A note on scope. Metanoia is a California 501(c)(3) public charity advancing the science of natural psychedelics for personal growth and human flourishing in healthy adults. Not treating illness. Expanding potential. Much of the strongest controlled work on perception used LSD, which is semi-synthetic, and some used intravenous DMT. This page names the compound in every study and does not carry a finding from one compound to another without separate evidence. Where a result comes from a method with no drug, such as meditation training or sensory restriction, the text says so.
What it is
Perception is the work the nervous system does to turn signals from the eyes, ears, skin and body into an experienced world, and Metanoia treats it as spanning vision, hearing, the sense of the body, and the felt passage of time. The word covers two layers that are easy to blur. One is performance: how faint a light can be and still be seen, how long an interval of a few seconds feels, how finely two tones can be told apart. The other is experience: whether a surface seems to shimmer, whether a sound seems to carry a color, whether a familiar room looks newly vivid.
Two kinds of measurement follow. Psychophysics studies how physical stimuli map onto a person's responses, and its tasks produce thresholds, switching rates and reproduced durations. Rating scales capture the experience itself, and the psychedelic literature leans heavily on them. The best-known family is the altered-states questionnaire, used in its 5D-ASC form in several studies below. In 2010 its lower-order scales were rebuilt from 591 drug sessions pooled from 43 experimental studies in healthy volunteers (327 of them psilocybin), using the 66 items that the older altered-states scale and the 5D-ASC share. The rebuilt scales behaved consistently across drugs and settings. The original three-dimension structure did not fit the data, though, the visual dimension was the most heterogeneous of the three, and the authors warned against using that visual scale as it stood [1]. The Hallucinogen Rating Scale was drafted from interviews with 19 experienced DMT users and tested on 11 experienced hallucinogen users given intravenous DMT (12 were enrolled). Its authors called their results preliminary [2]. Both rate the acute experience, and neither was built to track what remains afterwards.
Perception is among the dimensions in which the acute effects of classic psychedelics are best documented, and the research is some of the oldest. The acute alteration is not in doubt. What is open is whether anything useful or lasting follows for a healthy adult. An intense perceptual experience is a finding about the experience, and says nothing yet about perceptual skill, attention or well-being a month later.
The geometry of what people see, including Klüver's form constants, has its own page on this site, What People See. This page covers only the parts needed here.
What is known
The short version. Acute perceptual effects are the best established effects of the classic psychedelics. They are measured with questionnaires and laboratory tasks, depend in large part on the serotonin 2A receptor, and reach vision, hearing, time and the blending of the senses. Performance on perceptual and timing tasks mostly gets worse while the effect lasts. Evidence that anything perceptual remains afterwards in a useful direction is close to absent. The one documented lasting perceptual harm is a rare disorder with a small and dated evidence base.
Vision: a receptor, a rhythm and a brain signal
The most complete causal chain comes from a double-blind study of psilocybin in healthy volunteers. Seventeen people were enrolled and 15 were analysed. Psilocybin sharply reduced alpha, a resting rhythm thought to keep visual cortex quiet when nothing needs processing. It also reduced N170, an electrical response roughly 170 milliseconds after a picture appears. The drop in N170 tracked ratings of complex imagery (r = 0.61) and of audio-visual synesthesia (r = 0.54), but for simple geometric imagery the correlation (r = 0.49) did not reach significance (p = 0.067). When volunteers were pretreated with ketanserin, which blocks the serotonin 2A receptor, the alpha, N170 and visual rating changes were normalized. Ketanserin alone also lowered an early response called P1, so blocking the receptor is not a neutral manipulation of vision [3].
The fall in alpha did not significantly track hallucination intensity, which led the authors to say it may be insufficient on its own to produce a hallucination. Ketanserin is a preferential blocker of that receptor and is not perfectly selective. Participants could still do the task, though errors on illusory-triangle figures rose under the drug.
A review by the same group concludes that no single mechanism is likely to explain the diversity of visual effects, from intensified colors to complex scenes or figures [5]. In a single-blind imaging study of 15 volunteers given intravenous LSD, which is semi-synthetic, more blood flow to visual cortex, less alpha, and a greatly expanded pattern of connections from primary visual cortex tracked ratings of visual hallucinations, and the visual effects were not significantly linked to ego dissolution. A drop in alpha also correlated with ego dissolution, so it is not specific to the visual effects, and the blood-flow result ran opposite to an earlier psilocybin study from the same group, which was done with eyes open [6]. In a single-blind study with a fixed order, placebo first and DMT a week later, intravenous DMT, a naturally occurring tryptamine given here as a laboratory compound, lowered alpha and beta power and raised the diversity of the EEG signal. Thirteen volunteers took part, all experienced, 12 contributed EEG data, and they did not all receive the same amount. A slow delta and theta rhythm at the peak of the experience correlated with the rated visual component, and the authors call their reading of where that rhythm comes from speculative [7]. A mega-analysis of resting-state scans in healthy adults pooled 11 datasets and five drugs: psilocybin, LSD, mescaline, DMT and ayahuasca, which is a brew and not a single compound. The most consistent change was higher functional coupling between higher-order networks and the visual and somatomotor networks. The paper is inconsistent about how many participants and datasets it counted, and no scan included a perception task. Functional coupling does not show which way information flows [8].
