{"paper_id":"0e68ddad-5ef8-4c97-a6c6-11e7d4eb2100","body_text":"RESEA RCH ARTICL E\nEffect of frequency and rhythmicity on flicker\nlight-induced hallucinatory phenomena\nIoanna Alicia Amaya\nID\n1,2,3\n, Nele Behrens\n1,4\n, David John Schwartzman\nID\n5\n, Trevor Hewitt\n5\n,\nTimo Torsten Schmidt\nID\n1\n*\n1 Department of Education and Psycho logy, Freie Universita ¨ t Berlin, Berlin, Germany , 2 Charite ´ –\nUniversita ¨ tsmedizin Berlin, Einstein Center for Neurosciences Berlin, Berlin, Germa ny, 3 Berlin School of\nMind and Brain, Humboldt-Univ ersita ¨ t zu Berlin, Berlin, Germany , 4 Department of Psych ology, Sigmun d\nFreud Univers ity Berlin, Berlin, Germany, 5 Sackler Centre for Conscio usness Science and Department of\nInformatics , University of Sussex, Brighton, United Kingdom\n* timo.t.s chmidt@f u-berlin.de\nAbstract\nFlicker light stimulation (FLS) uses stroboscopic light on closed eyes to induce transient\nvisual hallucinatory phenomena, such as the perception of geometric patterns, motion, and\ncolours. It remains an open question where the neural correlates of these hallucinatory\nexperiences emerge along the visual pathway. To allow future testing of suggested underly-\ning mechanisms (e.g., changes in functional connectivity, neural entrainment) , we sought to\nsystematically characterise the effects of frequency (3 Hz, 8 Hz, 10 Hz and 18 Hz) and rhyth-\nmicity (rhythmic and arrhythmic conditions) on flicker-induced subjective experiences.\nUsing a novel questionnaire, we found that flicker frequency and rhythmicity significantly\ninfluenced the degree to which participants experienced simple visual hallucination s, partic-\nularly the perception of Klu ¨ ver forms and dynamics (e.g., motion). Participants reported their\nexperience of geometric patterns and dynamics was at highest intensity during 10 Hz rhyth-\nmic stimulation. Further, we found that frequency-match ed arrhythmic FLS strongly reduced\nthese subjective effects compared to equivalent rhythmic stimulation. Together, these\nresults provide evidence that flicker rhythmicity critically contributes to the effects of FLS\nbeyond the effects of frequency alone, indicating that neural entrainment may drive the\ninduced phenomenal experience.\nIntroduction\nFlicker light stimulation (FLS) reliably induces simple visual hallucinations in healthy partici-\npants via closed-eye ocular stimulation with stroboscopic light [1–3]. Simple visual hallucina-\ntions, synonymous with elementary visual hallucinations, refer to the subjective experience of\ncolours and geometric patterns that are devoid of semantic content. The experience is often\naccompanied by other phenomenological changes, such as altered mood, arousal, and sense of\ntime passing [2]. Simple visual hallucinations experienced under FLS display marked similari-\nties to the perceptual changes associated with migraine aura [4,5], epileptic seizures [6] and\nCharles Bonnet Syndrome (hallucinatory experiences due to sensory deprivation resulting\nfrom macular degeneration) [7] as well as drug-induced psychedelic experiences [2,8]. In\nPLOS ONE\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 1 / 18\na1111111111\na1111111111\na1111111111\na1111111111\na1111111111\nOPEN ACCESS\nCitation: Amaya IA, Behrens N, Schwartzm an DJ,\nHewitt T, Schmidt TT (2023) Effect of frequen cy\nand rhythmicity on flicker light-induced\nhallucinato ry phenomena. PLoS ONE 18(4):\ne0284271. https://d oi.org/10.1371/j ournal.\npone.028427 1\nEditor: Manuel Spitschan, Technical University of\nMunich, Germany, UNITED KINGDOM\nReceived: December 6, 2022\nAccepted: March 28, 2023\nPublished: April 11, 2023\nPeer Review History: PLOS recognize s the\nbenefits of transpar ency in the peer review\nprocess; therefore, we enable the publication of\nall of the content of peer review and author\nresponse s alongside final, published articles. The\neditorial history of this article is available here:\nhttps://doi.o rg/10.1371/jo urnal.pone.0 284271\nCopyright: © 2023 Amaya et al. This is an open\naccess article distributed under the terms of the\nCreative Commons Attribution License, which\npermits unrestricte d use, distribu tion, and\nreproduction in any medium, provided the original\nauthor and source are credited.\nData Availabilit y Statement: All questionnair e data\nfiles are available on the OSF data repository (URL:\nhttps://osf.i o/5d29g/).\n\nrecent years, FLS has been used as an experimental tool to study the neural underpinnings of\nvisual hallucinations [9]. However, in order to draw links between neural mechanisms under-\nlying specific forms of visual hallucinations, it is first important to establish a thorough charac-\nterisation of the experienced phenomena.\nFlicker-induced effects were first formally described by Purkinje in 1819 [10]; thereafter,\nthe phenomenon was relatively unexplored until the invention of the electroencephalogram\n(EEG) in the 1920s [11]. This allowed the observation of synchronised brain oscillations when\nFLS was presented in the alpha frequency range (8–12 Hz) [12]. Later, it was explored recrea-\ntionally in the 1960s with the creation of the “Dreamachine”, a low-fi method of delivering\nFLS using a record player [11]. Today, FLS can be delivered using specially programmed elec-\ntronic lamps with precise manipulation of flicker frequency, rhythmicity (i.e., the temporal\npattern of flashes), and brightness.\nInitial studies attempting to characterise visual experiences arising from altered and patho-\nlogical states employed illustrations and open report methods, which revealed a striking uni-\nversality in the types of visual patterns experienced [13,14]. Four of these commonly occurring\npatterns are collectively named the Klu ¨ ver form constants [15], which are comprised of grids,\nspirals, tunnels, and targets (see [16,17] for illustrations). FLS-induced perception of motion\nand colours are also shared across various pathologies and altered states. For example,\nmigraine sufferers often report seeing red, yellow, and blue in addition to bright white [18]\nand vivid colours are one of the most frequently reported characteristics of N, N-dimethyltryp-\ntamine (N, N-DMT) experiences [19]. In addition, FLS also induces other types of imagery\nthat have lower levels of pattern organisation and higher degrees of noise, such as TV static\nand floating, scattered blobs and dots. These are sometimes referred to as phosphene forms\nand likely occur due to retinal stimulation with a strong light source [20]. Aside from simple\nhallucinations, complex visual hallucinations (i.e., realistic scenes, objects, and faces) have also\nbeen reported during FLS, albeit less frequently [2,3]. Recent research further explored if FLS\nexperiences relate to person-specific factors, such as the personality trait of Absorption [2,21].\nIt was also found that people with Aphantasia (i.e., lack of mentally simulated visual imagery\n[22]) report fewer FLS-induced visual effects [23].\nRecent studies have used standardised methods to assess the flicker-induced subjective\nexperience, e.g., Bartossek et al. [2] administered the Altered States of Consciousness Rating\nScale (5D-ASC/11-ASC; [24]) and the Phenomenology of Consciousness Inventory (PCI) [25].\nThese questionnaires are well-established and validated to assess a whole spectrum of altered\nexperiences and thereby allow comparisons across different types of altered states of con-\nsciousness [8]. However, due to their breadth of measured phenomena, they are limited in cap-\nturing a high level of detail of visual effects that would enable differentiation between different\ntypes of visual hallucinations. Using an analogue slider to assess experience intensity,\nSchwartzman et al. [3] were able to differentiate the intensity of experiences between different\nfrequencies of FLS, which was otherwise not captured via 5D-ASC ratings. These observations\nhighlight the need for a careful and detailed assessment of the types of visual phenomena expe-\nrienced during FLS that extends beyond the currently available tools.\nThe link between phenomenology and neurophysiology can be used to shed light on the\nneural mechanisms underlying FLS-induced visual hallucinations. Using periodic flicker (i.e.,\nFLS with regular inter-flash intervals; also called rhythmic flicker), it was found that FLS at\nalpha frequency (8–12 Hz) induces stronger simple visual hallucinations than other frequen-\ncies [2,3,17] and additionally enhances the amplitude of EEG oscillations at the targeted fre-\nquency band of stimulation [26,27]. This indicates that entrainment (i.e., synchronisation of\nbrain oscillations with periodic external driving stimulation) may contribute to the generation\nof simple visual hallucinations. To further test whether entrainment is indeed a driving factor\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 2 / 18\nFunding: The investigator -initiated study was\nfinancially supporte d by a donation from Lumenate\nGrowth Ltd to Freie Universita ¨ t Berlin allocated to\nTTS. There was no additional external funding\nreceived for this study.