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.
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.
Aday and colleagues set out to synthesise research from the first era of psychedelic science (roughly 1895–1975) that investigated how classic psychedelics alter perception. The authors note that LSD, psilocybin, mescaline and N,N-dimethyltryptamine (DMT) produce pronounced alterations in perception—most prominently visual effects—and that extensive early-to-mid 20th century work explored these phenomena prior to regulatory closures of the field. Contemporary laboratories have recently renewed interest in perceptual effects and mechanisms (for example implicating 5-HT2A receptors in imagery and visual-evoked potentials), but there was no single source compiling the older literature. To address that gap, the study is a widespanning narrative review that collates human (and some animal) studies from the first research era examining perception. The authors organised the material into major perceptual domains—vision (subdivided into physiological changes, low-level processing, and complex imagery), auditory processing, body schema and tactile processing, time perception, and other modalities (taste, olfaction, synesthesia)—and emphasise that methodological standards varied widely across the period and are addressed in a dedicated limitations section.
Papers cited by this study that are also in Blossom
Aday, J. S., Mitzkovitz, C. M., Bloesch, E. K. et al. · Neuroscience and Biobehavioral Reviews (2020)
Anderson, E. W., Rawnsley, K. · European Neurology (1954)
Aronson, H., Silverstein, A. B., Klee, G. D. · JAMA Psychiatry (1980)
Barrett, F. S., Bradstreet, M. P., Leoutsakos, J. M. S. et al. · Journal of Psychopharmacology (2016)
This paper is a narrative review of studies from the first era of psychedelic research, defined by the authors as 1895 through 1975. The authors conducted a broad literature search using multiple search engines (they report using three search engines and an extensive Google Scholar search) and included published studies and self-experiments representative of that historical period. Human samples were the primary focus except where animal data were explicitly reported. Included material covered physiological, behavioural and electrophysiological investigations across perceptual domains; the review grouped studies into the categories listed above rather than following a formal systematic review protocol. The authors acknowledge that reporting of methodological details in many primary studies was inconsistent (for example, variable reporting of screening procedures, routes of administration, blinding and dosage standardisation). Because this is a narrative synthesis, no formal meta-analytic pooling or standard risk-of-bias assessment is reported. The authors also compiled a supplementary table pointing readers to additional primary sources they identified, while noting that their search may not be exhaustive.
Vision: The largest body of early research concerned vision, with evidence spanning peripheral (retinal/ocular) effects through subcortical and cortical changes, and including both simple form/depth/motion alterations and elaborate closed-eye imagery. Peripheral-eye findings included markedly elevated drug concentrations in ocular tissue in some animal studies (one report noted LSD levels 18 times higher in the iris than in cortex in monkeys), pupil dilatation and ERG changes such as a prolonged rod-cone break in dark adaptation. Some animal work found increased spontaneous discharge rates in retinal ganglion cells (peaking 20–30 minutes post-injection and persisting an hour), and cortical-evoked potentials were attenuated when the optic nerve was severed in one study, suggesting retinal contributions. Against a peripheral-only account, several human and animal studies indicated central origins: blockade of drug-induced pupil dilation did not abolish visual distortions, occipital lesions attenuated visual phenomena on LSD, and blind but non-congenitally blind subjects sometimes reported LSD-related visual experiences, particularly if they had a history of spontaneous visual imagery. Subcortically, effects on the lateral geniculate nucleus (LGN) were inconsistent—both inhibition and excitation were reported, sometimes varying with dose—whereas cortical effects were more robust. Cortical findings included altered visually evoked potentials (VEPs) with reports of increased amplitude and decreased latency in some studies but attenuation in others, mixed effects on alpha rhythms (increases in some high-imagery individuals, decreases in others), and early evidence that psilocybin increases P1 amplitude. Simple visual processing: Early studies documented distortions of form, depth and motion—examples include raised apparent horizon and altered apparent verticality under LSD, and contraction of nearby visual space under psilocybin. Low-level thresholds were sometimes affected: LSD elevated visual thresholds in pigeons and analogous tasks in humans suggested diminished sensitivity. Closed-eye elementary visual imagery (EVI) commonly manifested as vivid, dynamic geometric patterns (gratings, lattices, tunnels, spirals, etc.). Colour perception was systematically probed in several studies: in one set of comparisons, 47% (9/19), 85% (17/20) and 45% (9/20) of participants reported altered shapes or colours on psilocybin, mescaline and LSD respectively; all three drugs increased colour evoked on a flicker task and generally impaired colour discrimination relative to baseline, with psilocybin showing significant effects in some analyses. Complex visual imagery: Psychedelics frequently produced vivid, often meaningful closed-eye visions and longer episodic scenographic imagery. Self-experiment