A rivalry result that does not fit the receptor story
In binocular rivalry, a classic psychophysical test, two different pictures go to the two eyes and awareness alternates between them. In 12 healthy volunteers, psilocybin at two strengths slowed the rate and regularity of the alternations at the peak of the effect. Over the following hours the rate recovered, though a few individuals had not returned to baseline by 360 minutes [9]. A second study in ten volunteers confirmed the slowing and the greater share of mixed percepts. Ketanserin blocked most of the drug's hallucinogenic symptoms but not the slowing of rivalry, which tracked arousal and attention instead, and the authors judged it unlikely that the rivalry effect runs through the serotonin 2A receptor [10]. A drug can thus change perception through more than one route.
Hearing and music
Hearing has fewer studies than vision, and most of them record brain activity and leave auditory thresholds unmeasured. In a secondary analysis of a double-blind crossover study, 24 healthy volunteers (25 enrolled, 8 with earlier hallucinogen experience) listened to music in a scanner. LSD altered how closely brain regions followed the changing tonal structure of the music, mostly in higher regions. Basic auditory regions were not biased by the drug. Ketanserin pretreatment indicated that the serotonin 2A receptor was involved, though the authors also found effects they could not attribute to it [11]. In an auditory oddball paradigm, in which a rare deviant tone interrupts a series of standard tones, 20 volunteers on LSD showed less adaptation to repeated tones and a blunted surprise response to deviants [12]. Acoustic startle, the reflex jump to a loud sound, is normally dampened by a faint warning sound just before it. In a double-blind crossover study of 16 volunteers, 15 analysed for this reflex, LSD reduced that dampening when the warning came 30 or 60 milliseconds before the sound, and not significantly at 120 milliseconds. The reduction did not track the volunteers' own ratings, and audiovisual synesthesia was among the predominant effects [13].
Time
Duration is the perceptual quantity that psychedelics distort most reliably in the laboratory. In a double-blind study of 12 healthy volunteers, psilocybin impaired the reproduction of intervals longer than about 2.5 seconds and impaired tapping in time with beats spaced more than 2 seconds apart. Working memory was also worse, and participants reported depersonalization and derealization, including a disturbed sense of time [14]. A 2026 report from a double-blind trial in 24 healthy volunteers used temporal bisection, a task in which people decide whether a probe duration is nearer a short or a long standard. Under psilocybin the bisection point shifted rightward (Hedges g = -0.37 in the abstract), and the just noticeable difference grew (g = -0.47), meaning lower precision, both for durations beyond about 2 seconds. The authors attribute them to disrupted working memory and attention more than to a change in a dedicated clock. We could read only the abstract of this report [15]. A systematic review and meta-analysis of attention and executive tasks under psilocybin covered 13 studies that the authors describe as using healthy volunteers and found slower reaction times during the acute phase. That pooled estimate rests on 15 effect sizes from 6 studies (Hedges g = 1.13, 95% CI 0.57 to 1.7). Accuracy showed no reliable change. The authors call the analysis exploratory, it was not preregistered, they judged the risk of bias moderate to high, and reaction time showed clear publication bias. After a correction for four apparently missing studies the estimate fell to g = 0.96, which was still significant [16].
One randomized, double-blind, placebo-controlled trial gave very low LSD amounts, described by its authors as microdoses, to 48 healthy adults aged 55 to 75. Each person took six repeats of the same treatment, one every three days, and did the timing task on the day of the fourth. Forty-six were analysed. Self-reported changes in perception and thinking were not robust, although a slight tendency for the LSD groups to report more effect suggests some volunteers may have guessed their condition. The LSD groups over-reproduced intervals of about two seconds and longer (p = 0.017). The authors call the analysis underpowered, especially for comparisons between amounts, and note that the direction is opposite to the under-reproduction reported with psilocybin at active amounts, which they treat as unresolved. The trial was funded by a company for which several of the authors were paid consultants. The result concerns timing and carries no implication of benefit. We have not identified a replication [17].
