Psychedelic drugs and perception: a narrative review of the first era of research

This historic review (2021) examines a series of studies conducted in the first and second eras of psychedelic research which examine the perceptual effects of psychedelic drugs and highlights certain commonalities, such as a shared interest in the perception of music. While most studies investigated how psychedelics affect vision across every level of visual processing (e.g., retinal, cortical, subcortical, low-level visual processing, complex visual imagery), other studies investigated its effect on auditory discrimination, the neural correlates of auditory processing, and auditory hallucinations restricted to a subset of participants. Some studies also demonstrated that psychedelics can distort representations of body schema, time perception, taste, olfaction, and synesthesia, but these areas still remain understudied.

Abstract

“Psychedelic drugs are well-known for transiently altering perception, and in particular, for their visual effects. Although scientific interest into the substances’ effects on perception increased during the first era of psychedelic research during the early to mid-20th century, there is currently no source where these findings have been synthesized. In addressing this gap, the current narrative review found that psychedelics were examined for their influences across all levels of the visual system (e.g., retinal, cortical, subcortical, simple visual processing, complex imagery, hallucinations). Psychedelics were also shown to affect auditory discrimination/generalization, neural correlates of auditory processing, and led to auditory hallucinations in subsets of participants. Several studies demonstrated that psychedelics can distort representations of body schema and time perception. Concerns regarding methodological standards of this era are a limitation to the findings and are discussed. Collectively, this review preserves and increases the accessibility of the work done by pioneering psychedelic/perception researchers, synthesizes findings, and critically analyzes areas of discrepancy to inform future studies.“

Authors: Jacob S. Aday, Julia R. Wood, Emily K. Bloesch & Christopher C. Davoli

Summary

Jacob S. Aday*, Julia R. Wood, Emily K. Bloesch and Christopher C. Davoli Psychedelic drugs and perception: a narrative review of the first era of research

Psychedelic drugs are well-known for transiently altering perception, and in particular, for their visual effects. This review synthesizes findings from pioneering psychedelic/perception researchers, and critically analyzes areas of discrepancy to inform future studies.

Introduction

Psychedelic drugs can induce dramatic, transient shifts in perception, particularly vision. However, much of the early literature on the perceptual effects of psychedelic drugs is unavailable to the current generation of researchers.

Researchers have found that psychedelics alter mental imagery, N170 visually evoked potentials, and visual hallucinations, and have also been found to affect time perception, binocular rivalry, and synesthesia.

Given the increasing interest into the effects psychedelics can have on perceptual processing, it is important to synthesize the early research in this area. This review covers studies from the first era of psychedelic research (1895 – 1975) that looked at the relationship between psychedelics and perception.

Vision

Psychedelics can alter vision both with the eyes open and closed. Individuals on psychedelics often experience “pseudohallucinations” and can see unique patterns of geometric figures or vivid mental imagery.

Physiological changes

Researchers have argued that psychedelic-induced alterations in vision stem from changes in the peripheral eye versus changes in the brain. We will present research from both camps and reconcile their findings.

Researchers have found several reasons to link psychedelics’ effects on vision to the peripheral eye, including increased spontaneous discharge rate of two-thirds of retinal ganglion cells and prolonged rod-cone break in the dark adaptation curve of the ERG. Apter and Pfeiffer (1956) found that cats with LSD or mescaline had spontaneous action potentials on the sclera, optic nerves, and visual cortices, which may contribute to changes in vision.

The research on psychedelic drugs’ effects on the brain seemed to dwarf the research on the drugs’ effects on the retina. However, a closer inspection of the data revealed that the effects of psychedelic drugs on the retina were specific to individuals who had previously reported spontaneous visual experiences.

Researchers documented neurological changes at both the subcortical and cortical levels of the visual system, using electrodes implanted into animal models. The lateral geniculate nucleus (LGN) is the first relay center in the thalamus that broadly organizes retinal signals. However, some studies have noted that psychedelics increase activity in the LGN. McKay and Horn (1971) postulated that LSD affects the LGN dynamically, with dosage being a factor contributing to discrepant results. Buno et al. (1970) noted that changes in LGN activity were always less pronounced than alterations in cortical activity, suggesting that psychedelics’ influence on cortical processing may be more relevant to their visual effects.

Researchers found that LSD increased the amplitude and decreased latency of visually evoked potentials in rabbits and humans, and that DMT attenuated photic-evoked electrical activity in rabbits. Alpha rhythms were the most commonly investigated frequency, and fluctuations were thought to reflect changes in mental imagery and/or visual attention.

The neurological changes reported here can inform current work, and contradict modern findings that indicate suppressed alpha power while on psychedelics. Further research is needed to coalesce these discrepant results, but individual differences in mental imagery or task-level differences may be involved.

Alterations in simple visual processing

Psychedelics cause visual alterations in form, depth, and motion, which occur both with the eyes open and closed. These alterations include changes in the apparent horizon and apparent vertical, as well as curvature and tilt of nearby visual space. These studies support the hypothesis that psychedelics bias visual processing towards color at the expense of conventional form processing.

Low-level changes in visual thresholds may also contribute to EVI when the eyes are open. These findings seem to contradict recent models of psychedelic effects which argue that low-level processes are largely unaffected by the drugs.