reports described sharply focused, richly coloured scenes and symbolic content; the authors note that these effects are idiosyncratic and not readily reducible to standardised measures. Early investigators proposed cognitive contributors such as enhanced metaphorical or symbolic thinking. Auditory processing: Behavioural studies reported reduced auditory sensitivity, increased stimulus generalisation in some work, and typically slowed reaction times to auditory stimuli. Electrophysiological animal studies showed dose-dependent increases in auditory-evoked P1/N1 peak area, latency and amplitude with mescaline, and some human/auditory-cortex recordings reported increased auditory-evoked potentials under LSD. Notably, one study found that AEPs "disappeared" at the peak of DMT sessions and reappeared as effects waned. Auditory hallucinations were reported but appeared less frequent than visual phenomena; one early report cited about 35% incidence with mescaline in a sample, and many auditory effects resembled pseudohallucinations or distortions of veridical sounds. Early clinical commentators also emphasised the role of music in therapeutic sessions, reporting preferences for familiar music and noting that music could guide or deepen experiences. Body schema and tactile processing: Anecdotal distortions of body boundaries and size were common; quantitative data were limited but suggestive. For example, 10/14 participants given psilocybin reported changes in body size on a questionnaire, and dose-dependent body-image changes were reported from about 20 µg of LSD upward in one study. A controlled investigation using volumetric/distance tasks found LSD increased perceived head size while perceived arm length shortened, without analogous changes for external objects; drawings of bodies also increased in size under LSD in multiple reports. Tactile effects included paresthesia, altered temperature perception, numbness and tingling. Time perception: Findings were heterogeneous. The literature leans slightly towards overestimation of durations (a subjective slowing of time), but reports of time feeling faster, mixed effects, timelessness and null results also appear. Proposed reconciliations from the era included mood correlations and individual differences; the authors of the review emphasise that methodological variability (different tasks, possible motor confounds in tapping or reproduction tasks) likely contributed to inconsistency. Other modalities (taste, olfaction, synesthesia): Gustatory disturbances were reported in some studies—one LSD study recorded 33% reporting a "dry" taste, 19% "funny", and 8% "bitter"; another found altered taste in 6/23 (26%) participants. Olfactory hallucinations appear rare in the documented studies (for example, 6/59 patients with schizophrenia experienced olfactory hallucinations on mescaline in one report). Incidence of synesthesia varied greatly across reports: one analysis suggested roughly half of participants experienced synesthesia on LSD, whereas another study recorded 2/19, 2/20 and 3/20 reports of synesthesia for psilocybin, mescaline and LSD respectively. Experimental attempts to elicit consistent audio–visual pairings yielded visual brightening and dynamic patterns more often than stable synesthetic correspondences.
Aday and colleagues interpret the historical literature as demonstrating a concentrated research effort into the perceptual effects of psychedelics during the first era, with vision receiving by far the most attention. They conclude that both peripheral (retinal/ocular) and central (subcortical and cortical) mechanisms were investigated, but cortical effects—consistent with modern 5-HT2A-centred accounts—appear more prominent and more consistently reported. At the same time, the authors note unresolved discrepancies across studies, for example mixed findings on alpha oscillations and VEP directionality, which may reflect methodological heterogeneity, dose-dependent effects, and individual differences in imagery ability or attentional state. The authors emphasise several limitations of the primary literature that constrain confident interpretation: small sample sizes, variable or absent blinding and control procedures, inconsistent dosage reporting, diverse and sometimes rudimentary outcome measures, and the inclusion of self-experiments with attendant bias concerns. They stress that these historical practices can produce conflicting results and that contemporary researchers should examine primary sources closely before incorporating older findings. In terms of implications, the review is presented primarily as a resource to save modern investigators time and to prevent redundant research questions; the authors encourage revisiting specific unresolved topics (for example retinal/peripheral effects, the neural basis of closed-eye imagery, body-schema alterations, synesthesia and the role of music) using contemporary, preregistered, double-blind, placebo-controlled designs and modern neuroimaging and psychophysical methods. They also note potential clinical relevance for conditions such as hallucinogen persisting perception disorder (HPPD), while acknowledging that the early literature is insufficiently rigorous to support direct clinical claims.
The authors conclude that early psychedelic research produced a rich but methodologically varied body of evidence showing that classic psychedelics affect perception across multiple domains, especially vision. They highlight under-studied areas (body schema, synesthesia, gustation and olfaction) and call for modern, rigorous follow-up studies to reconcile discrepant findings and to elucidate mechanisms, while positioning their review as a curated guide to the historical literature for contemporary investigators.
