapheme-color association test and a sound-color association test. But the researchers found that the effects of LSD did not meet two of the accepted criteria for genuine synesthesia — consistency and specificity. The specific graphemes and sounds did not produce specific color experiences, and the associations between graphemes, sounds and colors were not more consistent under LSD than placebo.

The type of synesthesia produced by LSD could be “qualitatively different” from genuine synesthesia, the researchers said.

However, it is also possible that the effects of LSD failed to meet the accepted criteria for genuine synesthesia because the study was statistically underpowered due to the small number of participants.

“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,” Terhune explained to PsyPost via email. “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.”

LSD and other psychedelic drugs present a problem for synesthesia researchers. Genuine synesthesia is considered a reliable and consistent phenomenon. One specific perception consistently induces another specific perception. The tests designed to measure synesthesia reflect this consistency. But the synesthesia-like experiences produced by LSD appear to be inconsistent and transient.

“This is a difficult issue because standard criteria for synesthesia have served the field well and synaesthetes almost universally meet these criteria. Alternatively, it may be that current definitions of what qualifies as synesthesia are overly narrow. To be honest, we’re really not sure at this point which way to go on this,” Terhune told PsyPost.

The criteria for synesthesia may need to be updated, or the synesthesia-like experiences produced by LSD may need to be treated as a unique phenomenon.

“If they are qualitatively different, then yes, we’ll need to develop new ways to study the multisensory effects reported by people on LSD. 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,” Terhune explained to PsyPost.