Many individuals on psychedelics see patterns in their visual field that can be categorized into grating, lattice, fretwork, filigree, honeycomb, chessboard, cobweb, tunnel, funnel, alley, cone, or vessel. Some representative descriptions of closed-eye EVI include purple and green flashes and zigzags, and plain geometric figures. These visions differ from the more elaborate visions that will be described in the complex visual imagery section.

Alan M. Hartman and Leo E. Hollister conducted the most focused studies on psychedelics’ effects on color perception from this era, and found that all three drugs increased perception of color on a flicker task and decreased color discrimination equally, but only psilocybin significantly. Despite tentative variations, the authors concluded that the effects of psychedelic drugs on color perception are similar.

Complex visual imagery

Individuals on psychedelic substances often report having dynamic internal visions that are vivid and sometimes described as dreamlike. These visions are not easily quantified and reduced to averages, but are often most fully elaborated on in published self-experiments.

”I saw palaces with courts, arcades, gardens, and a mythological beast drawing a regal chariot. Later, I was suspended in mid-air viewing landscapes of mountains, with camel caravans advancing slowly across the slopes.”’

Auditory processing

The auditory effects of psychedelics were among the most studied during the first era of psychedelic research, and the effects of music in psychedelic-assisted psychotherapy sessions is an area of growing study.

Several studies indicated that psychedelics could reduce auditory sensitivity, leading to greater stimulus generalization. However, one research group consistently found no effect of psychedelic administration on stimulus generalization with rat models, and generally found that the drugs led to slower reaction times.

Several studies were conducted to assess the neural correlates of auditory changes in psychedelic states, and it was hypothesized that these changes were related to “functional disorganization or disruption” of the reticular formation. Key (1965) found that LSD increased AEP amplitude in the auditory cortex and suggested that the enhanced meaning/saliency of environmental stimuli may be driving increases in cortical processing. Arnold et al. 1971 found that DMT induced distinct changes in AEPs.

The psychedelic perceptual research of this era focused on auditory hallucinations, which were considered rare but were also common in some studies. These hallucinations were often considered pseudohallucinations, as they were distorted perceptions of veridical stimuli rather than being solely internal manifestations of sound.

The first era of psychedelic research postulated that music could help patients let go more easily, explore their inner mental experience more fully, and facilitate intense emotions. Recent work has validated this notion, and music’s role in enhancing meaning-making, emotionality, and mental imagery can be important mechanisms underlying therapeutic effects.

Many studies from this era reported changes in body schema and tactile perception while under the influence of psychedelic drugs, including feelings of changes in limb length/proportions, paresthesia, heaviness of limbs, and orientation of the body.

Although anecdotal reports of changes in body schema were common, empirical research into these changes was still in its infancy. Liebert et al. (1957) tested if LSD affected the perception of the size of one’s own body compared to the size of external objects. They found that LSD led to increases in perceived head size, whereas arm length was perceived as significantly shorter. Although empirical research into psychedelic bodily effects was sparse, anecdotal reports were common and substantiated by the limited research that was published in this area.

Time perception

Psychedelics were frequently reported to induce distortions in time perception, with a weight of evidence leaning slightly towards participants overestimating temporal durations while on psychedelics, but also null findings published on the effects of psychedelics on time perception.

There are several potential explanations for these conflicting findings. The authors of the first era of research suggested that time perception changes coincided with changes in mood, but this theory was later refuted.

Time perception can vary depending on whether a task requires participants to rate a temporal duration or reproduce it. Furthermore, some studies used tapping tasks to estimate time perception, which could have been contaminated by concomitant drug-induced changes in motor ability, body perception, and visuospatial perception.

Other (taste, olfaction, synesthesia)

In studies using LSD and other psychedelics, participants reported having a “dry”, “funny” or “bitter” taste in their mouths, and some noted a metallic, generally unpleasant flavor. However, there is little research examining psychedelics’ effects on taste or olfaction.

In a number of studies, participants reported experiencing synesthesia, or a blending of the senses. Incidence of synesthesia was quite variable in the first era of psychedelic research, with some participants reporting it and others noting it frequently. Hartman and Hollister (1962) investigated whether psychedelics affected synesthesia by presenting participants with 16 tones of varying frequencies and amplitudes while under the influence of psilocybin, mescaline, or LSD. Although colors were reported by less than half of participants, other visual changes seemed to emerge more consistently. Although some contemporary researchers appear to be intrigued by the synesthetic effects of psychedelics, the ASC questionnaire has not been a predominant focus of the field.

Limitations

Although there are a number of limitations to keep in mind when drawing conclusions from these results, this review can help direct investigators towards relevant work that may help reconcile disparate outcomes. Self-experiments were included in the research because they were representative of this early period of research, but conclusions from this research can be limited by concerns about ethics, experimenter biases, and generalizability.

We searched for studies on psychedelics and perception from the early-to-mid 20th century using three search engines, and a lengthy Google Scholar search was conducted. Our review is not comprehensive or exhaustive, but it does summarize the main findings.

Conclusion

This review demonstrated that interest in the perceptual effects of psychedelic drugs surged during the first era of psychedelic research. Psychedelics affected visual processing at all levels, including retinal, cortical, subcortical, low-level visual processing, complex visual imagery. Psychedelics altered perception in several ways, including changes in body schema, altered time perception, and synesthetic experiences. However, there was limited systematic inquiry into these effects, and there was no delineation of drug-specific effects.