The blending of the senses
Synesthesia is a condition in which a stimulus consistently triggers a second experience in another sense, such as letters that appear colored. People report something similar under psychedelics, most often sounds that produce visual sensations. A 2013 systematic review of chemically induced synesthesia found 35 studies, of which only four were direct experiments and 17 were case reports. Nearly all relied on self-report. The review, citing earlier pooled data, put auditory-visual synesthesia at roughly 37 percent of psilocybin sessions. Those are the same 327 psilocybin sessions analysed in the rating-scale study above [1], so they are not an independent sample. The review's authors conclude that chemically induced experiences do not yet meet the criteria for genuine synesthesia [18].
Whether these experiences are synesthesia or a vivid imitation of it is open. A within-group pilot of intravenous LSD involved ten healthy volunteers, all with earlier psychedelic experience, who always received placebo first and LSD second. They reported more spontaneous synesthesia-like experiences under LSD, but only on some items (visual motion, touch and Klüver-type forms), and their color choices were no more consistent over repeated tests and no more specific to the triggering stimulus. Two widely used criteria for genuine synesthesia were not met in this small pilot. Every participant correctly identified the condition, so blinding failed, and the authors do not consider the negative results conclusive, partly because the consistency criterion may be too strict for a fleeting form of synesthesia. Individual differences tracked absorption without correction for multiple comparisons [19]. The systematic review adds that placebo controls are rare and that psychedelics may raise suggestibility [18].
In one drug-free, uncontrolled study, 14 young adults without synesthesia (12 women, average age 19), selected for strong visual imagery and high motivation, practiced letter-color associations for about 30 minutes a day, five days a week, for nine weeks, under 24 hours in all. Immediately afterwards 9 of the 14 described seeing color on achromatic letters, inside and outside the laboratory, 3 were borderline and 2 reported none. Three months later much of that experience had faded, though a Stroop-type reaction-time effect remained. The authors state that they did not turn these adults into genuine synesthetes [20].
The prediction account
The most discussed theory of how psychedelics change perception is REBUS, short for relaxed beliefs under psychedelics, proposed by Carhart-Harris and Friston in 2019. It starts from predictive processing, the idea that the brain builds perception by sending predictions down a hierarchy and comparing them with signals coming up, with mismatches (prediction errors) travelling upward to revise the predictions. Each level weights its predictions by precision, roughly the confidence attached to them. REBUS proposes that classic psychedelics lower the precision of high-level predictions, so that bottom-up signals meet less top-down suppression and register more freely in experience [21].
The paper is a synthesis and a hypothesis. For perception it gathers observations: LSD blunted surprise and adaptation in the oddball study above [12], psilocybin reduced the N170 response to incomplete figures [4], and rivalry produced more mixed percepts [10]. The authors also suggest that the persisting disorder described below might arise if the hierarchy does not fully recover, and they describe that idea as natural speculation [21].
Several points limit how far the account can be pushed. The rivalry slowing was not blocked by the receptor antagonist, so it cannot simply be assigned to the receptor pathway the model emphasizes [10]. Kometer's own account, in which lowered alpha lets spontaneous cortical excitation overwhelm stimulus-driven excitation, fits relaxed priors and raised excitability equally [3]. We have not identified a study that measured the precision of high-level priors independently of the effects it is meant to explain. One imaging study did test the direction of influence within the visual system. In 20 healthy volunteers given oral psilocybin, modelling of resting scans showed stronger self-inhibition in four visual and association regions and weaker top-down suppression, which the authors read as more top-down influence on the visual regions and not as support for the bottom-up principle of REBUS. They call the link to imagery ratings preliminary, and the same connections also tracked ratings of disembodiment and unity [35]. A published commentary reports no data. It argues that context shapes extreme experiences and that the model's comparisons with other altered states may mislead, aiming mainly at its assumption about a brain network tied to the sense of self [22]. The pooled imaging evidence shows more integration between networks, which is compatible with the model without testing its hierarchy [8]. In our reading, REBUS organizes the findings well but has not yet been shown to predict anything the simpler accounts do not. That judgment is ours and the authors of the paper do not make it.
Do any perceptual changes outlast the effect?
Within hours, some do, as the rivalry recovery above shows [9]. Beyond that, the evidence splits into harm and benefit.