Lysergic acid diethylamide (LSD), psilocybin, mescaline, and ayahuasca/N,N-dimethyltryptamine (DMT) are considered classic psychedelic drugs). These substances can exert myriad acute effects including, but not limited to, alterations in sense of self, emotional breakthroughs, psychologically challenging experiences, and mystical states of consciousness that are described as being ineffable and meaningful. Most pertinently, psychedelics are notorious for their capacity to induce dramatic, transient shifts in perceptionparticularly vision. Although a wealth of studies was conducted during the early-to mid-20th century on the perceptual effects of psychedelics, rescheduling of the drugs to Schedule I category in the US in the late 1960s halted almost all ongoing research. Much of the psychedelics/perception literature from this early period of research is effectively unavailable to the current generation of researchers as there is presently no source where it has been synthesized. To this end, this review aimed to broadly coalesce the early literature on the perceptual effects of psychedelic drugs. Given the unique and robust perceptual changes induced by psychedelics, they may be a useful tool for transiently altering perceptual systems and studying their mechanisms. Franz X. Vollenweider's laboratory in Switzerland has been at the forefront of the reinvestigation of psychedelics' effects on perception in recent years. From this work, it has been determined that activation of serotonin 5-HT2A receptors underlies the dreamlike effects of psilocybin on mental imagery, as well as N170 visually evoked potentials and visual hallucinations. This laboratory also delineated psilocybin's effects on binocular rivalry, modal object completion, and time perception. A variety of researchers have also begun revisiting psychedelics' effects on synesthesia. Given the increasing interest into the effects psychedelics can have on perceptual processing, now is an important time to synthesize the early research in this area. Doing so can save contemporary researchers considerable time reviewing studies from over half a century ago and ensure that they are not asking questions that have already been examined. To remedy this gap, we conducted a widespanning narrative review on studies from the first era of psychedelic research ) that looked at the relationship between psychedelics and perception. Although the U.S. Controlled Substances Act was passed in 1970, manuscripts continued to be published over the next few years and we therefore set 1975 as our cutoff for this era of research. Additionally, 1895 was chosen becauseis considered the first scientific experiment published with psychedelics. We partitioned psychedelics and perception research into the following categories: vision, auditory processing, body schema and tactile processing, time perception, and other (e.g., taste, olfaction, synesthesia). Given that most of the perceptual research from this era was conducted on vision, this section is the most in-depth and was further subdivided into physiological changes, alterations in low-level visual processing, and complex imagery. Readers should keep in mind that methodological standards in experimental psychology and psychopharmacology tended to vary dramatically during this period of research; we address these considerations in further detail in the Limitations section. Studies used human samples except where otherwise noted.
The alterations in visual processing induced by psychedelics are the most marked and characteristic perceptual effects linked to the drugs. Changes in vision are ubiquitously found in descriptions of the phenomenology of the psychedelic experience. These changes can occur both with the eyes open and closed. In the former, individuals on psychedelic substances often experience "pseudohallucinations" (i.e., alterations in form and color of the external environment), but they do not typically see things that are not actually present, and they recognize the visual changes as being self-produced. When participants' eyes are closed, it is not uncommon for them to report visions of unique patterns of geometric figures or vivid mental imagery; these visions can be meaningful to some individuals and cultures. In this section, we will break down the early research on the effects of psychedelics on visual processing by examining the changes documented at each level of the visual system.
Our review uncovered what seems to be a forgotten debate from the first era of psychedelic research-to what degree do psychedelic-induced alterations in vision stem from changes in the peripheral eye versus changes in the brain? Given that psychedelic drugs consistently dilate the pupils, many researchers naturally hypothesized that these peripheral changes could have downstream effects on vision. However, pupil dilation is not specific to psychedelics and has been documented with other drugs that are non-hallucinogenic (e.g., stimulants; Koppanyi 1945); therefore, others argued that changes in vision are due to variations in cortical processing. Here, we will present research from both camps and reconcile their findings. Researchers had several justifications for linking psychedelics' effects on vision to the peripheral eye. First, compared to its concentration in the cortex, cerebellum, and midbrain, levels of LSD were found to be 18 times higher in the iris of monkeys. Further,argued that dilation of the pupils may contribute to blurring, difficulty in focusing, and the rainbow effect (i.e., rainbow-like colors at the edges of objects). Many studies of this era used an electroretinogram (ERG) to record electrical potentials directly from the eye while psychedelics were administered.observed that LSD produced a prolonged rod-cone break (i.e., when rods are equally as sensitive as cones after being in darkness for a period of time) in the dark adaptation curve of the ERG. Another study found that 50 µg/kg of LSD in cats did not significantly change ERG or visual cortical-evoked potentials in response to a light stimulus). However, the spontaneous discharge rate of two-thirds of retinal ganglion cells was increased, reaching a peak at 20-30 min and still present an hour after injection.found that spontaneous action potentials appeared