Effect of frequency and rhythmicity on flicker light-induced hallucinatory phenomena

Abstract

Flicker light stimulation (FLS) uses stroboscopic light on closed eyes to induce transient visual hallucinatory phenomena, such as the perception of geometric patterns, motion, and colours. It remains an open question where the neural correlates of these hallucinatory experiences emerge along the visual pathway. To allow future testing of suggested underlying mechanisms (e.g., changes in functional connectivity, neural entrainment), we sought to systematically characterise the effects of frequency (3 Hz, 8 Hz, 10 Hz and 18 Hz) and rhythmicity (rhythmic and arrhythmic conditions) on flicker-induced subjective experiences. Using a novel questionnaire, we found that flicker frequency and rhythmicity significantly influenced the degree to which participants experienced simple visual hallucinations, particularly the perception of Klüver forms and dynamics (e.g., motion). Participants reported their experience of geometric patterns and dynamics was at highest intensity during 10 Hz rhythmic stimulation. Further, we found that frequency-matched arrhythmic FLS strongly reduced these subjective effects compared to equivalent rhythmic stimulation. Together, these results provide evidence that flicker rhythmicity critically contributes to the effects of FLS beyond the effects of frequency alone, indicating that neural entrainment may drive the induced phenomenal experience.

Fig 1

(A) The experiment comprised a 3x4 factorial design with 3 levels of rhythmicity (Rhythmic, Arrhythmic_norm and Arrhythmic_pairs) and 4 levels of frequency (3 Hz, 8 Hz, 10 Hz, 18 Hz). (B) In the experimental setup, participants were seated in a dark room 150 cm away from the stroboscope (Lumenate Growth Inc., Bristol, United Kingdom). The initial assessment involved the completion of TAS questionnaire. In the training phase, participants were exposed to FLS and familiarised with the Stroboscopic Visual Experience Survey (SVES) and ASC-R items. The subsequent flicker session consisted of twelve two-minute stimulation periods presented in a fully randomised order of conditions. Following each stimulation period, participants rated their experience using SVES and ASC-R items. A second session took place within a week of the first.

Fig 2. Differential effects of rhythmicity and frequency on FLS-induced hallucinatory phenomena.

(A) Effects of rhythmicity and frequency on reports of simple visual hallucinations. Ratings are shown from the SVES Simple Visual Hallucinations and ASC-R Elementary Imagery scales. (B) Effects of rhythmicity and frequency on visual dynamics, which encompasses motion and how much the experience changes over time. (C) Effects of rhythmicity and frequency on different types of visual patterns. Klüver forms SVES subscale consists of spirals, cobwebs, targets and grids. Other forms include SVES items of rippling items and flowing lines. Phosphene forms refers to lower order forms and includes SVES items for TV snow and blobs of light or colour. Bar charts display the difference in ratings between arrhythmic controls and rhythmic stimulation for each frequency. Significance is determined by Tukey tests comparing ratings between rhythmicity conditions at each frequency level [See S1 Table for Tukey test results].

Fig 3. Effects of rhythmicity and frequency on colour selection, depicted as a probability of each colour being selected.

This is expressed as sum of selections over the n = 20 participants. The mean selection probability is displayed, where for each participant the probability was based on the average of the first and second session (0, 0.5 or 1).