On the harm side, hallucinogen persisting perception disorder (HPPD) is a recognized condition in which visual or other perceptual disturbances continue or recur after the substance has gone. A review of 20 quantitative studies from 1955 to 2001 found that the term "flashbacks" had been defined in so many ways that it was nearly useless, and concluded that HPPD appears to be a genuine but uncommon disorder, reported most often after illicit LSD use and less often after LSD given in research or treatment settings or after other hallucinogens, and that knowledge about it remained very limited [23]. A 2025 systematic review of medication studies found 31 reports covering 87 treated people, all in clinical populations: three observational studies, one known only from a conference poster abstract, and 28 case reports or small series. The percentages in the abstract come from the 36 people described in the case reports and series and not from all 87: 10 recovered fully (28 percent) and 22 partly (61 percent). The authors did not run a statistical analysis because the number of publications was small, no study had a control group, and they call for randomized trials. The natural course of the disorder is unknown [24]. A 2026 cohort study of 25,778 people diagnosed with HPPD in electronic health records found high rates of earlier depression (29.2 percent) and anxiety (26.2 percent). Among people coded as psychedelic users, anxiety (hazard ratio 1.5) and post-viral fatigue (1.9) predicted a later diagnosis. The methods and tables report these as hazard ratios, although the abstract calls them odds ratios. The groups were defined by diagnosis codes, and the psychedelic-user group by codes for hallucinogen use disorders, so it does not stand for healthy users. The authors acknowledge that mild or passing cases that never reach clinical care are not captured, and a cohort built from diagnosed patients cannot show how common HPPD is, or how great the risk is for a healthy adult [25]. These are clinical populations.
On the benefit side, lasting effects after a single laboratory session have been reported for well-being and meaning. The 16 healthy volunteers of the acute LSD study above [13] were followed for 12 months. They rated themselves as having more positive attitudes and greater well-being and attributed the change to the session. That was a retrospective self-rating against zero, with no control group, and on questionnaires with a baseline most personality traits and trait anxiety did not change. No validated perceptual measure was used. The authors did ask directly at one month, and none of the participants reported perceptual changes or flashbacks. There were no spontaneous reports of adverse effects at 12 months. With 16 people this is a weak safety signal [26]. We have not identified a controlled study that tested whether psychedelic exposure leaves healthy adults better at seeing, hearing, timing or sustaining perceptual attention.
What is not known
Durability is the largest gap. No study in the sources reviewed here followed perceptual performance in healthy adults after exposure, so questions about lasting acuity, timing, attention or aesthetic sensitivity have no data behind them, in either direction.
The second gap concerns which compounds were studied. Psilocybin and LSD supply most of the controlled evidence, with some from intravenous DMT. The best auditory and music results come from LSD, which is semi-synthetic. For mescaline, 5-MeO-DMT and ibogaine, the controlled perceptual literature in healthy adults is thin in the sources reviewed here, and Metanoia does not carry findings across compounds without separate evidence.
Ten of the sources cited here could be read only as abstracts, and the sources list marks each one. Claims that rest on them are limited to what those abstracts state.
Samples are small. The studies above typically enrolled between 10 and 50 people, many of them from a few laboratories in Zurich, Basel and London, some sharing authors and designs. Agreement among them is weaker confirmation than agreement among independent groups would be. Blinding is also hard when the effect is unmistakable. In the one pilot that reported it, every participant correctly identified the LSD session [19], and Metanoia did not extract this for the other studies.
The mechanism is unsettled. Receptor dependence is demonstrated for the alpha rhythm, the N170 response and the visual ratings, and is doubtful for rivalry. The theory that would unify them has not made a distinctive, tested prediction.
Measurement is a gap that research can work on directly. We have not identified a validated self-report instrument for durable, non-pathological perceptual change in healthy adults, one that would also capture a persisting visual symptom as an adverse event. That describes what is missing and is not a result. A drug-free direction, flickering light against closed eyelids, is described on the What People See page.
How it compares with other routes
The honest comparison is between a route that produces large acute change with no tested follow-through and routes that produce modest change with evidence of persistence. This page did not review physical exercise or psychotherapy for perceptual outcomes, and makes no claim about them.
Perceptual learning. Visual perceptual learning is a long-term improvement in performance on a visual task after training. One review describes growing evidence that implicit processes such as reinforcement and consolidation contribute [27], and a later theoretical review proposes a dual-plasticity model, one kind of plasticity changing the representation of the learned feature and another changing how the trained task is processed [28]. Both concern primitive visual features such as orientation, motion and contrast and do not address transfer to other kinds of perception, and the authors call the model schematic.
Action video games. A meta-analysis found that habitual players of action video games outperformed non-players on cognitive measures (g = 0.55), with the largest difference in perception (g = 0.78). Those are differences between groups and do not show that games caused the gap. In the intervention studies, which assigned young adults to train on action games or on other commercial games, the effect was smaller (g = 0.34), and for perception it was about 0.23 and not statistically significant. Most of the intervention effect came from one laboratory, and every training study longer than 30 hours came from that laboratory. The authors call the effect fragile. They estimate that published effects run about 30 percent larger than the full literature because of publication bias [29].