on the sclera, optic nerves, and visual cortices of cats 10 min after administration of LSD or mescaline, but not in control conditions with Nembutal or cannabis; the authors hypothesized that spontaneous firing originating in the retina may contribute to changes in vision. They also found that when the optic nerve was severed, LSD-induced cortical-evoked potentials were attenuated. Collectively, these studies indicate that psychedelics can have effects on the peripheral eye; however, researchers seldom fully delineated how these alterations lead to phenomenological changes in vision. Despite this research, the myriad studies documenting functional changes in brain activity seemed to dwarf the breadth of the retinal research. Researchers who downplayed the drugs' peripheral effects noted that even when pupil dilation was blocked with dibenzyline, participants experienced most of the visual distortions and illusions characteristic of LSD. Further, patients with lesions to the occipital lobe rarely experienced visual phenomena while on LSD, and those who did only had low-intensity visual effects.found that non-congenitally blind patients reported more visual hallucinations after 75 µg of LSD compared to placebo. Similarly,found that 1 µg/kg of LSD led to visual alterations in 13/24 non-congenitally blind subjects, suggesting that a normally functioning retina is not necessary for psychedelics to have visual effects. A closer inspection of the data, however, revealed that this effect seemed to be specific to individuals who had previously reported non-drug "spontaneous visual experiences", as 13/16 of that subset of participants experienced changes in vision, while 0/8 participants who did not endorse having spontaneous visual experiences had visual effects. The authors contended that LSD increases the frequency and intensity of spontaneous visual experiences in blind individuals, and that these changes were related to alterations in centralrather than peripheral-activity. Researchers documented neurological changes at both the subcortical and cortical levels of the visual system. However, since electroencephalography (EEG) was still the predominant neuroimaging modality during this period, it was difficult to localize activity to distinct parts of the cortex in human participants. To overcome EEG's limited spatial resolution, many researchers implanted electrodes directly into various regions of the cortex in animal models. Subcortically, the lateral geniculate nucleus (LGN), the first relay center in the thalamus that broadly organizes retinal signals, was widely studied and often noted to be inhibited after administration of psychedelics in animal models. However, in some studies, psychedelics increased activity in the LGN.postulated that LSD affects the LGN dynamically, where changes in sensory contrast could be explained through alterations in the balance of center/surround antagonism of LGN cells, such that center stimulation was increased. The researchers also noted that dosage may be a factor contributing to discrepant results as the direction of influence on LGN activity differed between low and high doses of LSD when using a cat model: they found more mixed effects with doses less than 100 µg compared to doses greater than 200 µg, which tended to suppress LGN activity.noted that changes in LGN activity were always less pronounced than alterations in cortical activity; given this as well as the paucity of studies showing change in other subcortical regions, research from this era suggests that psychedelics' influence on cortical processing may be more relevant to their visual effects. This is consistent with contemporary research noting that psychedelics exert their acute effects primarily through 5-HT2A receptors, which are most densely located in the cerebral cortex. At the cortical level, the most common region of interest was the visual cortex. In one study, researchers found that LSD increased the amplitude and decreased latency of visually evoked potentials (VEPs) in the visual cortex of rabbits. Similarly,found that VEP amplitudes were increased after administration of LSD in humans; they suggested this could relate to a decrease in the effectiveness of visual inhibitory mechanisms. In contrast,found that LSD and DMT attenuated photic-evoked electrical activity in the visual cortex of rabbits. When examining psychedelics' effects on vision-related neural oscillations, alpha rhythms were the most commonly investigated frequency, and fluctuations were thought to reflect changes in mental imagery and/or visual attention. Although some studies found that psychedelics increased alpha rhythms), others noted suppression, but the participants' activities varied across studies. This is an important distinction to keep in mind as alpha rhythms increase while relaxed and decrease with focused attention; further complicating the matter, alpha rhythms vary as a function of whether an individuals' eyes are open or closed. Individual differences may also play a role in psychedelic-mediated changes in alpha rhythms: two studies noted increased alpha rhythms in those with high mental imagery ability but not in the "non-visualizers" groups). Altogether, it seems likely that while changes in activity in the retina and peripheral eye may contribute to altered vision while on psychedelics, the changes in brain activity seem to be more robust, dynamic, and pertinent. The drugs' retinal/peripheral effects are not a focus of contemporary psychedelic researchers, but this may be an oversight given the inherent downstream neural effects resulting from functional changes in the peripheral eye. Despite the lack of contemporary retinal studies, the neurological changes reported here can inform current work. In particular, some of the changes in alpha activity from the first era of research 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 (e.g., differences in attentional demands) may be involved.found that psilocybin increased P1 amplitude, an automatic sensory-related event-related potential (ERP) component, which seems to be broadly in-line with the early findings we uncovered; however, delineation of specific ERP components was still in its infancy during this period, as the first component was not identified until 1964.