\nCompeting interests : I have read the journal’s\npolicy and the authors of this manuscript have the\nfollowing competing interests: TTS: This research\nwas supported by an unrestricted donation from\nLumenate Growth ltd to Freie Univers ita ¨ t Berlin\nallocated to TTS. This does not alter our adherenc e\nto PLOS ONE policies on sharing data and\nmaterials.\n\nin generating the subjective experience associated with FLS, one must compare the subjective\neffects between traditional rhythmic stimulation and frequency-matched arrhythmic stimula-\ntion. Theoretically, removing the rhythmicity of the stimulation should abolish entrainment.\nTherefore, if arrhythmic frequency-matched FLS produces fewer subjective effects, it would\nindicate that neural entrainment contributes to the generation of FLS-induced simple visual\nhallucinations.\nHere, we aim to determine the effects of frequency and rhythmicity on flicker-induced phe-\nnomenology. Based on previous findings, we expect that rhythmic FLS within the alpha fre-\nquency range (8–12 Hz) will lead to increased reports of simple visual hallucinations, Klu ¨ ver\nforms and visual experiences that are more dynamic (i.e., moving patterns, patterns changing\nfrequently over time) and visually detailed, compared to other frequencies. We further expect\nthat rhythmic FLS will generate more simple visual hallucinations compared to frequency-\nmatched arrhythmic stimulation. We utilise two arrhythmic conditions that vary in their\ndegree of arrhythmicity. We hypothesise that higher variability in inter-flash intervals (i.e.,\ngreater arrhythmicity) will lead to a greater reduction in subjective effects. In contrast, as all\nFLS conditions deliver the same total duration of light stimulation, we hypothesise that seeing\nphosphene forms (e.g., blobs, TV static) will be frequency- and rhythmicity-independent . This\nis because they have low levels of pattern organisation, making it likely they are caused by reti-\nnal stimulation and not higher-order neural mechanisms. We will also explore whether fre-\nquency and rhythmicity affect reports of complex visual hallucinations and types of colours\nthat are observed during the flicker experience.\nMethods\nParticipants\nHealthy participants were recruited (N = 20; 12 female, 7 male, 1 diverse; age range 20–37\nyears, M = 24.78, SD = 4.25) that met the following inclusion criteria, as established by Bartos-\nsek et al. [2]: no history of epilepsy, migraines, psychological problems (e.g., depression, anxi-\nety disorders), no current consumption of any psychotropic drugs (e.g., antidepressants,\nneuroleptics). To mitigate the risk of an adverse reaction, we only included subjects who had\npreviously used FLS for recreational purposes. Alternatively, an EEG examination was per-\nformed to screen for indicators of photosensitive epilepsy, which would lead to exclusion. The\nrecruitment took place via student mailing lists and through word-of-mouth. Participants gave\ntheir written consent before commencing the experiment. All materials and procedures were\napproved by the ethics committee at Freie Universita ¨ t Berlin (application reference: 045/2021).\nSeventeen participants filled out the questionnaires in German, of which fifteen were native\nspeakers, while the remaining three participants preferred English, two of which were native\nspeakers.\nMaterials\nFlicker light stimulation. A custom stroboscope was constructed by Lumenate Growth\nInc. (Bristol, United Kingdom) to generate light stimulation. It consists of twelve 4500k J2 6V\nwhite LEDs organised in a three-by-four grid with dimensions 128 x 176mm (width x height).\nThe lamp was set to deliver 5,520 Lumens over participants’ eyes (maximum capacity is 10,360\nLumens). Positioned approximately 150cm from participants, the LEDs were within a visual\nangle of approximately 6.5˚, while the setup assured that illumination of the visual field was\nexperienced as homogenous. The lamp was interfaced with an Arduino (v1.8.16) to deliver\nFLS at different frequencies and rhythmicities. Three rhythmicity conditions were used across\nfour levels of frequency (3 Hz, 8 Hz, 10 Hz and 18 Hz) [Fig 1A]. Rhythmicity levels were:\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 3 / 18\n\nRhythmic, which consists of periodic light stimulation following a 0.3 duty cycle (30% ON\ntime); normally distributed arrhythmic stimulation (Arrhythmic\nnorm\n), where inter-flash inter-\nvals (IFIs) were sampled from a normal distribution with mean IFI equal to one OFF time dur-\ning periodic stimulation at matched frequency (e.g., 70 ms at 10 Hz) and standard deviation\nequal to 0.45*OFF time at matched frequency; and paired arrhythmic stimulation (Arrhyth-\nmic\npairs\n), which involved paired high frequency flashes (similar to the luminance control con-\ndition used by Ffytche [28]). The inter-pair OFF time was calculated as 100/freq ms and\nadjusted to 10 ms if the value would otherwise be lower. The flash pairs were embedded within\na set of intervals that were sampled from an exponential probability distribution, where the\nmean IFI was equal to one OFF time at frequency-matched periodic stimulation. As IFIs were\ncalculated for every second, 3 Hz Arrhythmic\npairs\nused one pair and one single flash for every\nsecond. All conditions delivered 300 ms of light stimulation per second (30% ON time). While\nprevious studies used 50% ON time [2,3], we decided to shorten the ON period as this allowed\nfor a greater degree of variation in the arrhythmic IFIs. The Arrhythmic\npairs\ncondition contains\nIFIs with higher variability than the Arrhythmic\nnorm\nat each frequency level (excluding the\nconstant inter-pair interval). This was determined using the root mean squared of successive\ndifferences (RMSSD), which is commonly used for calculating heart rate variability [29] (e.g.,\nat 10Hz, Arrhythmic\npairs\n: RMSSD = 166.57, Arrhythmic\nnorm\n: 64.22). For an illustration of the\nON/OFF flicker sequences see Fig 1A.\nQuestionnaires\nThree questionnaires were used in the study: The Tellegen Absorption scale (TAS), selected\nitems from the Altered States of Consciousness Rating Scale (ASC-R) and an abridged version\nof the novel Stroboscopic Visual Experience Survey (SVES). Participants were able to answer\nthe questionnaires in English or German.\nFig 1. (A) The experime nt comprise d a 3x4 factorial design with 3 levels of rhythmicity (Rhythm ic, Arrhyth mic\nnorm\nand Arrhyth mic\npairs\n) and 4 levels of frequenc y (3 Hz, 8 Hz, 10 Hz, 18 Hz). (B) In the experime ntal setup, participants\nwere seated in a dark room 150 cm away from the strobosc ope (Lumenate Growth Inc., Bristol, United Kingdom) . The\ninitial assessment involved the completion of TAS questionn aire. In the training phase, participant s were exposed to\nFLS and familiarised with the Stroboscopic Visual Experience Survey (SVES) and ASC-R items. The subsequent flicker\nsession consisted of twelve two-minu te stimulation periods presented in a fully randomised order of conditions.\nFollowin g each stimulation period, participa nts rated their experi ence using SVES and ASC-R items. A second session\ntook place within a week of the first.\nhttps://d oi.org/10.1371/j ournal.pon e.0284271.g0 01\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 4 / 18\n\nTAS. TAS assesses the personality trait Absorptiveness. It captures the openness to\nexperiencing mind-altering states with 34 items rated on a five-point Likert scale (0 = “Not at\nall” to 4 = “Very much”) [21].\nASC-R. Eleven questions were taken from the Altered State of Consciousness Scale [24].\nThe ASC-R is a standardised and validated research tool widely used to investigate ASCs [8,24].\nItems are rated using a visual analogue scale (VAS) ranging from “no, not more than usually” to\n“yes, much more than usually”. The 96-item questionnaire can be decomposed into 11 factors,\nincluding subscales from the main five dimensions: Oceanic Boundlessness, Visionary Recon-\nstruction, Dread of Ego Dissolution, Auditory Alterations, Vigilance Reduction) [30]. As FLS\nprimarily induces visual effects, we selected all items from the Elementary Imagery subscale: (“I\nsaw regular patterns [with closed eyes or in complete darkness.]”; “I saw colors [with closed\neyes or in complete darkness.]”), excluding the item “I saw brightness or flashes of light with\neyes closed or in complete darkness” as the experience is inherent to FLS. Further, we selected\nall items from the Complex Imagery subscale of the Visionary Reconstruction scale: “I saw\nwhole scenes roll by [with closed eyes or in complete darkness]”; “I could see images from my\nmemory or imagination with extreme clarity”; “My imagination was extremely vivid”. As it has\nbeen reported that other altered state phenomena can additionally arise from FLS [2], we also\nselected two items from the Positive Derealisation subscale: “I felt as if in a wonderful other\nworld.”; “The boundaries between myself and my surroundings seemed to blur”, the item “My\nsense of space and time was altered as if I was dreaming” from Altered Perception of Time sub-\nscale and “I had the impression I was out of my body.” from the Positive Depersonalisation sub-\nscale of the Oceanic Boundlessness scale. Finally, to measure participants alertness during each\ntrial we included “I felt sleepy” from the Reduction of Vigilance scale.