Meditation. In a randomized waiting-list trial, 60 experienced meditators attended a three-month retreat of about five hours of practice a day, the first half beginning at once and the second half waiting. The trained group improved on a visual discrimination threshold, with the effect present at the end of training (p = 0.019) and about five months later (p = 0.037). Retention correlated with how much people kept meditating each day. The sample was unusually committed and the retreat setting could have contributed. The comparison group was a waiting list and not an active control, and the correlation with daily practice does not show cause. In the first retreat the training did not improve vigilance. A smaller decline in vigilance appeared only in the second retreat, where the target length was fixed, and the authors say they found no compelling evidence of a direct benefit for sustaining attention [30]. In a small study of 40 students, duration judgments shifted toward "long" after a 10-minute guided meditation but not after an audiobook. The direct comparison between the groups was not significant (p = 0.100), the groups were assigned in order of arrival and not at random, and the effect came from separate tests within each group. The authors link the shift to attention but note the experiment could not settle between explanations [31].
Sensory restriction. A brief period of complete isolation from sound and light produced perceptual disturbances, more in those prone to hallucinations, according to the abstract of one study [32]. Ganzfeld, an unstructured uniform visual field, elicits pseudo-hallucinatory percepts in most observers according to one review [33]. In 47 adults with normal sight, five days of continuous blindfolding together with intensive tactile training was compared with the same training and a blindfold worn only during sessions. Braille discrimination improved in both groups, more in the blindfolded group in a very small sample. What reversed within 24 hours of removing the blindfold was the extra response of visual cortex to touch, while the Braille gain stayed. Most blindfolded participants also reported complex visual hallucinations in the first days [34].
Psychedelics are strongest at producing acute, vivid and measurable perceptual change, and that change typically comes with worse task performance. The only controlled evidence of a durable improvement in a perceptual skill in this review comes from sustained meditation training, compared with a waiting list, and the only evidence of trained, newly acquired perceptual experience comes from a drug-free learning program. For the psychedelic route, durable perceptual benefit is untested. That is a different statement from saying it is absent.
Traditional and historical accounts of visions are covered on What People See and are not repeated here.
Sources
Grade key: 1 = peer-reviewed primary study of any design, systematic review, meta-analysis or official record. 2 = peer-reviewed narrative, integrative or scoping review. 3 = preprint, press, community or grey source. 4 = traditional text. Grade describes the type of source, and the design and its limits are given in the text. "Full text" means the main text was read in full by Metanoia on 2 October 2026, and the entry names what was not read. Online supplements were not read for any source. "Abstract only. Full text not accessible" means the paper could not be opened, and nothing beyond the abstract was used from it.
- Studerus E, Gamma A, Vollenweider FX. Psychometric evaluation of the altered states of consciousness rating scale (OAV). PLoS One 2010, 5(8):e12412. PMID 20824211. DOI 10.1371/journal.pone.0012412 Grade 1. Psychometric analysis of 591 sessions pooled from 43 studies (psilocybin n = 327, ketamine 162, MDMA 102), healthy volunteers, 66 shared items. Full text. Supplementary tables not read. Note: The 591 sessions came from 43 experimental studies in healthy volunteers: 327 psilocybin, 162 ketamine and 102 MDMA.
- Strassman RJ, Qualls CR, Uhlenhuth EH, Kellner R. Dose-response study of N,N-dimethyltryptamine in humans. II. Subjective effects and preliminary results of a new rating scale. Arch Gen Psychiatry 1994, 51(2):98-108. PMID 8297217. DOI 10.1001/archpsyc.1994.03950020022002 Grade 1. 11 experienced hallucinogen users analysed (12 enrolled), intravenous DMT, 126-item scale. Full text, from a repository copy. The item appendix and some tables were not in the copy. Note: The Hallucinogen Rating Scale was tested in a 126-item version. In the early DMT work, auditory effects were reported less often than visual ones.
- Kometer M, Schmidt A, Jäncke L, Vollenweider FX. Activation of serotonin 2A receptors underlies the psilocybin-induced effects on alpha oscillations, N170 visual-evoked potentials, and visual hallucinations. J Neurosci 2013, 33(25):10544-10551. PMID 23785166. DOI 10.1523/JNEUROSCI.3007-12.2013 Grade 1. Double-blind, placebo-controlled, within-subject, 15 analysed. Full text, from the PMC copy PMC6618596. Figures read through their captions. Note: The 15 analysed were 11 men and 4 women, mean age 26.6. Alpha is a resting rhythm of about 8 to 12 cycles per second over the back of the head. N170 is linked to assembling local edges into whole objects. Error rates on the task were between 1.1 and 3.4 percent.