Many of the visual alterations induced by psychedelics seem to stem from changes in form, depth, and motion (i.e., elementary visual imagery; EVI)-these occur both with the eyes open and closed. In the former, for example, it is not uncommon for individuals to report that stationary stimuli appear as though they are bulging, waving, or breathing when under the influence of the drugs. Representative examples of EVI include changes in the apparent horizon (i.e., point in space perceived to be at eye-level) and apparent vertical (i.e., the point at which a stimulus is perceived as vertical). LSD was found to raise the apparent horizon in a task which required the participants, with their head position fixed in an apparatus, to indicate when a black line appeared at eye-level. Distortion of apparent verticality was noted with LSD in a similar task involving participants adjusting a luminescent rod until it appeared vertical. In a study measuring perceptions of curvature and tilt by having participants direct six movable rods to a single frontal plane in line with a reference rod, psilocybin produced contraction of nearby visual space. These studies support that changes in form and depth contribute to visual transformations.suggested that psychedelics bias visual processing towards color at the expense of conventional form processing. In line with this hypothesis, another study found that LSD impaired form discrimination more than color discrimination. Low-level changes in visual thresholds may also contribute to EVI when the eyes are open.found that LSD elevated visual thresholds in pigeons in a task which required the birds to peck one of two keys in response to whether a stimulus patch appeared dark. This finding was concurrently verified in human samples using a similar task involving the pressing of a response switch to indicate whether or not a stimulus was perceived. These results seem to contradict recent models of psychedelic effects which argue that low-level processes are largely unaffected by the drugs (Carhart-Harris and Friston 2019); therefore, current researchers should follow up on these findings with studies using contemporary methodological protocols (i.e., preregistered, double-blind, placebo-controlled trials) to coalesce the literature. When the eyes are closed, on the other hand, many individuals on psychedelics see forms of unique geometric figures and kaleidoscope-like designs in their visual field. These forms seem to be distinctly vivid, rarely static, and commonly repeated or combined into patterns.grouped these patterns into (a) grating, lattice, fretwork, filigree, honeycomb, or chessboard; (b) cobweb; (c) tunnel, funnel, alley, cone, or vessel; (d) and spiral designs (seefor detailed illustrations). Some representative descriptions of closedeye EVI include "…colour visions consisting of beautiful purple and green flashes and zigzags") and, "The figures constantly changed in form and color, but always remained a series of fantastic curves, revolving rapidly back and forth upon their own axis. The forms changed through rich arabesques, Syrian-carpet patterns, and plain geometric figures, and with each new form came a new flush of color, every shade appearing, from pure white to deepest purple. When the eyes opened and the light was turned up, the visions faded like stars going out in daylight".and Kluver (1942) speculated these forms stem from an emergent capacity for individuals to see intraocular structures of the eye on psychedelics, such that retinal vessels can be observed overlaying the visual field, but this was only discussed conceptually and was never empirically supported; the hierarchical structure of information processing in the early visual cortex may be a more deductive underlying mechanism. Closed-eye EVI of basic geometric patterns differs from the more elaborate visions (i.e., which are more prolonged, episodic, and often involve scenery) that will be described in the complex visual imagery section. Alterations in color perception were another common visual effect of psychedelics, with many reporting that colors appeared more vivid or saturated). Alan M. Hartman and Leo E. Hollister conducted what seemed to be the most focused studies on psychedelics' effects on color perception from this era, as they systematically compared the effects of LSD, psilocybin, and mescaline on a variety of color perception measures. They first found that 9/19 (47%), 17/20 (85%), and 9/20 (45%) participants administered psilocybin, mescaline, or LSD, respectively, subjectively reported altered shapes or colors on a questionnaire. Relative to baseline, all three drugs equally increased perception of color on a flicker task which entailed observing an episcotister rotating at various speeds, and all three equally impaired performance on a color discrimination task relative to baseline, but only psilocybin significantly. A follow-up study found that all three drugs increased color evoked in the flicker task and, once again, they generally decreased color discrimination equally, but only psilocybin significantly). Here, however, they found evidence that the drugs may affect different hues. Specifically, LSD impaired performance with red-yellow and yellow-orange hues, whereas mescaline affected a wider range of hues, approaching significance with red-yellow and blue-green discriminations. Psilocybin increased errors in all areas, approaching significance with violet hues. Despite these tentative variations, the authors concluded that the similarities of LSD, psilocybin, and mescaline's effects on color perception are more prominent than any differences. Given that each of their groups included a relatively small number of participants, future better-powered studies are needed to delineate the nuances of psychedelic drugs' effects on color perception.
In addition to low-level visual alterations, individuals on psychedelic substances often report having dynamic internal visions that are vivid and sometimes described as dreamlike (i.e., complex visual imagery; CVI). Although these effects are not easily quantified and reduced to averages, anecdotes appeared throughout the literature in our review. CVI seems to be a salient aspect of the psychedelic experience, with crosscultural evidence dating to the beginnings of psychedelic science. The CVI induced by psychedelics were often most fully elaborated on in published self-experiments (e.g.,. In his influential Life magazine article,described his psilocybin-induced visions as "…vivid color, always harmonious. They began with art motifs, angular such as might decorate carpets or textiles or wallpaper or the drawing board of an architect. Then they evolved into palaces with courts, arcades, gardens-resplendent palaces all laid over with semiprecious stones. Then I saw a mythological beast drawing a regal chariot. Later it was though the walls of our house had dissolved, and my spirit had flown forth, and I was suspended in mid-air viewing landscapes of mountains, with camel caravans advancing slowly across the slopes, the mountains rising tier above tier to the very heavens." He went on to assert, "The visions were not blurred or uncertain. They were sharply focused, the lines and colors being so sharp that they seemed more real to me than anything I had ever seen with my own eyes." The idiosyncratic nature of CVI precludes a completely representative description, but in general, Wasson's account underscores that the visions can be vivid, meaningful, and vastly different than the mental imagery typically experienced in waking states of consciousness. Psychedelics effects on enhancing creative thinkingas well as increasing thinking in metaphors and symbols) may be cognitive factors related to CVI.
