Summary: A new study reveals how flickering lights can trigger hallucinations by causing standing waves in the brain’s visual cortex. Researchers observed that high-frequency flickering lights create ripple-like waves of neural activity, which turn into standing waves at higher frequencies.
These standing waves might explain the geometric patterns people often perceive during flicker-induced hallucinations. While the exact mechanism is still under study, this research offers the first strong evidence for how brain waves contribute to visual hallucinations.
Key Facts:
- Flickering lights create standing waves in the visual cortex, altering perception.
- High-frequency light leads to finer, more detailed hallucinations.
- Study provides the first direct evidence linking brain waves to hallucinations.
Source: KNAW
You’re sitting on the bus or train and close your eyes. Sunlight flickering through the trees suddenly fills your mind with kaleidoscopic hallucinatory patterns.
This is what Brion Gysin experienced during his trip to Marseille in the late 1960s. The fact that flashing lights can cause hallucinations was not surprising to scientists.
Stroboscopic light, familiar to many from dancefloors, 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 to spontaneously perceive geometric patterns and images.
Flickering-light stimulation in the scientific community was picked up by members of the 1960s underground—the Beat Generation—who sought mind-altering experiences and manufactured their own stroboscopes that could induce vivid hallucinations without drugs.
Both scientists and artists were fascinated by how stroboscopic light creates vivid images that are not there. What is the mechanism behind flicker-induced hallucinations?
Traveling wave versus standing wave
Mathematicians hypothesized that these hallucinatory patterns could be standing waves, or striped patterns, of neural activity in the visual cortex. Due to specific wiring of our visual system, direction of these striped patterns would determine what is perceived: a pinwheel, bullseye or rotating spiral. There are different types of waves: traveling and standing waves.
Traveling waves appear as ripples spreading from a raindrop in a still pond, while standing waves occur when two people shake a skipping rope at both ends synchronously. This creates a pattern of waves moving up and down.
But is there evidence that standing waves can form in our brain?
Waves in the mouse brain
To investigate this, Rasa Gulbinaite and her colleagues looked at the formation of standing wave patterns in the mouse brain. Rasa Gulbinaite explains: “I study brain waves and the effect rhythmic lights, sounds, and touch have on our brain rhythms.
“In humans, this is difficult to measure because our brain has folds and what happens on the bottom of the lake is not necessarily 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 the 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 high-speed camera to take pictures of the brain while the animals looked at the flickering light.”
Ripples in a pond
“When we stimulate a specific location in the visual field, we expect to see activity in the corresponding area of the visual cortex that represents this location. This is precisely what we observed. However, we also noticed waves of neural activity propagated through the visual cortex, originating from the stimulated spot.”
“These waves resembled the ripples created 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.
“Our findings prove the earlier hypothesis that flickering light can cause standing waves in the visual cortex. Whether mice also hallucinated geometric patterns, we cannot tell because we cannot ask: this is the most challenging part of our research.
“However, there is good reason to believe that 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.”
About this hallucinations and visual neuroscience research news
Author: Eline Feenstra
Source: KNAW
Contact: Eline Feenstra – KNAW
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“Spatiotemporal resonance in mouse primary visual cortex” by Rasa Gulbinaite et al. Current Biology
Abstract
Spatiotemporal resonance in mouse primary visual cortex
Highlights
- Widefield imaging of spatiotemporal responses to visual flicker in iGluSnFR mice
- Mouse V1—just like human—resonates in response to specific flicker frequencies
- Flicker-induced cortical patterns form standing waves confined to the visual cortex
- These patterns correspond to standing-wave solutions of the linear wave equation
Summary
Human primary visual cortex (V1) responds more strongly, or resonates, when exposed to ∼10, ∼15–20, and ∼40–50 Hz rhythmic flickering light.
Full-field flicker also evokes the perception of hallucinatory geometric patterns, which mathematical models explain as standing-wave formations emerging from periodic forcing at resonant frequencies of the simulated neural network.
However, empirical evidence for such flicker-induced standing waves in the visual cortex was missing.
We recorded cortical responses to flicker in awake mice using high-spatial-resolution widefield imaging in combination with high-temporal-resolution glutamate-sensing fluorescent reporter (iGluSnFR).
The temporal frequency tuning curves in the mouse V1 were similar to those observed in humans, showing a banded structure with multiple resonance peaks (8, 15, and 33 Hz).
Spatially, all flicker frequencies evoked responses in V1 corresponding to retinotopic stimulus location, but some evoked additional peaks.
These flicker-induced cortical patterns displayed standing-wave characteristics and matched linear wave equation solutions in an area restricted to the visual cortex.
Taken together, the interaction of periodic traveling waves with cortical area boundaries leads to spatiotemporal activity patterns that may affect perception.