- Kometer M, Cahn BR, Andel D, Carter OL, Vollenweider FX. The 5-HT2A/1A agonist psilocybin disrupts modal object completion associated with visual hallucinations. Biol Psychiatry 2011, 69(5):399-406. PMID 21126732. DOI 10.1016/j.biopsych.2010.10.002 Grade 1. n = 17. Abstract only. Full text not accessible. Note: The study in 17 volunteers found N170 reduced most when incomplete figures needed to be filled in.
- Kometer M, Vollenweider FX. Serotonergic hallucinogen-induced visual perceptual alterations. Curr Top Behav Neurosci 2018, 36:257-282. PMID 27900674. DOI 10.1007/7854_2016_461 Grade 2. Narrative review. Abstract only. Full text not accessible. Note: The review sorts the visual effects into intensified colors and contrast, illusions of size, simple patterns and flashes, images triggered by sound, and complex scenes or figures.
- Carhart-Harris RL, Muthukumaraswamy S, Roseman L, et al. Neural correlates of the LSD experience revealed by multimodal neuroimaging. Proc Natl Acad Sci USA 2016, 113(17):4853-4858. PMID 27071089. DOI 10.1073/pnas.1518377113 Grade 1. LSD (semi-synthetic). Full text. Supplement not read.
- Timmermann C, Roseman L, Schartner M, et al. Neural correlates of the DMT experience assessed with multivariate EEG. Sci Rep 2019, 9:16324. PMID 31745107. DOI 10.1038/s41598-019-51974-4 Grade 1. 13 participants (12 analysed for EEG), intravenous DMT against placebo, single-blind, fixed order. Full text. Supplement not read. Note: A slow delta and theta rhythm at the peak of the experience correlated with the visual component of the experience as rated in interviews and scales.
- Girn M, Doss MK, Roseman L, et al. An international mega-analysis of psychedelic drug effects on brain circuit function. Nat Med 2026, 32(4):1543-1554. PMID 41942645. DOI 10.1038/s41591-026-04287-9 Grade 1. 11 resting-state fMRI datasets, five drugs. Full text. Supplement not read. Note: Reductions inside networks were weaker and more variable than the coupling increase.
- Carter OL, Pettigrew JD, Hasler F, et al. Modulating the rate and rhythmicity of perceptual rivalry alternations with the mixed 5-HT2A and 5-HT1A agonist psilocybin. Neuropsychopharmacology 2005, 30(6):1154-1162. PMID 15688092. DOI 10.1038/sj.npp.1300621 Grade 1. n = 12, placebo-controlled. Abstract only. Full text not accessible. Note: At recovery some individuals ended above their starting level.
- Carter OL, Hasler F, Pettigrew JD, et al. Psilocybin links binocular rivalry switch rate to attention and subjective arousal levels in humans. Psychopharmacology 2007, 195(3):415-424. PMID 17874073. DOI 10.1007/s00213-007-0930-9 Grade 1. n = 10, ketanserin pretreatment arm. Abstract only. Full text not accessible.
- Barrett FS, Preller KH, Herdener M, Janata P, Vollenweider FX. Serotonin 2A receptor signaling underlies LSD-induced alteration of the neural response to dynamic changes in music. Cereb Cortex 2018, 28(11):3939-3950. PMID 29028939. DOI 10.1093/cercor/bhx257 Grade 1. n = 24 analysed (25 enrolled), LSD (semi-synthetic), ketanserin arm, secondary analysis. Full text, from PMC6887693. Supplementary tables not read. Note: The altered regions were in the frontal and temporal lobes, the angular gyrus, the amygdala and the cerebellum. In one primary auditory area tracking was higher on placebo. The report gives brain responses and no listening thresholds.
- Timmermann C, Spriggs MJ, Kaelen M, et al. LSD modulates effective connectivity and neural adaptation mechanisms in an auditory oddball paradigm. Neuropharmacology 2018, 142:251-262. PMID 29101022. DOI 10.1016/j.neuropharm.2017.10.039 Grade 1. n = 20, magnetoencephalography, LSD (semi-synthetic). Abstract only. Full text not accessible. Note: The oddball study also reported altered backward connectivity in a frontal and temporal network.
- Schmid Y, Enzler F, Gasser P, et al. Acute effects of lysergic acid diethylamide in healthy subjects. Biol Psychiatry 2015, 78(8):544-553. PMID 25575620. DOI 10.1016/j.biopsych.2014.11.015 Grade 1. n = 16, double-blind crossover, LSD (semi-synthetic). Full text, from the authors' accepted manuscript.