Beyond the visual distortions, the auditory changes induced by psychedelics were among the most studied perceptual effects during this period of research. The alterations in auditory processing can broadly be broken into three areas of study: behavioral research, electrophysiological studies, and hallucinations. Although research into the drugs' effects on audition is not a predominant focus of the current psychedelic renaissance, the role that music plays in psychedelic-assisted psychotherapy sessions is an area of growing study. Thus, we conclude this section by touching on this area of research and connecting it with findings from the first era of psychedelic research. The behavioral studies of this era involving auditory processing largely examined psychedelic drugs' effects on auditory discrimination/generalization and basic reaction time to auditory stimuli. Several studies indicated that psychedelics could reduce auditory sensitivity, leading to greater stimulus generalization). On the contrary, one research group consistently found no effect of psychedelic administration on stimulus generalization with rat models, but made a point of noting that their null results did not mean that their subjects were not experiencing perceptual distortions; their measures or subjects may have just not been sensitive to these effects. They also generally found that the drugs led to less responding overalland slower reaction times. Slowed reaction time to auditory stimuli was a common finding when participants/subjects were under the influence of psychedelics.found that LSD slowed reaction times for auditory stimuli, but not to the same extent as for visual stimuli. During this era, several studies were also undertaken to assess the neural correlates of auditory changes in psychedelic states. Some of the cleanest experiments conducted in this area were that of, who recorded auditory evoked potentials (AEPs) directly from electrodes implanted in the cat auditory cortex while dosages of 5, 10, and 15 mg/kg of mescaline were administered and tones were played. They found that, in a generally dose-dependent manner, mescaline increased the peak area, latency, and amplitude of the P1 and N1 AEP components. The authors hypothesized that these changes were related to "functional disorganization or disruption" of the reticular formation, which was thought to generally inhibit AEPs.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; it should be noted, however, that the directionality of this suggestion cannot be verified, and it is also possible that increased meaning is a result (rather than cause) of increased cortical activity. Another study compared the effects of LSD and DMT on AEPs and found that the drugs induced distinct changes: while LSD seemed to have no effect, AEPs "disappeared" during the peak of DMT sessions and gradually reappeared as the drug wore off. This effect seems to be consistent with the intense dissociation from one's surroundings commonly reported by individuals on DMT, and which can also occur with other intravenous hallucinogens. Another focus of the psychedelic perceptual research of this era centered on auditory hallucinations. Although some considered these hallucinations to be rare aspects of the psychedelic experience, others argued they were common. The handful of studies that quantitatively assessed the frequency of auditory hallucinations generally found them to be uncommon (e.g., seen in 35% of participants with mescaline;. More often than not, it seems that these hallucinations would be considered pseudohallucinations as they were distorted perceptions of veridical stimuli rather than being solely internal manifestations of sound. For example,reported that one participant mistook the sound of a typewriter for symphony. Conversely, a participant fromreported, "I thought I heard the sound of violins, as if the air itself was singing." These results are in line with contemporary research documenting that psychedelics can induce auditory alterations), but they are less common than visual changes. Finally, a commonality between the first and current eras of psychedelic research regards the central role of music in psychedelic therapy sessions.postulated that music could help patients let go more easily, explore their inner mental experience more fully, and facilitate intense emotions. Indeed, this notion has been validated by recent work demonstrating that LSD generally enhances emotional responses, and music's role in enhancing meaning-making, emotionality, and mental imagery can be important mechanisms underlying therapeutic effects. In an early article published in Journal of Music Therapy,came to several conclusions regarding music in psychedelic therapy, including: music with which the patient is familiar with is most effective, music was requested more frequently as sessions progressed, religious and romantic music was considered "most important", and at-times music could "guide" the patients' experiences. Contemporary investigators should delineate this research given the central role music can play in psychedelic experiences. The findings regarding familiarity, in particular, will be important to address as psychedelic therapy expands to new populations and cultures.
