In Psychedelic Journal Club, the BCSP brings you essential reads to keep you up to date with the leading topics in psychedelic science. This month, postdoctoral researcher Sean Noah explains “Enhanced visual contrast suppression during peak psilocybin effects: Psychophysical results from a pilot randomized controlled trial,” published in 2024 in the Journal of Vision.
A new article by Link Ray Swanson and colleagues, published in the Journal of Vision, reports that, in healthy human volunteers, psilocybin increases the strength of a visual illusion called contrast surround suppression. The article, titled “Enhanced visual contrast suppression during peak psilocybin effects: Psychophysical results from a pilot randomized controlled trial,” describes that in a small sample of six volunteers, a high dose of psilocybin strengthened the contrast surround suppression effect by about 7 percent.
What is contrast surround suppression?
Visual contrast surround suppression is a phenomenon in which a visual stimulus appears to have slightly less contrast when it is located within a higher contrast surrounding stimulus, compared to when it is presented in isolation. Here, “contrast” is a quantifiable physical property of a visual stimulus, calculated from the range of its luminance values. For example, an image made up of black and white has higher contrast than an image made from only shades of gray. If that black and white image were affected by contrast surround suppression, it would appear a little more gray.
Contrast surround suppression has been extensively studied in visual neuroscience and its mechanisms are well understood. It is thought to result from mechanisms in the brain’s visual system that incorporate contextual information from the broader visual field into the processing of local visual information. Contextual processing mechanisms are fundamentally important and beneficial for perception, and multiple different kinds have been discovered throughout the visual system, from the retina to the highest levels of the visual cortex.
A visual task to probe visual alterations
In their experiment, Swanson and colleagues sought to determine whether the psychedelic drug psilocybin affected contrast surround suppression. They reasoned that some of the characteristic visual effects of psychedelics might be attributable to this basic visual mechanism. Numerous previous studies of surround suppression without any drug manipulation have highlighted its important role in the brain’s ability to identify object boundaries (for examples, see: Nothdurft, Gallant, and Van Essen, 2000 and Lamme, 1995). Identifying object boundaries may be at the root of many of the hallmark visual effects of psychedelics, like shifting visual textures and distorted movements. If the brain’s best guesses about the boundaries of an object are changing from moment to moment, this could result in a perception of that object whose edges are drifting back and forth, reminiscent of psychedelic visual effects like moving surfaces and “breathing” walls.
The researchers administered 25 milligram capsules of psilocybin to healthy participants. The participants waited for three hours after ingesting the drug before performing the visual experiments, so that the perceptual measurements could capture the peak effects of the drug, and so that the participants could have time to acclimate to the psychedelic effects before attempting to perform the experimental task. Then, the participants performed the experiment.
In the experiment, two circular visual stimuli were presented side by side. One was presented in isolation, and one was presented inside a surrounding higher contrast ring. Participants reported which circular stimulus appeared to have higher contrast. Once they made a judgment, two new stimuli appeared in the place of the old ones, requiring another judgment, and so on, for about 30 minutes. By varying the actual contrasts of the presented stimuli and recording the participant’s perceptual judgments, the researchers were able to determine the strength of the contrast suppression illusion, or how much higher contrast a surrounded stimulus had to be than an isolated stimulus in order for the two stimuli to appear to have the same contrast.
The researchers reported that in their small sample of six participants, psilocybin increased the strength of the surround suppression effect by about 7 percent compared to a placebo in the same group of participants. Moreover, the enhancement of surround suppression in each participant correlated with the intensity of visual perceptual alterations measured with the Altered States of Consciousness questionnaire, a survey that is commonly used to quantify the subjective effects of psychedelics. In other words, the more a participant reported experiencing visual phenomena such as complex imagery or audiovisual synesthesia, the stronger their contrast surround suppression.
Notably, the researchers examined whether psilocybin affected the participant’s basic ability to follow the instructions of the experiment. In a study of the effects of psychoactive drugs, it is important to ensure that any observed differences between active drug and placebo conditions are not due to factors irrelevant to the scientific question at hand, such as a participant’s motivation or their basic ability to follow instructions. To this end, the researchers included catch trials, or perceptual judgements designed to be easy to answer correctly, to assess whether participants were performing the task properly. Participants performed the catch trials with almost perfect accuracy whether they were given a placebo or psilocybin, indicating that psilocybin did not impair the overall ability to perform the experimental task, and bolstering the finding that psilocybin had a specific effect on visual contrast surround suppression.
The bigger picture
Why is it interesting that psychedelics marginally increase the potency of this specific visual illusion? Visual effects are a hallmark feature of psychedelics and are very diverse: Psychedelics commonly alter perception of color, texture, motion, and object identity. However, it’s not clear how this variety of visual phenomena might be caused in the brain, or even whether various psychedelic visual effects can be attributed to any common neural mechanisms. By carefully recording the effect of psilocybin on a well-studied visual phenomenon like contrast surround suppression, the researchers are gathering important new clues that will contribute to a clearer explanation of how psychedelics cause profound perceptual effects.
Swanson and colleagues describe their experiment as a pilot study, meaning that it is a small preliminary trial designed to test the feasibility of an experiment. In this small pilot study with six participants, the researchers were specifically interested in ascertaining whether it would be safe to administer a high dose of psilocybin to healthy volunteers and then ask those volunteers to perform a visual experiment during the drug’s peak effects. In their paper, the researchers reported no unexpected or serious adverse events resulting from their experiment. Moreover, this pilot study serves as an example of how the effects of psychedelics on basic visual processes can be carefully studied in a laboratory setting. The more that researchers characterize the effects of psychedelics on visual processes with well-understood neural mechanisms, the better our understanding of psychedelic visual alterations will become.
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