\nSVES. An abridged version of the Stroboscopic Visual Experience Survey, which is cur-\nrently under development, was used. The SVES is a computer-based questionnaire designed to\nallow participants to capture aspects of their FLS experience more accurately, implemented\nusing the SoSci Survey platform. It was originally constructed in English and was translated\ninto German for the purposes of this study. The SVES begins with an instruction page that\nexplains how to answer each item. Thereafter, participants are asked “How well do you recall\nyour visual experience right now” and “What colours did you see? Select all that apply”. There\nare twenty-four colour options, which correspond to the following Natural Colour System\n(NCS) IDs: S1040-R, S2070 Y80R, S4050 Y90R, S2050-Y50R, S1070-Y70R, S3050-Y80R,\nS0550-Y20R, S0580-Y30R, S1060-Y40R, S2070-G70R, S2070-G60Y, S5040-B70G, S0520-B,\nS3050-R70B, S4050-B10G, S1020-R50B, S5020-R70B, S4050-R50B, S2050-R20B, S5010-B70G,\nS6010-G10Y, S4050-R20B as well as black and white. Participants are then asked to rate the\noccurrence of different patterns and forms during the preceding FLS experience (See S1\nAppendix for full list of items). The patterns used in these questions are based on geometric\npatterns that were reported in previous FLS studies [1,16,28] and additional piloting. The pat-\nterns vary between Klu ¨ ver form constants, phosphenes forms and other possible geometric\npatterns that could appear, as well as one geometric pattern that is unlikely to occur (akin to a\ncontrol pattern). In addition, overarching visual aspects of the FLS experience are assessed\nwith another ten items, such as “Did your visual experience continuously change or evolve\nover time?” and “Did your visual experience contain a high level of randomness or chaos?”.\nThese items use a visual analogue scale (VAS) ranging from 0 (no, not at all) to 100 (yes, very\nmuch), which was used to increase comparability of effect sizes with the ASC-R. Three exam-\nple pictures are given to demonstrate the range of possibilities across the scale. Item 1 was\nexcluded due to technical difficulties. Item 16 and Item 22 were excluded due to high response\nvariability. For the subsequent analysis, items were grouped together that conceptually mea-\nsured the same visual phenomena. Items were grouped into the following scales: colors (Item\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 5 / 18\n\n2), simple visual hallucinations (Item 3—Item 10), phosphene forms (Item 11 & Item 12),\ndetail (Item 13), dynamics (Item 14 & Item 15), paisley (Item 17), complex visual hallucina-\ntions (Item 18) and absorption (Item 21). The paisley pattern represents a geometrically simple\npattern that is unlikely to occur. Two subscales of the simple visual hallucinations scale were\nalso determined: Klu ¨ ver (Item 3 –Item 6) and other (Item 8 and Item 10) in order to assess\nwhether there were differences in reported pattern subtypes.\nExperimental procedure\nInitial assessment. A semi-structured interview, which followed the guidelines published\nby Bartossek et al. [2], took place to screen participants for eligibility. During the initial assess-\nment [Fig 1B], participants were given an information sheet and then filled out the consent\nform. A pseudo-anonymised subject ID was created to link data from the two experimental\nsessions. Participants completed the TAS via tablet.\nTraining phase. Participants wore noise-cancelling headphones and were seated on a\ncomfortable chair with headrest 150 cm away from the lamp in a dark room. The training\nphase consisted of four one-minute stimulation periods: constant light, 3 Hz, 10 Hz and 18 Hz\nof rhythmic flicker light. This allowed participants to accustomise to the light intensity and\ntype of experience. Next, participants were asked to evaluate a static image using the SVES to\ngain familiarity with the questionnaire items.\nExperimental sessions. If participants had no further questions, the experimental phase\ncould begin. This involved presentation of twelve two-minute stimulation periods with a fully\nrandomised order of conditions. The conditions were comprised of three levels of rhythmicity\n(Rhythmic, Arrhythmic\nnorm\nand Arrhythmic\npairs\n) and four levels of frequency (3 Hz, 8 Hz, 10\nHz, 18 Hz). Following each stimulation period, participants answered the SVES and ASC-R\nitems to evaluate their phenomenal experience. A second experimental session took place at\nthe same time of day 1–7 days after the first.\nStatistical analysis\nAll statistical analysis was conducted using Rstudio (v1.4.1103). To test whether participant\nratings differed across the two test sessions, 3x4x2 ANOVAs with rhythmicity, frequency and\ntest session as factors were run. For each participant, the mean rating of each scale between the\ntwo sessions was used for further analysis. To test the effects of rhythmicity and frequency, we\nran 3x4 ANOVAs with rhythmicity and frequency as factors. Post-hoc Tukey HSD-Tests were\nused to compare the distribution of ratings in different conditions. As there was insufficient\nevidence to assume normality of data for some of the assessed scales, shown with Shapiro-\nWilks normality tests, we used Kruskal-Wallis tests to confirm the ANOVA results with non-\nparametric testing. To test the effect of rhythmicity and frequency on colour selection, we ran\na repeated measures logistic regression model for each colour using the lme4 package in R. In\nthe regression model, Participant ID was included as a random effect term while frequency\nand rhythmicity were fixed effect terms. Further, we used Pearson product-moment correla-\ntion to explore whether there were associations between the personality trait Absorptiveness\nand the occurrence of complex visual hallucinations.\nResults\nEffects of test session order on flicker-induced phenomena\nTo test for the effects of test session order, we performed a 3x4x2 ANOVA for each SVES scale\nwith frequency, rhythmicity, and test session as factors. We found effects of test session for the\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 6 / 18\n\nSimple Visual Hallucination (F(1, 456) = 11.93, p < 0.001, η\n2\np\n= 0.03), Dynamics (F(1, 456) =\n16.41, p < 0.001 η\n2\np\n= 0.03) and Paisley SVES scales (F(1, 456) = 3.87, p = 0.05, η\n2\np\n< 0.01),\nalbeit with small effect sizes, where ratings were higher in the first session compared to the sec-\nond. When using nonparametric Wilcoxon Rank Sum tests to assess differences in ratings for\neach condition, there was no significant differences between test sessions for any SVES scale.\nTherefore, for subsequent analysis, the mean score was calculated from the first and second\nsession for each participant.\nEffect of frequency and rhythmicity on simple visual hallucinations\nWe sought to test if frequency and rhythmicity affects how participants rated, between 0 and\n100, the occurrence of simple visual hallucinations in their flicker experience. First, we ran a\n3x4 ANOVA on the Simple Visual Hallucination scale of the SVES. We found main effects of\nfrequency (F(3, 228) = 46.19, p < 0.001, η\n2\np\n= .38) and rhythmicity (F(2, 228) = 27.91,\np < 0.001, η\n2\np\n= .19) and a significant interaction effect (F(6, 228) = 3.78, p = 0.001, η\n2\np\n= 0.09).\nNonparametric Kruskal-Wallis testing confirmed a significant effect of frequency and rhyth-\nmicity (H(3) = 79.3, p < 0.001; H(2) = 26.5, p < 0.001). Post-hoc Tukey tests found that 8 Hz,\n10 Hz and 18 Hz stimulation elicited higher ratings of simple visual hallucinations than 3 Hz\n(all p < 0.001). Furthermore, all rhythmicity levels were significantly different from each\nother, where rhythmic was higher than Arrhythmic\nnorm\n(p = 0.001) and Arrhythmic\npairs\n(p < 0.001) and Arrhythmic\nnorm\nwas higher than Arrhythmic\npairs\n(p < 0.001). For interaction\neffects, post-hoc Tukey tests showed that the highest increase in reports of simple visual hallu-\ncination was between Rhythmic and Arrhythmic\npairs\nstimulation at 10Hz (p < .001) and\nremained significant for 18Hz (p = .003) and 8Hz (p = .01) [See S1 Table for full report of\nTukey tests]. Furthermore, during Rhythmic stimulation, reports of simple visual hallucina-\ntions are significantly higher during 8Hz, 10Hz and 18Hz compared to 3Hz (all p < .001) [See\nS1 Table for full report of Tukey tests]. Fig 2A summarises the interaction between rhythmicity\nand frequency on ratings of simple visual hallucinations. Secondly, we tested for effects on sim-\nple visual hallucinations via scores of the ASC-R Elementary Imagery scale. Here, a 3x4\nANOVA revealed a main effect of frequency (F(3, 228) = 5.89, p < 0.001, η\n2\np\n= 0.07), which\nwas further confirmed by Kruskal-Wallis testing (H(3) = 18.6, p < 0.001). Again, post-hoc\nTukey tests found that 8 Hz, 10 Hz and 18 Hz stimulation generated higher ratings of simple\nvisual hallucinations than 3 Hz (all p < 0.01).