Low‑dose LSD alters how people see, but not contrast perception

A team from the University of Basel tested whether very small doses of LSD would change how people see in a basic visual test. The study used 31 healthy adults who completed three sessions: a placebo, 10 µg of LSD, and 20 µg of LSD, each at least a week apart. Four hours after taking the drug, participants performed a contrast‑discrimination task in which they had to decide which of two striped patches looked brighter. The patches were arranged so that a surrounding ring normally makes the centre patch look less bright. The results showed that LSD did not alter the strength of this illusion, indicating that the drug did not change the brain’s early filtering of visual contrast.

Even though the objective task was unchanged, participants reported subtle visual changes. Those who received 20 µg said colors felt more saturated, the world seemed more dynamic, and their vision felt a little blurrier. The 10 µg dose produced only slight blur, while brightness was unchanged. On a 100‑point scale, the overall drug‑effect rating rose by about 17 points for the higher dose. Participants could correctly guess when they had taken the placebo or the 20 µg dose, but not the 10 µg dose.

Earlier studies with psilocybin, the active ingredient in magic mushrooms, found that higher doses increase the contrast‑suppression illusion. In a 2024 pilot, 4 mg of psilocybin made the illusion stronger, and a 2025 study showed that low to moderate psilocybin doses increased the Ebbinghaus size illusion. The present study suggests that the 20 µg of LSD is roughly equivalent to 4 mg of psilocybin – far below the doses used in those earlier experiments – and that LSD also targets dopamine receptors, which may counteract changes in early visual processing.

The findings imply that low‑dose LSD can alter conscious visual experience without affecting the brain’s automatic filtering of contrast. The study had limitations: the task was run four hours after dosing, possibly after the peak effect; caffeine withdrawal may have affected focus; and the doses were very low. Future research could test higher doses of LSD or assess participants earlier in the drug’s timeline to see whether surround‑suppression changes emerge. The research is part of a larger project on how psychedelics influence visual context in decision making.