"Gradually, the feeling of the body vanished, the position of the limbs could not be localized; the posture of the body could hardly be determined; it could scarcely be separated from its surroundings". Many studies from this era reported distortions in body schema and tactile perception while under the influence of psychedelic drugs. Some of these anecdotal reports, such as the one quoted above, seem to be similar in phenomenology to changes in body schema reported during meditative states, where the distinction between one's own bodily boundaries and the surrounding environment becomes less pronounced. Beyond the changes in self/ environment boundaries, anecdotes about changes in bodily perception were quite varied, ranging from feelings of changes in limb length/proportions, paresthesia, heaviness of limbs), and orientation of the body (Guttman 1936). Tactile changes included tingling sensations, numbness, and alterations in perception of temperature. Although these changes were often anecdotally noted and mentioned briefly in manuscripts, empirical research into alterations in body schema was still in its infancy during this period. One of the few studies that quantitatively assessed these changes found that 10/14 participants administered psilocybin reported changes in body size on a questionnaire. A study looking at dose effects found that changes in body image emerged at 20 µg of LSD, but not smaller doses. The most rigorous and focused investigation into psychedelics' effects on body perception came from. In this study, the authors tested if LSD differentially affected the perception of the size of one's own body compared to the size of external objects. They hypothesized that LSD would lessen the boundaries between the environment and the body, and that the increased incorporation of the environment into the body schema would lead to an enlargement of the perceived size of the body. To test these changes in bodily perception, the researchers used a task that is generally comparable to body size methodology used today. Participants were instructed to manipulate the distance between two panels until they believed the distance was approximately the same size as various stimuli (e.g., their own head, a dollar bill, pack of cigarettes, etc.). LSD led to increases in perceived head size, whereas arm length was perceived as significantly shorter, and nonbody objects did not change relative to baseline. When asked to draw person figures, the size of the drawings increased on LSD.similarly found that drawings of bodies increased in size on LSD; they interpreted this as a reflection of corollary changes in body image, but changes in motor function could also account for the results. Although empirical research into these bodily effects was sparse, anecdotes were common and substantiated by the limited research that was published in this area. Investigation into these effects is again not a large focus of the current era of research, but given the wealth of anecdotal reports about bodily changes, psychedelics could potentially be a unique tool for studying the phenomenology and neural correlates of changes in bodily perception.
Psychedelics were also frequently reported to induce distortions in time perception, although the direction of this change (i.e., underestimation or overestimation) seemed to be inconsistent. The weight of the evidence leans slightly towards participants overestimating temporal durations while on psychedelics, indicative of a sense of time slowing down. However, reports of time feeling faster or mixed effects appeared in the literature as well. It was not uncommon for participants to also experience feelings of timelessness or as though time stood still. Finally, there were also null findings published on the effects of psychedelics on time perception. There are several potential explanations for these conflicting findings. First, we will elaborate on reconciliations proposed by the researchers from the first era of research.noted that distortions in time perception often coincided with changes in mood, although this was not statistically examined. Rebutting this theory,administered a variety of mood surveys and found that none correlated with changes in time perception. Others argued that individual differences accounted for the varied changes in time perception. For example,had participants complete a handwriting task several times pre-drug to determine "perceptual stability," as measured by the standard deviation of the handwriting area. They found that the larger the handwriting standard deviation, the more pronounced the perception of time contraction under psilocybin. The authors proposed that those with small standard deviations were considered to be generally "perceptually stable individuals" and, thus, experienced less robust time distortions on psilocybin. However, given that handwriting ability is a complex motor-based task, the use of this paradigm was a poor proxy for pure perceptual stability. A parsimonious explanation for the discrepant time perception findings, viewed through a modern researcher's lens, is that these changes may be task-dependent and/or some of these tasks may have been poor measures of time perception. For example, changes in time perception have been shown to vary as a function of whether the task requires participants to rate a temporal duration or reproduce it. Early researchers did not have this nuanced understanding regarding different facets of time perception; it was often unclear with their methodologies if time perception was affected during the encoding or retrieval of temporal information. Furthermore, some studies used tapping tasks to estimate time perception (e.g.,, but these indirect measures of time perception could have been contaminated by concomitant drug-induced changes in motor ability, body perception, and/or visuospatial perception. Thus, it is also often unclear whether time perception, or a different psychological construct, was being affected by psychedelics.
A variety of other perceptual changes were documented by study participants during this era but were not as fully elucidated. For instance, in a study using LSD, some participants reported a "dry" (33%), "funny" (19%), or "bitter" (8%) taste in their mouths during the session. Similarly,found that taste perception was altered in 6/23 (i.e., 26%) participants while on LSD. Several noted a metallic, generally unpleasant flavor, and one reported that the flavors of the meal he was eating would not blend together, each having a coarse essence to it. Although only one participant reported gustatory disturbances in another studynoted that gustatory hallucinations occurred frequently, and participants also reported having a "funny" taste in their mouths. However, this was the extent of the research we documented examining psychedelics' effects on taste. The only work examining olfactory hallucinations included a passing account of a participant believing tobacco smoke was really opiumand another study that found 6/59 patients with schizophrenia experienced olfactory hallucinations while under the influence of mescaline). Thus, changes in gustation and olfaction as a result of psychedelics seem to be either uncommon, or possibly understudied. In a number of studies, participants reported episodes of synesthesia, or a blending of the senses. These reports date back to Native American use of psychedelic compounds, as demonstrated by, who noted that the Kiowas used drumming to increase the beauty and variety of visual hallucinations during mescaline rituals. Incidence of synesthesia was quite variable in the first era of psychedelic research, with one study reporting that "some" participants experienced it, and another noting that synesthesia was "frequent" (Kluver 1942). A quantitative analysis found that roughly half of participants experienced synesthesia on LSD, but only 2/19, 2/20, and 3/20 participants reported synesthesia on psilocybin, mescaline, and LSD respectively in another study. The most direct experimental investigation into psychedelics' effects on synesthesia came from. Participants, with their eyes shut, were presented with 16 tones of varying frequencies and amplitudes while under the influence of psilocybin, mescaline, or LSD. If they reported visual imagery or color, they were asked to match the perceived color to a comparator scale. Although colors were reported by less than half of the participants, other visual changes seemed to emerge more consistently. These effects included brightening of the visual field and perception of dynamic patterns. Yet, beyond these studies, synesthesia was rarely the main focus of the experiments from this era, and quantitative analyses were often rudimentary. One quality of true synesthesia is stable perceptual pairings within a given individual (e.g., the number "2" is always blue); the early research described here does not provide enough information to ascertain whether this stability was present in participants experiencing synesthesia as a result of psychedelic use. Indeed, a recent study found that synesthesia-like experiences were not any more consistent with LSD compared to placebo. Although some contemporary researchers appear to be intrigued by the synesthetic effects of psychedelics, this once again has not been a predominant focus of the field. Several studies have administered the Altered States of Consciousness questionnaire (ASC), which includes a sub-dimension assessing audio/visual synesthesia, but reporting and discussion of these perceptual results has often been secondary to the clinical effects that are generally the main focus of the studies.