\nEffects of frequency and rhythmicity on dynamics and detail\nTo test if frequency and rhythmicity would affect the visual dynamics and detail of hallucina-\ntory phenomena, we ran 3x4 ANOVAs on the Dynamics and Detail SVES scales. We found a\nmain effect of rhythmicity (F(2, 228) = 33.83, p < 0.001, η\n2\np\n= .23) and frequency (F(3, 228) =\n46.32, p < 0.001, η\n2\np\n= .38) as well as an interaction effect (F(6, 228) = 3.50, p = .0025, η\n2\np\n=\n.08) on visual dynamics (Fig 2B). Kruskal-Wallis testing confirmed the significant effect of fre-\nquency and rhythmicity on visual dynamics (H(3) = 76.3, p < 0.001; H(2) = 33.1, p < 0.001).\nPost-hoc Tukey tests revealed that 8 Hz, 10 Hz and 18 Hz stimulation elicited higher ratings of\nvisual dynamics than 3 Hz (all p < 0.001). Moreover, ratings were higher for Rhythmic com-\npared to Arrhythmic\nnorm\nand Arrhythmic\npairs\n(p < 0.001). For interaction effects, post-hoc\nTukey tests showed that, during Rhythmic stimulation, 8 Hz, 10 Hz and 18 Hz elicited higher\nratings of visual dynamics than 3 Hz (p < .001). Moreover, ratings were higher for Rhythmic\ncompared to Arrhythmic\npairs\nat 8 Hz, 10 Hz and 18 Hz (all p < .001) [See S1 Table for Tukey\ntest results]. At 10Hz, visual dynamics were also higher for Rhythmic compared to Arrhyth-\nmic\nnorm\n(p = .03) [See S1 Table for full report of Tukey tests]. Further, we found a significant\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 7 / 18\n\neffect of rhythmicity (F(2, 228) = 9.25, p < 0.001, η\n2\np\n= .08) and frequency (F(3, 228) = 26.60,\np < 0.001, η\n2\np\n= .26) on visual detail. Kruskal-Wallis testing confirmed the significant effect of\nfrequency and rhythmicity on ratings of visual detail (H(3) = 57.9, p < 0.001; H(2) = 13.5,\np = 0.001). Post-hoc Tukey tests revealed that visual detail was higher at 8 Hz, 10 Hz and 18 Hz\ncompared to 3 Hz (p < 0.001) [See S1 Table for Tukey test results]. Ratings of visual detail\nwere also higher for Rhythmic compared to Arrhythmic\npairs\n(p < 0.001).\nEffects of frequency and rhythmicity on seeing different pattern types\nTo test if frequency and rhythmicity would affect the types of patterns experienced during sim-\nple visual hallucinations, we ran 3x4 ANOVAs on the Klüver forms, Other forms, Phosphene\nforms and Paisley SVES scales. We found that frequency had a main effect on ratings of seeing\nKlu ¨ ver forms (F(3, 228) = 34.92, p < 0.001, η\n2\np\n= .31) and Other forms (F(3, 228) = 24.78,\np < 0.001, η\n2\np\n= .13) (Fig 2C]. Kruskal-Wallis testing confirmed significant effects of frequency\non seeing Kluver forms (H(3) = 69.6, p < 0.001) and Other forms ((H(3) = 57.6, p < 0.001).\nANOVA testing also revealed a significant main effect of rhythmicity on Klu ¨ ver form (F(2,\n228) = 17.47, p < 0.001, η\n2\np\n= .13) and Other form ratings (F(2, 228) = 16.69, p < 0.001, η\n2\np\n=\n.13), which was further confirmed by Kruskal-Wallis testing (Kluver forms: H(2) = 20.3,\np < 0.001); Other forms: H(2) = 22.1, p < 0.001). Additionally, there was a significant interac-\ntion effect on ratings of Klu ¨ ver forms (F(6, 228) = 3.26, p = 0.004, η\n2\np\n= 0.08). Post-hoc Tukey\ntests found that Klu ¨ ver and Other forms generated higher ratings at 8 Hz, 10 Hz and 18 Hz\nthan at 3 Hz (all p < 0.001). Ratings of Klu ¨ ver and Other forms were also significantly higher\nFig 2. Differen tial effects of rhythmi city and frequency on FLS-induc ed hallucinato ry phenomena . (A) Effects of rhythmicity and frequency on reports of simple\nvisual hallucinations. Ratings are shown from the SVES Simple Visual Hallucinations and ASC-R Elementary Imagery scales. (B) Effects of rhythmicity and frequency on\nvisual dynamics, which encompass es motion and how much the experience changes over time. (C) Effects of rhythmic ity and frequency on differe nt types of visual\npatterns. Klüver forms SVES subscale consists of spirals, cobwe bs, targets and grids. Other forms include SVES items of rippling items and flowing lines. Phosphene forms\nrefers to lower order forms and includes SVES items for TV snow and blobs of light or colour. Bar charts display the differe nce in ratings between arrhythmic controls\nand rhythmic stimulatio n for each frequency . Significance is determined by Tukey tests comparing ratings between rhythmic ity conditions at each frequency level [See S1\nTable for Tukey test results].\nhttps://doi.o rg/10.1371/j ournal.pone .0284271.g002\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 8 / 18\n\nin the Rhythmic condition compared to arrhythmic controls [See S1 Table for Tukey test\nresults]. While ANOVA testing identified a small effect of rhythmicity on ratings of Phosphene\nforms (F(2,228) = 3.07, p = 0.05, η\n2\np\n= .03), Kruskal-Wallis testing found no effect of rhythmic-\nity or frequency on Phosphene forms. Similarly, ANOVA testing identified a small effect of fre-\nquency (F(3, 228) = 2.71, p = 0.05, η\n2\np\n= .03) and rhythmicity (F(2, 228) = 4.83, p = 0.009, η\n2\np\n=\n.04) on perception of Paisley patterns, which was not supported by Kruskal-Wallis testing (i.e.,\nno significant effects were found).\nEffect of rhythmicity and frequency on observed colours\nNext, we explored whether there were categorical shifts in the spectrum of perceived colours\nduring FLS at different frequencies and rhythmicity. No a priori hypotheses were set. The\nprobability of each colour being selected is shown in Fig 3. From this, it appears there were no\nmajor shifts in the proportions of colours that were perceived across conditions. Descriptively,\nprominent peaks for reds, white and black can be observed, which increase in amplitude as the\nfrequency increases. Logistic regression models were used to assess the relationship between\nfrequency and rhythmicity and colour selection. The alpha threshold was Bonferroni corrected\nto 0.002 (0.05/24 due to 24 colours being tested). We found that frequency affected colour\nselection of white, light yellow and bright blue. The odds of participants selecting white were\n6.0, 5.6 and 10.7 times greater during 8 Hz, 10 Hz and 18 Hz, respectively, compared to 3 Hz\nstimulation (all p < 0.001). White was chosen in 53.3% of trials at 3 Hz, 78.3% of trials at 8 Hz,\n76.7% of trials at 10 Hz and 82.5% of trials at 18 Hz. Additionally, during 8 Hz stimulation, the\nodds of selecting light yellow were increased by 3.2-fold compared to 3 Hz stimulation\n(p < 0.001). At 18 Hz, the odds of selecting bright blue were 6.7 times higher than at 3 Hz\n(p < 0.001). Rhythmicity had a significant interaction with frequency on selection of white,\nwhereby odds were 7.1 times higher during rhythmic stimulation at 10 Hz (p = 0.002).\nEffect of frequency and rhythmicity on complex visual hallucinations\nNext, we tested the effects of frequency and rhythmicity on the occurrence of complex visual\nhallucinations. To this end, we performed a 3x4 ANOVA on the ratings of the SVES Complex\nImagery scale. We found a main effect of rhythmicity (F(2, 228) = 4.35, p = 0.01, η\n2\np\n= .04),\nwhich was not found using nonparametric testing. However, nonparametric Kruskal-Wallis\ntesting identified a significant effect of frequency on SVES complex imagery ratings (H(3) =\nFig 3. Effects of rhythmici ty and frequency on colour selection, depicted as a probabi lity of each colour being\nselected. This is express ed as sum of selections over the n = 20 participa nts. The mean selection probabili ty is displayed,\nwhere for each participant the probability was based on the averag e of the first and second session (0, 0.5 or 1).\nhttps://d oi.org/10.1371/j ournal.pon e.0284271.g0 03\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 9 / 18\n\n11.6, p = 0.009). When testing for complex hallucinations via the ASC-R Complex Imagery\nscale, no significant effect was found. Note that the ratings for complex visual hallucinations\nwere overall relatively low, however showing a relevant variability (i.e., for SVES scale ratings:\n3 Hz: 5.2 ± 12.4 M ± SD; 8 Hz: 10.3 ± 19.0; 10 Hz: 12.7 ± 19.4; 18 Hz: 11.1 ± 16.3), which moti-\nvates further exploration to identify factors that determine if a participant will experience com-\nplex hallucinations or not.