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The acute influence of psilocybin slows down human reaction times and mildly impairs parts of cognition that coordinate behavior. These short-term effects scale with the dose of the drug, highlighting a need for supervision and safety measures while the substance is active in the body. The findings were recently published in the journal Psychopharmacology.
Psychedelic substances are increasingly being evaluated for their therapeutic potential in treating conditions like anxiety and depression. As these drugs move toward broader clinical acceptance, researchers are attempting to understand exactly how they alter immediate thinking processes. Cognitive functions, specifically executive functions, are essential mental skills that help people plan, focus attention, remember instructions, and juggle multiple tasks.
Executive functions act as a sort of traffic control system for the brain, coordinating basic abilities to achieve a specific goal. Impairments in this system are common across various psychiatric conditions. Observing how psychedelics temporarily alter these domains helps scientists gauge basic safety and the potential side effects of future clinical treatments.
P. Yousefi, a researcher at Leiden University, and Morten P. Lietz, a researcher at the University of Freiburg, led a group to quantify the acute cognitive effects of psilocybin. The acute phase refers to the timeframe when a person is actively experiencing the direct physiological and psychological effects of the drug. The research team wanted to observe how executive functions respond to different doses of psilocybin and how the timing of the drug administration affects mental performance.
To evaluate these details, the research group conducted a systematic review and a multilevel meta-analysis. A meta-analysis is a statistical method that combines data from multiple independent studies to identify broader, more reliable trends. The researchers pooled data from 13 empirical studies that included a total of 42 separate measurements of cognitive performance.
These individual studies assessed different components of executive function. The researchers evaluated tasks that measure conflict monitoring, which is the brain’s ability to process contradictory information. A common way to test this is the Stroop task, where a participant might see the word “green” printed in red ink and must state the color of the ink rather than read the word.
They also evaluated working memory, which is often tested by asking participants to remember a sequence of numbers or locations that updates continuously as the test goes on. Finally, the analysis included studies of response inhibition, which is the ability to suppress a habitual reaction. Attention spans and cognitive flexibility were also evaluated.
By combining the gathered data, the scientists evaluated how psilocybin affected two main outcomes during these cognitive tests. They looked at reaction time, which is how fast a participant responds to a prompt. They also measured accuracy, which is the percentage of correct answers provided during a given testing session.
The team categorized the data by the dose given to volunteers, treating anything under five milligrams as a microdose and anything over 30 milligrams as a high dose. They also grouped the data by the specific cognitive function being tested. Finally, they evaluated the timing of the test relative to when the drug typically peaks in the bloodstream, usually between 90 and 180 minutes after ingestion.
The results showed that participants took longer to respond to cognitive tasks when taking the drug compared to when they operated under an inactive placebo. The researchers found a linear relationship between the dose of the drug and the delay in reaction time. Microdoses produced a minor slowing effect that was barely measurable, while low and medium doses created mild to moderate delays.
The highest doses produced the most extreme delays in processing speed. A notable finding from the pooled data was that working memory seemed slightly more resilient to these delays than basic attention, showing severe slowing only at higher doses. In contrast, basic attention tasks suffered delays even at the lower dose ranges.
Changes in accuracy were less pronounced. While there was a slight trend toward decreased accuracy across the studies, the results were not statistically significant. This means the observed drop in correct answers was small enough that it could have been driven by statistical noise or random chance rather than the drug itself.
The slowing of reaction times occurred across the board. The researchers did not find that psilocybin impaired one specific area of executive function substantially more than another. The delays were relatively uniform whether the participant was suppressing a reflex, updating their memory, or switching between conflicting rules.
The timing of the tests within the acute window also did not change the outcome. Reaction times were consistently delayed both during the peak window of the drug’s effect and in the adjacent hours before and after the peak. This finding establishes that cognitive slowing is a persistent feature of the entire acute psychedelic experience.
The analysts explored how task design affected these outcomes. Some cognitive tests try to isolate pure mental tasks by subtracting baseline reaction speeds from the final score, while others rely on raw scores that require a broad mix of sensory, motor, and attention skills. The analysis revealed that general tests requiring multiple cognitive skills showed stronger delays compared to tasks designed to isolate a pure mental process.
The authors propose a few explanations for this uniform pattern of delayed reactions. First, psilocybin might influence basic sensory and motor systems directly. It could cause a lag in how the visual cortex processes incoming light and how the motor cortex commands the hand to press a button, which would slow down performance on any physical test.
Second, the drug is known to alter baseline attention. Attention acts as a foundational building block for all higher level mental operations. If an individual struggles to focus on simple instructions, they will logically execute demanding rules at a slower pace.
A third mechanism involves dual task interference. When people navigate the intense subjective experience of a psychedelic trip, their brain is heavily engaged in processing emotional and sensory distortions. Asking a participant to simultaneously perform a laboratory test splits their mental resources, leading to cognitive fatigue and slower execution speeds.
There are a few caveats to consider regarding the underlying data. The researchers noted a moderate to high risk of bias across the studies they analyzed. This was primarily driven by the difficulty of blinding participants to whether they received a psychedelic or a placebo, as the physical effects of the active drug are highly noticeable.
The team also detected clear signs of publication bias in the data concerning reaction times. Publication bias occurs when studies with dramatic results are more likely to be published than those showing minor effects. As a result, the existing scientific literature might slightly overestimate the true extent of how much psilocybin slows down cognitive processing.
The available data focuses exclusively on the acute phase of the drug. Very few studies have tracked cognitive performance days, weeks, or months after the drug leaves the body. Some recent standalone studies have hinted at potential long-term cognitive benefits following psychedelic use, but the mechanisms bridging an acute impairment and a long-term benefit remain unexplored.
Future research might benefit from transitioning to tasks that better mimic real-world activities. Traditional laboratory tests can feel detached and abstract, which might reduce participant motivation while they are actively experiencing an altered state. Testing attention and memory in more natural settings could help separate a true cognitive impairment from a simple lack of motivation to complete a boring computer test.
Because psilocybin consistently slows reaction times during its active phase, the researchers emphasize the need for robust supervision in medical settings. Patients should avoid situations that require rapid processing speeds, such as driving or operating heavy machinery, until the immediate effects of the drug have fully resolved. Clinical guidelines should ensure patients remain in secure environments for the duration of the drug’s active window.
The study, “Acute effects of psilocybin on attention and executive functioning in healthy volunteers: a systematic review and multilevel meta-analysis,” was authored by P. Yousefi, Morten P. Lietz, F. J. O’Higgins, R. C. A. Rippe, G. Hasler, M. van Elk, and S. Enriquez-Geppert.