- Wittmann M, Carter O, Hasler F, et al. Effects of psilocybin on time perception and temporal control of behaviour in humans. J Psychopharmacol 2007, 21(1):50-64. PMID 16714323. DOI 10.1177/0269881106065859 Grade 1. n = 12, double-blind. Abstract only. Full text not accessible. Note: Psilocybin also slowed the tempo at which people preferred to tap.
- Scholle P, Wenke S, Nekovářová T, et al. The effects of psilocybin on time perception in humans: a comparative analysis of subjective and objective measures. J Psychopharmacol 2026, 40(5):818-830. PMID 41479142. DOI 10.1177/02698811251389552 Grade 1. n = 24, double-blind, placebo-controlled, EudraCT 2012-004579-37. Abstract only. Full text not accessible. Note: The bisection shift is consistent with time seeming to slow.
- Yousefi P, Lietz MP, O'Higgins FJ, et al. Acute effects of psilocybin on attention and executive functioning in healthy volunteers: a systematic review and multilevel meta-analysis. Psychopharmacology 2025, 242(6):1171-1196. PMID 39847068. DOI 10.1007/s00213-024-06742-2 Grade 1. 13 studies and 42 effect sizes in the review, 6 studies and 15 effect sizes behind the reaction-time estimate, exploratory, publication bias reported. Full text.
- Yanakieva S, Polychroni N, Family N, et al. The effects of microdose LSD on time perception: a randomised, double-blind, placebo-controlled trial. Psychopharmacology 2019, 236(4):1159-1170. PMID 30478716. DOI 10.1007/s00213-018-5119-x Correction notice: Psychopharmacology 2020, 237(12):3803 (not read). Grade 1. n = 48 healthy adults aged 55 to 75 (46 analysed), LSD (semi-synthetic). Full text, from PMC6591199. The 2020 correction was not read. Note: Each person belonged to one group only. The over-reproduction was clearest between 2.8 and 3.6 seconds (drug effect F(1,44) = 6.18, p = 0.017, partial eta squared 0.12). A correction to the paper was issued in 2020. If the result replicates, it would mean timing can shift without an obvious altered state.
- Luke DP, Terhune DB. The induction of synaesthesia with chemical agents: a systematic review. Front Psychol 2013, 4:753. PMID 24146659. DOI 10.3389/fpsyg.2013.00753 Grade 1 as a systematic review, with a design note: most included reports are case reports and surveys. Full text, from PMC3797969. Note: The review put auditory-visual synesthesia at roughly 37 percent of psilocybin sessions in pooled laboratory data, rising with the amount given.
- Terhune DB, Luke DP, Kaelen M, et al. A placebo-controlled investigation of synaesthesia-like experiences under LSD. Neuropsychologia 2016, 88:28-34. PMID 27059209. DOI 10.1016/j.neuropsychologia.2016.04.005 Grade 1. Pilot, within-groups, n = 10, fixed order, LSD (semi-synthetic). Full text, from the authors' manuscript. Note: Participants were asked to report colors for letters and sounds. Absorption is the tendency to become deeply immersed in experience.
- Bor D, Rothen N, Schwartzman DJ, Clayton S, Seth AK. Adults can be trained to acquire synesthetic experiences. Sci Rep 2014, 4:7089. PMID 25404369. DOI 10.1038/srep07089 Grade 1. Uncontrolled training study, 14 young adults, no drug. Full text, from PMC4235407.
- Carhart-Harris RL, Friston KJ. REBUS and the anarchic brain: toward a unified model of the brain action of psychedelics. Pharmacol Rev 2019, 71(3):316-344. PMID 31221820. DOI 10.1124/pr.118.017160 Grade 2. Theory and synthesis, no new data. Full text, from PMC6588209, apart from the reference list and glossary. Note: A further study, cited in the paper and not read by Metanoia in the original, reportedly found impaired high-level but intact low-level motion perception.
- Noorani T, Alderson-Day B. Spotlight commentary: REBUS and the anarchic brain. Neurosci Conscious 2020, 2020(1):niaa007. PMID 32550008. DOI 10.1093/nc/niaa007 Grade 2. Commentary, no data. Full text.
- Halpern JH, Pope HG. Hallucinogen persisting perception disorder: what do we know after 50 years? Drug Alcohol Depend 2003, 69(2):109-119. PMID 12609692. DOI 10.1016/S0376-8716(02)00306-X Grade 2. Review of 20 quantitative studies, 1955 to 2001. Abstract only. Full text not accessible.
- Neven A, Blom JD. Pharmacological treatment of hallucinogen persisting perception disorder (HPPD): a systematic review. Harv Rev Psychiatry 2025, 33(5):264-275. PMID 40938689. DOI 10.1097/HRP.0000000000000439 Grade 1 as a systematic review, with a design note: mostly case reports and small series, clinical populations. Full text. Note: Follow-up varied between reports. The authors report recoveries and not measured effects.
