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 what the neuronal correlates of these experiences are and where they emerge within 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.

Thalamocortical interactions reflecting the intensity of flicker light-induced visual hallucinatory phenomena

“…Together, the basic visual features that activates these cortical regions make up the key features of elementary visual hallucinations, namely coloured geometric patterns that move around the visual field. Indeed, 10 Hz rhythmic FLS was shown to increase ratings of perceived motion, shapes and patterns, found here and shown extensively in previous research (Smythies, 1959(Smythies, , 1960Allefeld et al, 2011;Schwartzman et al, 2019;Bartossek et al, 2021;Amaya et al, 2023bAmaya et al, , 2023aMontgomery et al, 2023), suggesting that the activation of these visual cortical areas may contribute to the hallucinatory perception of elementary features.…”

“…On the contrary, flicker light stimulation (FLS) can reliably and selectively induce visual hallucinatory effects in healthy participants via closed-eye application of stroboscopic light. The flicker-induced effects are mainly constituted of elementary visual hallucinations, referring to the perception of geometric patterns and colours devoid of semantic content (Bartossek et al, 2021;Amaya et al, 2023a). These elementary phenomena are also frequently reported during psychedelic experiences (Schmidt and Berkemeyer, 2018;Prugger et al, 2022), Charles Bonnet Syndrome (Jan and Castillo, 2012) and for some patients with psychosis (Ommen et al, 2019).…”

Neural entrainment induced by periodic audiovisual stimulation: A large-sample EEG study

“…We also observed weaker evidence of neural entrainment to higher frequencies ( > 40 Hz, strobe B) which was limited to parietal and occipital channels. Alongside these entrainment effects, the stroboscopic or “flicker” stimulation utilized in the current study is known to induce geometric perceptions through closed eyes [27, 20, 11, 12], an effect which has received increased attention and research in recent years [16, 7, 5, 8, 17, 18]. Taken together, these findings underscore the potential of PAVS to induce powerful neural effects.…”

“…Like tACS [2], PAVS entrains neural oscillations to the frequency of stimulation. In fact, PAVS reliably and consistently induces geometric phosphenes in participants with little effort on the part of the experimenter [7, 18], whereas visual phosphenes require precise and careful targeting of visual cortex with tACS and TMS [2]. Given that at least one study has suggested that PSS has plasticity-promoting effects [5], we advocate for future work that will compare PAVS (or broader forms of PSS) to TMS as a means of enhancing neural plasticity and potentially treating mood disorders.…”

“…To better understand this novel approach to brain stimulation, we studied periodic audiovisual stimulation (PAVS) in a large sample. While many recent studies have described the visual effects of stroboscopic stimulation [16, 17, 8, 18], the neural effects are comparatively understudied, with just a handful of published reports [19, 20, 21] which have largely focused on examining EEG frequency changes. We thus sought to fill a gap in the literature by rigorously searching for entrainment effects, which have so far only been observed in small, uncontrolled studies [19, 22].…”