There are a number of limitations to keep in mind when drawing conclusions from these results. The first and foremost concerns the at-times unrefined methodological standards in experimental psychology and psychopharmacology during this era of research. Sample sizes tended to be quite small in a majority of these studies. Blinding procedures, control groups, and detailed reporting of methodology (e.g., screening procedures, route of drug administration, etc.) was inconsistent between many studies. Further, perhaps consequently, a number of experiments produced conflicting findings about psychedelics' effects on perceptual processing. Although these conflicting results can be a limitation, they are also an opportunity for future psychedelic/perception researchers -this review can help direct investigators towards this relevant work so that they may incorporate it into their research and reconcile disparate outcomes. Doing so can strengthen our understanding of the perceptual effects of psychedelics, which may be relevant for clinical conditions such as hallucinogen persisting perception disorder (HPPD;. Self-experiments were included because they were representative of this early period of research and they can be used to provide qualitative information which statistical analyses cannot; however, conclusions from this research can be limited by concerns about ethics, experimenter biases, and generalizability. Other limitations include unstandardized dosages and a lack of direct comparisons between different psychedelic drugs-problems that persist in psychedelic science today. Additionally, although researchers today have developed perceptual rating scales with components that parse the constitutive components of the experience (e.g.,, this was typically not the case with earlier research and scales often varied across research groups. Finally, given the breadth of psychedelic research conducted during the first era of study and the substantial time that has since passed, we cannot claim that our review is comprehensive or exhaustive. There are undoubtedly studies from this era that our search missed or that have been lost to time. We attempted to compensate for this inherent limitation by conducting a rigorous search with a broad scope; the use of three search engines, and a lengthy Google Scholar search in particular, increased our confidence in the thoroughness of the review. In any case, the primary goals of the review were to curate this fruitful period of perception research and broadly synthesize the research questions and findings. Because it was not possible to discuss every single study on psychedelics and perception from the early-to-mid 20th century that we uncovered in this single paper, we have added a resource (Supplementary Table) directing readers to further research in this area. Finally, current investigators should follow-up on the primary sources to consider the methodological details of any individual study before incorporating it into their research.
Altogether, this review demonstrated that interest into the perceptual effects of psychedelic drugs surged during the first era of psychedelic research. In particular, the effects of psychedelics were investigated across all levels of visual processing (e.g., retinal, cortical, subcortical, low-level visual processing, complex visual imagery), and the breadth of the research on their visual effects dwarfed any other perceptual modality. These findings are in line with the prominence that vision takes in phenomenological descriptions of psychedelic experiences. Psychedelics were also shown to affect auditory discrimination/generalization, neural correlates of auditory processing, and led to auditory hallucinations in subsets of participants. There were, however, distinctly under-studied aspects of perception during the first era of research and areas with clear needs for further delineation. One example is the drugs' effects on representations of body schema-a number of participant accounts included reports of feeling alterations in bodily boundaries. In some cases, participants felt as though their bodily proportions were changing, and at other times, participants would report losing the distinction between themselves and the surrounding world. Researchers also noted altered time perception while on psychedelics, although the direction of this change varied. Psychedelics induced synesthetic experiences in some individuals, but there was limited systematic inquiry into this effect. The research on the drugs' olfactory and gustatory effects was scarce. Issues with methodological rigor, unstandardized dosages, and lack of delineation of drug-specific effects are important limitations to keep in mind. Nonetheless, this review can be a valuable resource for contemporary researchers who have interest in the perceptual effects of psychedelics by directing them to this relevant work, providing a synthesis of findings, and noting areas of discrepancy which future experiments can work to resolve.
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