- Butler M, Moore E, Rucker JJ, et al. Characterising the clinical associations of hallucinogen persisting perception disorder: a retrospective cohort study. Transl Psychiatry 2026, 16(1). PMID 42031693. DOI 10.1038/s41398-026-04042-1 Grade 1. Retrospective cohort of diagnosed patients (clinical population), electronic health records. Full text. Supplement not read.
- Schmid Y, Liechti ME. Long-lasting subjective effects of LSD in normal subjects. Psychopharmacology 2018, 235(2):535-545. PMID 28918441. DOI 10.1007/s00213-017-4733-3 Grade 1. n = 16, the same volunteers as source 13, follow-up at 1 and 12 months, LSD (semi-synthetic). Full text. Supplement not read.
- Sasaki Y, Nanez JE, Watanabe T. Advances in visual perceptual learning and plasticity. Nat Rev Neurosci 2010, 11(1):53-60. PMID 19953104. DOI 10.1038/nrn2737 Grade 2. Review. Full text.
- Watanabe T, Sasaki Y. Perceptual learning: toward a comprehensive theory. Annu Rev Psychol 2015, 66:197-221. PMID 25251494. DOI 10.1146/annurev-psych-010814-015214 Grade 2. Review and theory. Full text, apart from the reference list.
- Bediou B, Adams DM, Mayer RE, Tipton E, Green CS, Bavelier D. Meta-analysis of action video game impact on perceptual, attentional, and cognitive skills. Psychol Bull 2018, 144(1):77-110. PMID 29172564. DOI 10.1037/bul0000130 Erratum: Psychol Bull 2018, 144(9):978-979, PMID 30148383, DOI 10.1037/bul0000168 (not read). Grade 1. Meta-analysis. Read in part, from the published version in an open repository. The study selection, main results, publication-bias analysis and discussion were read. The introduction, moderator coding, appendix tables and closing implications were not.
- MacLean KA, Ferrer E, Aichele SR, et al. Intensive meditation training improves perceptual discrimination and sustained attention. Psychol Sci 2010, 21(6):829-839. PMID 20483826. DOI 10.1177/0956797610371339 Grade 1. Randomized waiting-list trial, n = 60 experienced meditators. Full text, from PMC3132583. Note: The training effect at the end was F(1,58) = 5.80, p = 0.019, partial eta squared 0.09, and about five months later F(1,56) = 4.58, p = 0.037. Retention correlated with daily practice at r = -0.36 with threshold, p = 0.007.
- Kramer RSS, Weger UW, Sharma D. The effect of mindfulness meditation on time perception. Conscious Cogn 2013, 22(3):846-852. PMID 23778017. DOI 10.1016/j.concog.2013.05.008 Grade 1. Within-subjects, temporal bisection, 40 students. Full text, from the authors' accepted manuscript.
- Mason OJ, Brady F. The psychotomimetic effects of short-term sensory deprivation. J Nerv Ment Dis 2009, 197(10):783-785. PMID 19829208. DOI 10.1097/NMD.0b013e3181b9760b Grade 1. Perceptual effects of short sensory isolation. Abstract only. Full text not accessible.
- Wackermann J, Pütz P, Allefeld C. Ganzfeld-induced hallucinatory experience, its phenomenology and cerebral electrophysiology. Cortex 2008, 44(10):1364-1378. PMID 18621366. DOI 10.1016/j.cortex.2007.05.003 Grade 2. Review including the authors' own EEG work. Cited only for the description of ganzfeld percepts. Abstract only. Full text not accessible.
- Merabet LB, Hamilton R, Schlaug G, et al. Rapid and reversible recruitment of early visual cortex for touch. PLoS One 2008, 3(8):e3046. PMID 18728773. DOI 10.1371/journal.pone.0003046 Grade 1. Five days of blindfolding with tactile training, 47 adults. Full text. Figures read through their captions. Note: Touch thresholds improved equally in both groups. Visual cortex responded more to touch in the blindfolded group on day five.
- Stoliker D, Preller KH, Novelli L, Anticevic A, Egan GF, Vollenweider FX, Razi A. Neural mechanisms of psychedelic visual imagery. Mol Psychiatry, online 11 June 2024, volume 30, page 1259. DOI 10.1038/s41380-024-02632-3 PMID 38862674. Grade 1. Secondary analysis of a double-blind placebo-controlled crossover study, 20 healthy volunteers analysed, oral psilocybin. Full text. Supplement not read. Note: The authors read the result as more top-down influence on the visual regions and not relaxed priors, and say it does not support the bottom-up principle of REBUS there.
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