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

## TL;DR

This study shows that rhythmic flicker light causes stronger brain connections linked to visual hallucinations, offering insights into how the brain creates these experiences.

## Contribution

The study provides novel evidence linking rhythmic flicker-induced hallucinations to thalamocortical hyperconnectivity in higher-order visual areas.

## Key findings

- Rhythmic flicker light increases activation in higher-order visual cortices compared to arrhythmic stimulation.
- Stronger connectivity between ventroanterior thalamic nuclei and visual cortices correlates with the intensity of visual hallucinations.
- Thalamocortical changes are linked to higher-order functions rather than primary visual processing.

## Abstract

Aberrant thalamocortical connectivity occurs together with visual hallucinations in various pathologies and drug-induced states, highlighting the need to better understand how thalamocortical interactions may contribute to hallucinatory phenomena. Flicker light stimulation (FLS) at 10-Hz reliably and selectively induces transient visual hallucinations in healthy participants. Arrhythmic flicker elicits fewer hallucinatory effects while delivering equal amounts of visual stimulation, together facilitating a well-controlled experimental setup to investigate the neural correlates of visual hallucinations driven by flicker rhythmicity. Using rhythmic and arrhythmic FLS during fMRI scanning, we found that rhythmic FLS elicited stronger activation in higher order visual cortices compared with arrhythmic control. Consistently, we found that rhythmic flicker selectively increased connectivity between ventroanterior thalamic nuclei and higher order visual cortices, which was also positively associated with the subjective intensity of visual hallucinatory effects. As these thalamic and cortical areas do not receive primary visual inputs, it suggests that the thalamocortical connectivity changes relate to a higher order function of the thalamus, such as in the coordination of cortical activity. In sum, we present novel evidence for the role of specific thalamocortical interactions with ventroanterior nuclei within visual hallucinatory experiences. Importantly, this can inform future clinical research into the mechanistic underpinnings of pathologic hallucinations.

Flicker light stimulation induces an intense transient experience of elementary visual hallucinations, such as colored geometric patterns, that has phenomenal similarities to the visual experience induced by psychedelic drugs or during psychopathology. During fMRI scanning, we found that increased connectivity between ventroanterior thalamic nuclei and higher order visual cortices was associated with the reported intensity of the flicker-induced visual effects. Our results suggest that the role of thalamocortical hyperconnectivity during hallucinatory experiences may relate to a higher order function of the thalamus, such as the regulation of cortical activity. This novel finding results from a highly controlled experimental setup, which can be extended to inform the mechanistic underpinnings of hallucinatory experiences during psychopathology.