\nTesting for a relationship between the personality trait absorptiveness and\nFLS-effects\nIn our sample, TAS scores, which indicate the personality trait Absorptiveness, ranged\nbetween 33 and 110 (maximum possible range: 0–136) across participants (M = 64.61,\nSD = 19.09). We used Pearson product-moment correlation to test if absorptiveness relates to\nthe occurrence of simple and complex visual hallucinations. Across the four frequency condi-\ntions, no correlations (p<0.05) were found for SVES nor ASC-R ratings of simple visual hallu-\ncinations. Following the literature suggestion that the occurrence of complex hallucinations\nmight be driven by a persons’ absorptiveness, we tested within the four rhythmic frequency\nconditions for correlations of TAS with Complex Imagery scales of the SVES and ASC-R. We\nfound a positive correlation for the 10 Hz condition, which was significant when assessed with\nthe SVES (r = .61, p = .004) even after correction for multiple comparisons (Bonferroni: 0.05/\n8 = 0.00625). When testing with the ASC-R scores, this correlation was also present (r = .49, p\n= .03), however did not survive correction for multiple comparisons.\nDiscussion\nIn this study, we aimed to determine the effects of flicker frequency and rhythmicity on FLS-\ninduced hallucinatory effects. We used the well-established ASC-R in combination with an\nabridged version of the novel SVES to quantitatively assess the visual experience elicited by dif-\nferent FLS frequencies. This combination provided a direct comparison to previous data while\nalso allowing a more thorough assessment of visual phenomena than previously attempted.\nWe found effects of frequency on occurrence of simple visual hallucinations, especially percep-\ntion of Klu ¨ ver forms, as well as visual dynamics (e.g., motion) and the degree of visual detail.\nHere, participants reported that they experienced the most geometric patterns (e.g., Klu ¨ ver\nforms) and visual dynamics during 10 Hz FLS. Furthermore, to test the influence of FLS rhyth-\nmicity on inducing simple visual hallucinations, we compared the subjective effects of rhyth-\nmic versus arrhythmic stimulation. We found that, even though arrhythmic stimulation\ndelivered the same amount of physical light stimulation per second as rhythmic stimulation, it\nresulted in substantially reduced visual effects, including reduced perception of geometric pat-\nterns and visual dynamics. This may suggest that neural entrainment, elicited by rhythmic\nFLS, plays a significant role in the generation of simple visual hallucinations. The reduction in\nvisual effects was most pronounced for the Arrhythmic\npairs\ncondition at 10 Hz, supporting its\nfuture use in investigations of the neuronal mechanisms underlying the flicker experience.\nAssessment of phenomenology\nTo draw conclusions from phenomenological data, it is first important to establish whether\nthe employed assessment tools provide an accurate representation of the subjective experience.\nBased on recent work [1,2,17], we designed and administered an abridged version of the novel\nSVES to assess FLS-induced visual effects. We found that ratings of simple visual hallucina-\ntions were similar across ASC-R and SVES measures. Given that the ASC-R is well validated\n[22], the parity of these results indicates construct validity of the SVES, which should be\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 10 / 18\n\nformally tested in future studies. Furthermore, larger effect sizes were found using SVES rat-\nings compared to ASC-R ratings. This is likely because the SVES was designed to capture spe-\ncific details and pattern subtypes within simple visual hallucinations while ASC-R targets an\noverarching altered state experience. By differentiating pattern subtypes, we found that some\nvisual phenomena occur irrespective of frequency and rhythmicity (i.e., phosphene forms; see\nFig 2C), thus the SVES preserves this information while the ASC-R only captures gross visual\nphenomena: “I saw patterns”. Indeed, recent studies did not find differences in ASC-R ratings\nof simple visual hallucinations at different FLS frequencies [2,3], even though the experience\nintensity was rated differently [3]. While the ASC-R questionnaire remains useful for compari-\nsons across altered state induction methods [2,8], the SVES enables a more detailed assessment\nof visual hallucinatory characteristics.\nEffects of frequency and rhythmicity on visual hallucinatory phenomena\nFirst, we tested how the phenomenal characteristics of FLS-induced hallucinatory phenomena\nwere affected by flicker frequency. We found that simple visual hallucinations, such as percep-\ntion of Klu ¨ ver forms, were experienced most intensely at 10 Hz rhythmic FLS. This not only\nconfirms previous findings, where 10 Hz FLS was identified as generating the greatest halluci-\nnatory effects [2,3,17,28], but offers an extension by distinguishing phenomenal components\nwithin the experience. For example, by differentiating simple visual hallucinations into pat-\nterns subtypes, we found that Klu ¨ ver forms (i.e., grids, cobwebs, spirals, tunnels) were the\nmost reported pattern subtype during rhythmic FLS at 8 Hz, 10 Hz and 18 Hz. Furthermore,\nwe found that 10 Hz rhythmic stimulation elicited the most visually dynamic experiences.\nVisual dynamics encompass perceived motion and how much the experience changes over\ntime. FLS-induced moving patterns have been previously documented [1,31,32]. We extend\nthis by finding that flicker frequency had the largest effect on dynamics compared to all other\nFLS-induced subjective qualities, emphasising that it constitutes a highly relevant characteris-\ntic of FLS effects. Future studies could incorporate eye tracking sensors that monitor partici-\npant eye movements during the flicker experience to explore whether participants’ eye\nposition and movement adds to variability in the subjective experience. Altogether, out of the\ntested frequencies, our results identify 10 Hz FLS as the frequency that induced the greatest\nperceptual changes.\nNext, we investigated whether rhythmicity affected the phenomenal characteristics induced\nby FLS. We found that arrhythmicity significantly reduced simple visual hallucinations and\nvisual dynamics. The relative reduction of effects was largest for Arrhythmic\npairs\nat 10 Hz,\ncompared to rhythmic 10 Hz. The use of paired flashes as a control condition was first utilised\nby Ffytche [28], where it was found that paired flashes led to significant decreases in occipito-\ntemporal activity, measured via EEG, compared to periodic FLS. However, it should be noted\nthat Ffytche did not include a phenomenal characterisation of the flicker conditions. Further-\nmore, we applied an arrhythmic version of the paired flash stimulation whereby inter-pair\nintervals were sampled from an exponential probability distribution. In case of higher frequen-\ncies (i.e., 18 Hz), flicker trains of higher frequency due to IFI randomization were more likely\nto occur than in the other conditions, with local frequency of up to 37 Hz (compare Fig 1).\nDue to the randomization of IFIs within one second, these trains were very short and inter-\nrupted by longer IFIs, making it unlikely that they were majorly driving the subjective experi-\nences. Overall, we present considerable evidence to show that frequency-matched arrhythmic\nFLS reduces hallucinatory effects compared to rhythmic stimulation, underscoring the impor-\ntance of rhythmicity in determining the intensity of FLS effects.\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 11 / 18\n\nSimple visual patterns, such as those reported in our study, are also commonly reported fol-\nlowing administration of a range of psychedelic drugs (serotonin-2A receptor agonists)\n[19,33]. Indeed, Klu ¨ ver forms were first identified in the context of mescaline-induced halluci-\nnations [15] and have also been reported in migraine aura [5] and Charles Bonnet Syndrome\n(i.e., reported mosaic patterns as a form of grid) [34]. Similarly, perceived motion of visual\nimagery also occurs in epileptic seizures [35] and during migraine aura [4]. The similarities in\nreports of simple visual hallucinations across aetiologically distinct origins (including FLS)\nindicate shared underlying neural mechanisms. Seminal computational modelling work sug-\ngests that the structure of simple visual hallucinations is to some extent determined by the neu-\nronal architecture between the retina and brain [14,16,36,37]. Therefore, simple visual\nhallucinations may reveal the hidden architecture of visual areas of the brain. Future neuro-\nphysiology research can therefore draw upon research from various domains to formulate a\nbetter understanding of how hallucinatory phenomena are generated.