Based on fresh research from the Netherlands Institute for Neuroscience1, flickering light generates “standing waves” of brain activity that lead to hallucinations in our brain.
Picture yourself sitting on the bus or train and closing your eyes. Sunlight flickering through the trees suddenly fills your mind with kaleidoscopic hallucinatory patterns.
Brion Gysin, co-inventor of the Dreamachine, had this experience when visiting Marseille in the late 1960s. The idea that flashing lights can produce hallucinations did not surprise scientists.
Stroboscopic Stimulation
Strobe light, familiar to many from dance floors, has been used in neuroscience research for 200 years. In 1819, neuroscientist Jan Purkinje discovered that bright full-field light flashes can make our brain spontaneously perceive geometric patterns and images.
Flickering-light stimulation in the scientific world was picked up by members of the 1960s underground and the Beat Generation in their search for mind-altering experiences. Some built their own stroboscopes that could cause dramatic hallucinations without using drugs.
The way that stroboscopic light creates vivid, unrealized visuals has attracted both scientists and artists. What is the mechanism for flicker-induced hallucinations?
Travelling vs Standing Waves
Mathematicians proposed that these hallucination patterns might be standing waves, or striped patterns, of neuronal activity in the visual cortex. Because of the wiring of our visual system, the direction of these striped patterns determines whether we perceive a pinwheel, bullseye, or revolving spiral.
There are two sorts of waves: travelling and standing. Travelling waves show as ripples spreading from a raindrop in a still pond, whereas standing waves form when two persons shake a skipping rope synchronously at both ends.
This results in a pattern of waves travelling up and down. But is there any proof that standing waves can arise in the brain?
Rasa Gulbinaite and colleagues examined standing wave pattern formation in the mouse brain in order to investigate this.
Flicker-induced Cortical Patterns
Gulbinaite studies brain waves and the effect rhythmic lights, sounds, and touch have on our brain rhythms. This is difficult to measure in humans because our brains have folds, and what happens at the bottom of the lake is not always what we can measure on the surface.
“But mice have a flat brain, making it easier to map the activity on the surface. In our experiments, we exposed mice to flickering lights. These mice were genetically modified and had a fluorescent label attached to specific neurons. When these neurons were active, they fluoresced, allowing us to track brain activity. We used a high-speed camera to take pictures of the brain while the animals looked at the flickering light,”
Gulbinaite said.
When the researchers stimulate a specific spot in the visual field, they expect to find activity in the visual cortex area associated with that location. This is exactly what they noticed. However, they saw waves of neuronal activity propagating through the visual cortex, beginning from the stimulated region.
Convincing Visual Cortex Evidence
These waves looked like the ripples left by a raindrop falling into a pond.
“When raindrops fall at regular intervals, their ripples spread out, bounce off the banks, interfere with each other, and can create patterns similar to standing waves. Some parts of the pond’s surface appear still, while others oscillate with maximum amplitude. This is exactly what occurred at higher strobe light frequencies in our experiment. The traveling waves transformed into standing waves, with some regions of the visual cortex becoming more active and others less so,”
Gulbinaite said.
The results support the earlier concept that flickering light can induce standing waves in the visual brain. Scientists cannot detect if mice hallucinated geometric patterns since they cannot ask: this is the most problematic component of this investigation.
“However, there is good reason to believe that the standing waves we observed could be the mechanism behind flicker-induced hallucinations. People report that when the flickering light frequency is higher, they perceive finer hallucinatory patterns. And that is exactly what we also saw in the brains of mice. As the frequency increased, the patterns in the visual cortex became finer. We don’t have a definitive answer yet, but we are now showing convincing evidence for the first time,”
Gulbinaite added.
- Gulbinaite, R., Nazari, M., Rule, M. E., Bermudez-Contreras, E. J., Cohen, M. X., Mohajerani, M. H., & Heimel, J. A. (2023). Spatiotemporal resonance in mouse primary visual cortex. bioRxiv, 2023-07.