\nAside from simple hallucinatory phenomena, there were small effects of rhythmicity and\nfrequency on reports of complex visual hallucinations. Complex hallucinations involve the\nperception of realistic objects, scenes, and faces (i.e., containing semantic value). While previ-\nous reports found them to be more prevalent at 3 Hz stimulation [3], we found that ratings\nincreased with frequency. Still, complex visual hallucinations remained relatively low through-\nout all FLS conditions and occurred to a lesser extent than simple hallucinations, which is in\nline with previous work [2]. This reinstates that FLS reliably induces simple visual hallucina-\ntions, while phenomena that involve semantically meaningful content occur only occasionally.\nTo explore what factors may influence the extent of experiencing complex hallucinations,\nwe tested their occurrence in relation to the personality trait “absorptiveness”, following from\nprevious work [2]. We found a positive correlation between absorptiveness and complex imag-\nery ratings for 10 Hz rhythmic FLS. Absorptiveness positively correlates with hypnotisability\n[21,38], a term that precedes the recently introduced concept of “phenomenological control”\n[39], which describes one’s capacity to alter their subjective experience in order to meet expec-\ntations. This could suggest that participants with high absorptiveness experience more halluci-\nnations due to expectation that they will occur. Interestingly, however, absorptiveness did not\ncorrelate with simple visual hallucinations. Following from this, predictive coding models sug-\ngest that altered hierarchical processing, more specifically prior distributions, on either lower\nor higher levels of the visual hierarchy relate to simple or complex hallucinations, respectively\n[40–42]. In light of this distinction, it is plausible that personality traits like absorptiveness and\nphenomenological control influence hierarchically higher regions and thereby increase likeli-\nhood of experiencing complex hallucinations. Further research could expand the scope of\nassessing how inter-individual differences influence FLS-induced phenomenology and neural\nprocessing by measuring a wider variety of participant traits, such as phenomenological\ncontrol.\nExploration of FLS-induced perception of colours\nWe explored the types of colours that participants reported during different levels of flicker\nfrequency and rhythmicity. We aimed to decipher whether there were categorical shifts in the\nspectrum of perceived colours depending on the type of FLS. We found that increasing flicker\nfrequency increased the chances of participants reporting the perception of white, light yellow\nand bright blue. Rhythmicity also influenced the selection of white, whereby rhythmic stimula-\ntion led to higher chances of perceiving white during 10 Hz stimulation. Previous research\nidentified that colours are often experienced during rhythmic FLS [2,3], Ganzfeld stimulation\n[43], psychedelic drug-induced experiences [19,33], epileptic seizures [6] and Charles Bonnet\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 12 / 18\n\nSyndrome [34], however we are not aware of literature that has further classified the specific\ncolours experienced or their respective proportions within an experience. The VES enables\ngroup-level quantification of each colour perceived during different FLS conditions. It is inter-\nesting to note that colour perception is only weakly modulated by frequency and rhythmicity,\nespecially when compared against simple patterns and visual dynamics. This could indicate\nthat hallucinatory colour perception arises in the lower levels of the visual pathway (e.g., from\nretinal stimulation; entoptic phenomena), while patterns and other hallucinatory phenomena\ndepend on frequency and rhythmicity-dependent neural mechanisms, such as neural entrain-\nment. These exploratory findings can be used to formulate hypotheses of flicker-induced col-\nour perception in future studies.\nPotential underlying neural mechanisms\nUltimately, it is of interest to explain phenomenal characteristics in relation to their underlying\nneural mechanisms. While there have been some neuroimaging studies of FLS [3,26–28], the\ndirect link between FLS-induced phenomena and neural activity is yet to be established. Cur-\nrently, there are three main views addressing the neural mechanisms that lead to FLS-induced\nvisual hallucinatory phenomena.\nFirstly, it is likely that neural entrainment plays an important role. Haegens [44] defined\nentrainment as the phase alignment of existing brain oscillations to an external periodic stimu-\nlus, which continues for several cycles after stimulus termination. Previous EEG studies found\nthat rhythmic flicker at alpha frequency increases neural entrainment at that frequency\n[3,12,45–47]. Further, it was found that rhythmic flicker produced stronger phase locking than\narrhythmic stimulation when presented with high light intensity at a stimulation frequency\nclose to the individual’s dominant intrinsic frequency [45–47]. In our study, we found the\ngreatest differences in reported hallucinatory phenomena between Rhythmic and the Arrhyth-\nmic\npairs\ncontrol, which has more arrhythmicity than the normally distributed control, as deter-\nmined by the RMSSD of IFIs (see Methods). This finding could suggest that the relationship\nbetween rhythmicity and visual effects exists as a continuum where the degree of arrhythmicity\naffects the extent to which effects are reduced. However, it is important to note that we did not\ndirectly assess the neural effects of arrhythmic stimulation. In this light, it should be considered\nthat other mechanisms can also contribute to oscillatory activity, such as the superposition of\nevent-related responses, which are evoked cortical responses to visual stimulation that add\nonto, but do not interact with, ongoing oscillations [48]. Moreover, a recent study using rhyth-\nmic flickering checkerboards found evidence for both frequency-specific neural responses,\nsupporting the entrainment model, and frequency-independent resonance phenomena, sup-\nporting the superposition model [49]. As we found that intensity of FLS effects was affected by\nfrequency, it is likely that underlying frequency-specifi c neural responses, such as entrainment,\ncontribute to FLS effects. However, future EEG research is necessary to test whether FLS-\ninduced neural responses satisfy the criteria for entrainment [44] and if there is markedly less\nentrainment elicited by arrhythmic conditions. In doing so, evidence can be provided to deter-\nmine whether neural entrainment mediates the effect of frequency and rhythmicity on the\nflicker-induced subjective experience.\nSecondly, the Ermentrout-Cowan model proposes that the perception of Klu ¨ ver forms cor-\nresponds to self-organised striped cortical activity in the primary visual cortex (V1)\n[14,16,17,50]. Due to the nonlinear transformation of retinal to cortical coordinates, the model\ndemonstrates that striped activation in V1 translates into spirals, tunnels and other Klu ¨ ver\nforms when mapped onto retinal coordinates. The model incorporates anatomical knowledge\nof the visual cortex, such as the size of V1 hypercolumns, their lateral inhibitory connections\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 13 / 18\n\nand orientation selectivity [14,16]. Nevertheless, it cannot explain the entirety of reported sim-\nple visual hallucinations as there are other pattern types and characteristics that it does not\naccount for. For example, Ffytche [28] found that FLS led to increased V4 activity, which may\ncorrespond to perceived colour or motion of patterns. Still, our findings lend some support to\nthe model as we found that Klu ¨ ver forms are reported to a greater degree than other patterns\nsubtypes, reinforcing their relevance in the study of simple visual hallucinations. Moreover, as\nexpected, flicker frequency and rhythmicity did not influence whether phosphene forms were\nperceived. This supports the notion that phosphene forms, such as TV snow and blobs, are\ngenerated from retinal stimulation with a bright light source [17] as all frequency and rhyth-\nmicity levels produced the same amount of light input (300ms of light stimulation per second).\nThe additional patterns, such as Klu ¨ ver forms, are more likely to be a result of frequency-\ndependent modulation of neural activity via periodic light stimulation.\nFinally, one can look to models developed in other domains to inform predictions of how\nFLS-induced phenomena could arise. The cortico-striato-thalamo-cortical (CSTC) model pro-\nposes that drug- and pathology-induced hallucinations are associated with aberrant modulation\nof thalamus activity leading to thalamocortical dysconnectivity [51,52]. This is supported by\nstudies that found drug-induced alterations in sensory perception to be positively correlated\nwith the functional connectivity between thalamus and primary sensory cortices [53]. Such\nincreased thalamocortical functional connectivity has also been found during flicker-induced\nhallucinations [28] and psychosis [54–56]. Further, thalamocortical dysconnectivity has been\nimplicated in Ganzfeld-induced altered states [57] and thalamocortical functional and structural\ndysconnectivity is present in patients with epilepsy [58–60] and migraine [61–63]. These find-\nings hint that thalamocortical dysconnectivity may also play a key role in FLS-induced effects.\nIt is likely that the three views are not mutually exclusive but that their proposed mecha-\nnisms interact or influence each other at different levels. For example, thalamocortical dyscon-\nnectivity may arise from neural entrainment at specific frequencies. Furthermore, the\nErmentrout-Cowan model may explain specific properties of the visual experience, such as\nperception of Klu ¨ ver forms, but requires additional inputs from other models to encapsulate\nthe entire phenomenal experience. The aim of future research should be to tie together the\ninteracting mechanisms in order to formulate an overarching model of how simple visual hal-\nlucinations are generated in the brain.\nOutlook\nHere, we have presented the effects of flicker light rhythmicity and frequency on aspects of the\nflicker-induced experience, such as simple visual hallucinations, visual dynamics and per-\nceived colours. The applied SVES generated similar ratings to the ASC-R, but with larger effect\nsizes, which suggests that SVES can capture the FLS-induced phenomenology with a higher\nlevel of detail. Further, we found that flicker arrhythmicity significantly reduced visual effects,\nwhich implies that neural entrainment may be critical to the generation of simple visual hallu-\ncinations. From two frequency-matched arrhythmic control conditions, we identified the\nArrhythmic\npairs\ncondition as most effective in reducing simple visual hallucinations when\ncompared against rhythmic stimulation, especially at 10 Hz. Using this, future neuroimaging\nstudies can investigate the neural mechanisms that mediate the effects of rhythmicity on the\nflicker-induced hallucinatory experience.\nSupporting information\nS1 Table. Tables for full report of Tukey test results.\n(PDF)\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 14 / 18\n\nS1 Appendix. Abridged version of Stroboscopic Visual Experience Survey.\n(PDF)\nAcknowledgmen ts\nWe would like to thank Tom Galea, Jay Conlon and Lumenate Growth Inc. for the helpful dis-\ncussions and generous provision of experimental hardware.\nAuthor Contributions\nConceptualization: Ioanna Alicia Amaya, David John Schwartzman, Timo Torsten Schmidt.\nData curation: Ioanna Alicia Amaya.\nFormal analysis: Ioanna Alicia Amaya, Nele Behrens, Timo Torsten Schmidt.\nFunding acquisition: Timo Torsten Schmidt.\nInvestigation: Ioanna Alicia Amaya.\nMethodology: Ioanna Alicia Amaya, David John Schwartzman, Trevor Hewitt.\nProject administration: Timo Torsten Schmidt.\nResources: Timo Torsten Schmidt.\nSoftware: Ioanna Alicia Amaya.\nSupervision: Timo Torsten Schmidt.\nVisualization: Ioanna Alicia Amaya, Timo Torsten Schmidt.\nWriting – original draft: Ioanna Alicia Amaya.\nWriting – review & editing: David John Schwartzman, Trevor Hewitt, Timo Torsten\nSchmidt.\nReferences\n1. Allefeld C, Pu ¨ tz P, Kastner K, Wackermann J. Flicker-light induced visual phenome na: Freque ncy\ndependenc e and specificity of whole percepts and percept features. Conscio us Cogn. 2011; 20: 1344–\n1362. https://d oi.org/10.101 6/j.concog.2 010.10.0 26 PMID: 211230 84\n2. Bartossek MT, Kemmere r J, Schmidt TT. Altered states phenomena induced by visual flicker light stim-\nulation. Plos One. 2021; 16: e0253779. https:/ /doi.org/10.13 71/journal.p one.025 3779 PMID: 341975 10\n3. Schwartzm an DJ, Schartner M, Ador BB, Simone lli F, Chang AY-C, Seth AK. Increased spontane ous\nEEG signal diversity during stroboscopi cally-ind uced altered states of consciou sness. Biorxiv. 2019;\n511766. https:// doi.org/10.11 01/5117 66\n4. Schott GD. Exploring the visual hallucina tions of migraine aura: the tacit contribu tion of illustration.\nBrain. 2007; 130: 1690–1 703. https://doi.or g/10.109 3/brain/aw l348 PMID: 17264093\n5. Cowan JD. Visual Hallucinatio ns and Migraine Aura. Encyclope dia of Computationa l Neuroscien ce.\n2013; 1–11. https://doi.or g/10.1007/ 978-1-4614 -7320-6_51 5–1\n6. Panayiot opoulos CP. Elementary visual hallucin ations in migraine and epilepsy. J Neurology Neurosur g\nPsychiatry. 1994; 57: 1371. https://do i.org/10.1136 /jnnp.57 .11.1371 PMID: 796481 4\n7. Jan T, Castillo J del. Visual Hallucinatio ns: Charles Bonnet Syndrom e. West J Emerg Medicine Integra t-\ning Emerg Care Popul Heal. 2012; 13: 544–547. https://d oi.org/10.581 1/westjem .2012.7.1 2891 PMID:\n23357937\n8. Schmidt TT, Berkemeye r H. The Altered States Database: Psych ometric Data of Altered States of Con-\nsciousnes s. Front Psychol. 2018; 9: 1028. https://doi. org/10.3389/fp syg.2018. 01028 PMID: 30013493\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 15 / 18\n\n9. Rogers S, Keogh R, Pearson J. Hallucinat ions on demand: the utility of experiment ally induced phe-\nnomena in hallucination research . Philosophica l Transact ions Royal Soc B. 2021; 376: 20200233.\nhttps://doi.or g/10.109 8/rstb.202 0.0233 PMID: 3330807 6\n10. Purkinje von J. Beitra ¨ ge zur Kenntnis s des Sehen s in subjectiver Hinsicht. Prag: In Commiss ion bei\nJohann Gottfried Calve; 1819.\n11. Meulen BC ter, Tavy D, Jacobs BC. From Strobos cope to Dream Machine: A History of Flicker-Induc ed\nHallucinatio ns. Eur Neurol. 2009; 62: 316–32 0. https://doi.or g/10.115 9/000235945 PMID: 197299 29\n12. Adrian ED, Matthew s BHC. The Berger Rhythm: Potenti al Changes from the Occipital Lobes in Man.\nBrain. 1934; 57: 355–38 5. https://doi.or g/10.109 3/brain/57.4.3 55\n13. Ffytche DH. Visual hallucinations and the charles bonnet syndrome. Curr Psychiat Rep. 2005; 7: 168–\n179. https://do i.org/10.1007 /s11920- 005-0050- 3 PMID: 15935130\n14. Ermentrout GB, Cowan JD. A mathematical theory of visual hallucin ation patterns. Biol Cybern. 1979;\n34: 137–150. https://doi.or g/10.1007/ BF00336965 PMID: 486593\n15. Klu ¨ ver H. Mescal and Mechanism s of Hallucinat ions. Chicago: University of Chicago Press; 1966.\n16. Bressloff PC, Cowan JD, Golubitsky M, Thomas PJ, Wiener MC. What Geomet ric Visual Hallucinatio ns\nTell Us about the Visual Cortex. Neural Compu t. 2002; 14: 473–49 1. https://doi.or g/10.1162 /\n08997660231 725086 1 PMID: 11860679\n17. Billock VA, Tsou BH. Elementary Visual Hallucinatio ns and Their Relations hips to Neural Pattern-For m-\ning Mechanism s. Psychol Bull. 2012; 138: 744–774. https://doi. org/10.1037/a 0027580 PMID:\n22448914\n18. Richards W. The Fortificati on Illusions of Migraines. Sci Am. 1971; 224: 88–96. https://doi.or g/10.103 8/\nscientificam erican0571- 88 PMID: 555258 1\n19. Lawrence DW, Carhart-H arris R, Griffiths R, Timmermann C. Phenom enology and content of the\ninhaled N, N-dimethyltr yptamine (N, N-DMT) experie nce. Sci Rep-uk. 2022; 12: 8562. https://doi.o rg/\n10.1038/ s41598-022- 11999-8 PMID: 3561023 0\n20. Billock VA, Tsou BH. Neural interacti ons between flicker-indu ced self-organiz ed visual hallucin ations\nand physical stimuli. Proc National Acad Sci. 2007; 104: 8490–84 95. https://doi.or g/10.1073 /pnas.\n0610813104 PMID: 17470794\n21. Tellegen A, Atkinson G. Openness to absorbing and self-alte ring experiences (“absorption ”), a trait\nrelated to hypnotic suscept ibility. J Abnorm Psychol. 1974; 83: 268–27 7. https://doi.or g/10.1037/\nh0036681 PMID: 4844914\n22. Keogh R, Pearson J. The blind mind: No sensory visual imagery in aphanta sia. Cortex. 2018; 105: 53–\n60. https://doi. org/10.1016/j .cortex.2017 .10.012 PMID: 29175093\n23. Ko ¨ nigsmark VT, Bergma nn J, Reeder RR. The Ganzfli cker experie nce: High probability of seeing vivid\nand complex pseudo- hallucinations with imagery but not aphanta sia. Cortex. 2021; 141: 522–534.\nhttps://doi.or g/10.101 6/j.cortex.20 21.05.00 7 PMID: 34172274\n24. Dittrich A. The Standa rdized Psychomet ric Assessme nt of Altered States of Consciousne ss (ASCs) in\nHumans . Pharmacop sychiatry. 1998; 31: 80–84. https://doi.or g/10.105 5/s-2007-979 351 PMID:\n9754838\n25. Pekala RJ. Quantif ying Conscious ness, An Empirical Approach. 1991; 127–143. https://doi. org/10.\n1007/978-1- 4899-0629- 8_8\n26. Becker C, Gramann K, Mu ¨ ller HJ, Elliott MA. Electro physiologic al correlates of flicker-i nduced color hal-\nlucination s. Conscious Cogn. 2009; 18: 266–276. https://doi.or g/10.1016/j. concog.200 8.05.001 PMID:\n18602838\n27. Herrmann CS. Huma n EEG response s to 1–100 Hz flicker: resonan ce phenome na in visual cortex and\ntheir potential correlation to cognitive phenomena. Exp Brain Res. 2001; 137: 346–353. https://doi.or g/\n10.1007/ s00221010068 2 PMID: 11355381\n28. Ffytche DH. The hodology of hallucinations . Cortex. 2008; 44: 1067–1 083. https://doi.or g/10.101 6/j.\ncortex.2008 .04.005 PMID: 18586234\n29. Shaffer F, Ginsbe rg JP. An Overview of Heart Rate Variability Metrics and Norms. Frontiers Public\nHeal. 2017; 5: 258. https:// doi.org/10.33 89/fpubh .2017.00258 PMID: 29034226\n30. Studerus E, Gamma A, Vollenw eider FX. Psychom etric Evaluation of the Altered States of Conscio us-\nness Rating Scale (OAV). Plos One. 2010; 5: e12412. https://d oi.org/10.137 1/journal.po ne.0012 412\nPMID: 208242 11\n31. Smythies JR. The Stroboscop ic Patterns. Brit J Psychol. 1960; 51: 247–255. https://doi.or g/10.1111/ j.\n2044-8295.1 960.tb00 747.x\n32. Herrmann CS, Elliott MA. Fechner’ s colors are induced by flickering monochro matic light. In: Sommer-\nfeld E, Kompass R, Lachmann T, editors. Pabst Science Publish ers; 2001. pp. 427–43 1.\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 16 / 18\n\n33. Zamberla n F, Sanz C, Vivot RM, Pallavicini C, Erowid F, Erowid E, et al. The Varieties of the Psyche-\ndelic Experien ce: A Preliminar y Study of the Association Between the Reported Subjective Effects and\nthe Binding Affinity Profiles of Substitute d Phenethy lamines and Tryptamine s. Frontiers Integr Neu-\nrosci. 2018; 12: 54. https://d oi.org/10.338 9/fnint.20 18.00054 PMID: 30467466\n34. Vacchian o V, Tonon C, Mitolo M, Evange listi S, Carbonelli M, Liguori R, et al. Functiona l MRI study in a\ncase of Charles Bonnet syndrome related to LHON. Bmc Neurol. 2019; 19: 350. https://doi.or g/10.\n1186/s12 883-019-1 579-9 PMID: 31888524\n35. Wilkinson F. Auras and other hallucinations : windows on the visual brain. Prog Brain Res. 2004; 144:\n305–320. https:// doi.org/10.10 16/S0079-6 123(03)14 421-4 PMID: 14650857\n36. Butler TC, Benayoun M, Wallace E, Drongelen W van, Goldenfeld N, Cowan J. Evolutionary constraints\non visual cortex architectur e from the dynamics of hallucinations . Proc National Acad Sci. 2012; 109:\n606–609. https:// doi.org/10.10 73/pnas.1 118672109 PMID: 22203969\n37. Rule M, Stoffrege n M, Ermentrout B. A Model for the Origin and Properties of Flicker-In duced Geome t-\nric Phosph enes. Plos Comp ut Biol. 2011; 7: e1002158. https://doi.or g/10.1371/ journal.pcbi.1 002158\nPMID: 219802 69\n38. Glisky ML, Tataryn DJ, Tobias BA, Kihlstrom JF, McConkey KM. Absorption, openne ss to experience,\nand hypnotizab ility. J Pers Soc Psychol. 1991; 60: 263–272. https:// doi.org/10.10 37//0022 -3514.60.2.\n263 PMID: 201666 9\n39. Lush P, Scott RB, Seth AK, Dienes Z. The Pheno menologic al Control Scale: Measurin g the Capac ity\nfor Creating Illusory Nonvolition , Hallucinat ion and Delusion. Collabra Psychology . 2021; 7. https://do i.\norg/10.1525/ collabra.2954 2\n40. Suzuki K, Roseboom W, Schwartzm an DJ, Seth AK. A Deep-Drea m Virtual Reality Platform for Study-\ning Altered Perceptual Phenom enology. Sci Rep-uk. 2017; 7: 15982. https:/ /doi.org/10.10 38/s4159 8-\n017-16316- 2 PMID: 291675 38\n41. Corlett PR, Horga G, Fletcher PC, Alderson-D ay B, Schmack K, Powers AR. Hallucinatio ns and Strong\nPriors. Trends Cogn Sci. 2019; 23: 114–127. https:/ /doi.org/10.10 16/j.tics.2 018.12.001 PMID:\n30583945\n42. Reichert DP, Seriès P, Storkey AJ. Charles Bonnet Syndrom e: Evidence for a Generative Model in the\nCortex? Plos Comput Biol. 2013; 9: e1003134. https:// doi.org/10.13 71/journal.p cbi.10031 34 PMID:\n23874177\n43. Schmidt TT, Prein JC. The Ganzfeld experienc e—A stably inducible altered state of consciousne ss:\nEffects of different auditory homogen izations. Psych J. 2019; 8: 66–81. https://doi.or g/10.100 2/pchj.262\nPMID: 306093 22\n44. Haegens S. Entrainm ent revisited: a commentary on Meyer, Sun, and Martin (2020). Lang Cognition\nNeurosci. 2020; 35: 1119–1123. https://d oi.org/10.108 0/232737 98.2020.17583 35 PMID: 33718510\n45. Mathews on KE, Prudhomm e C, Fabiani M, Beck DM, Lleras A, Gratton G. Making Waves in the Stream\nof Conscio usness: Entraining Oscillations in EEG Alpha and Fluctuation s in Visual Awarenes s with\nRhythmic Visual Stimulati on. J Cognitive Neurosc i. 2012; 24: 2321–23 33. https://doi.or g/10.116 2/jocn_\na_00288 PMID: 22905825\n46. Notbohm A, Herrmann CS. Flicker Regularity Is Crucial for Entrainment of Alpha Oscillati ons. Front\nHum Neurosci. 2016; 10: 503. https://doi.o rg/10.3389/fnh um.2016.0 0503 PMID: 27790105\n47. Notbohm A, Kurths J, Herrmann CS. Modification of Brain Oscillations via Rhythmic Light Stimulati on\nProvides Evidence for Entrainm ent but Not for Superpositio n of Event-Rel ated Respo nses. Front Hum\nNeurosci. 2016; 10: 10. https://doi.or g/10.338 9/fnhum.20 16.00010 PMID: 26869898\n48. Capilla A, Pazo-Alvare z P, Darriba A, Campo P, Gross J. Steady-State Visual Evoked Potenti als Can\nBe Explained by Temporal Superpos ition of Transient Event-Rel ated Responses . Plos One. 2011; 6:\ne14543. https:// doi.org/10.13 71/journal.p one.001 4543 PMID: 2126708 1\n49. Nuttall R, Ja ¨ ger C, Zimme rmann J, Archila-Mele ndez ME, Preibisch C, Taylor P, et al. Evoked\nresponse s to rhythmic visual stimulation vary across sources of intrinsic alpha activity in humans. Sci\nRep-uk. 2022; 12: 5986. https://doi.or g/10.103 8/s41598-022- 09922-2 PMID: 353965 21\n50. Ermentrout GB, Billock VA. Flicker-Ind uced Phosphe nes. Encycloped ia of Comp utational Neurosci-\nence. 2018; 1–6. https://doi.o rg/10.1007/97 8-1-4614- 7320-6_511 –4\n51. Vollenwei der FX, Geyer MA. A systems model of altered conscious ness: integrati ng natural and drug-\ninduced psychos es. Brain Res Bull. 2001; 56: 495–507. https://doi.or g/10.1016 /s0361-92 30(01)0064 6-3\nPMID: 11750795\n52. Geyer MA, Vollenweider FX. Serotonin research : contribution s to understa nding psychoses . Trends\nPharmac ol Sci. 2008; 29: 445–45 3. https://doi.or g/10.101 6/j.tips.200 8.06.006 PMID: 19086254\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 17 / 18\n\n53. Mu ¨ ller F, Lenz C, Dolder P, Lang U, Schmidt A, Liechti M, et al. Increased thalamic resting-s tate con-\nnectivity as a core driver of LSD-indu ced hallucination s. Acta Psych iat Scand. 2017; 136: 648–657.\nhttps://doi.or g/10.111 1/acps.1281 8 PMID: 289403 12\n54. Ramsay IS. An Activation Likelihood Estimat e Meta-analy sis of Thalamocor tical Dysconne ctivity in Psy-\nchosis. Biologica l Psychiat ry Cognitive Neurosci Neuroimag ing. 2019; 4: 859–869. https://doi.o rg/10.\n1016/j.bps c.2019.04 .007 PMID: 312028 21\n55. Avram M, Rogg H, Korda A, Andreou C, Mu ¨ ller F, Borgwardt S. Bridging the Gap? Altered Thalamocor -\ntical Connec tivity in Psychot ic and Psychedeli c States. Frontiers Psychiatry. 2021; 12: 706017. https://\ndoi.org/10.33 89/fpsyt.2021 .706017 PMID: 34721097\n56. Sheffield JM, Huang AS, Rogers BP, Giraldo-Ch ica M, Landman BA, Blackford JU, et al. Thalamoc orti-\ncal Anatomica l Connect ivity in Schizoph renia and Psychotic Bipolar Disorder. Schizoph renia Bull.\n2020; 46: 1062–1 071. https://doi.or g/10.109 3/schbul/sbaa 022 PMID: 32219397\n57. Schmidt TT, Jagannathan N, Ljubljanac M, Xavier A, Nierhau s T. The multimodal Ganzfeld-in duced\naltered state of consciousn ess induces decrease d thalamo-c ortical coupling . Sci Rep-uk. 2020; 10:\n18686. https:// doi.org/10.10 38/s4159 8-020-750 19-3 PMID: 33122651\n58. Chen Y, Fallon N, Kreilkamp BAK, Denby C, Bracewel l M, Das K, et al. Probabilist ic mapping of tha-\nlamic nuclei and thalamocor tical functional connectiv ity in idiopathic generalis ed epilepsy. Hum Brain\nMapp. 2021; 42: 5648–56 64. https://doi.or g/10.1002 /hbm.2564 4 PMID: 34432348\n59. Wang Z, Zhang Z, Jiao Q, Liao W, Chen G, Sun K, et al. Impairmen ts of Thalamic Nuclei in Idiopathic\nGeneralize d Epilepsy Revealed by a Study Combining Morphologic al and Functiona l Connec tivity MRI.\nPlos One. 2012; 7: e39701. https://doi.or g/10.137 1/journal.po ne.0039 701 PMID: 22808050\n60. Kim JB, Suh S, Seo W, Oh K, Koh S, Kim JH. Altered thalamocor tical functional connectiv ity in idio-\npathic generalized epilepsy. Epilepsia. 2014; 55: 592–600. https://doi.or g/10.111 1/epi.12580 PMID:\n24650142\n61. Bolay H. Thalamocor tical Networ k Interruption: A Fresh View for Migrain e Symptoms. Turk J Med Sci.\n2020; 50: 1651–1 654. https://doi.or g/10.390 6/sag-2005-2 1 PMID: 32421284\n62. Tu Y, Fu Z, Zeng F, Maleki N, Lan L, Li Z, et al. Abnorm al thalamocor tical network dynamics in migraine .\nNeurolo gy. 2019; 92: e2706–e2716 . https://doi.o rg/10.1212/WN L.000000 0000007607 PMID:\n31076535\n63. Martinelli D, Castellazz i G, Icco RD, Bacila A, Allena M, Faggioli A, et al. Thalamocor tical Connectivity\nin Experime ntally-Induc ed Migraine Attacks: A Pilot Study. Brain Sci. 2021; 11: 165. https://doi.or g/10.\n3390/brains ci11020165 PMID: 335140 29\nPLOS ONE\nFlicker light phenome nology\nPLOS ONE | https://doi.or g/10.137 1/journal.po ne.02842 71 April 11, 2023 18 / 18","source_license":"CC-BY-4.0","license_restricted":false}