Researchers Use fMRI To Map Impacts Of Flicker On The Brain

This study used ultra–high-field 7T fMRI to examine how temporal light modulation (TLM) (aka flicker) at different frequencies activates or deactivates brain regions in healthy adults, with a particular focus on whether non-perceptible flicker still drives visual cortex responses. Participants viewed LED stimuli modulated at 50 Hz (clearly perceived as flicker) and at ≥100 Hz (subjectively steady light), allowing direct comparison of neural responses below and above the critical flicker fusion threshold under tightly controlled, static viewing conditions.

The authors ground their work in the modern LED context, where dimming via pulse‑width modulation and complex, dynamic lighting systems have reintroduced TLM‑related concerns once largely solved by high‑frequency electronic ballasts. They review the three main temporal light artifacts—flicker, stroboscopic effect, and phantom array effect—and emphasize that the current experiment isolates flicker‐type conditions only: static observer, static environment, no motion and no eye‑movement‑dependent artifacts. Prior electroretinogram and EEG findings show that neural tissues respond to TLM well above conscious detection thresholds and that such stimulation can alter arousal, eye movements, and cortical activity, motivating a spatially resolved imaging approach.

Methodologically, the study implemented a blocked design in which TLM conditions were alternated with a constant‑light baseline, enabling the authors to map BOLD signal increases (activation) and decreases (deactivation) across cortical and subcortical regions. A 50 Hz run served as a manipulation check, since this frequency is easily perceived as flicker and is known from earlier fMRI work to strongly engage the occipital lobe. Two additional runs used high‑frequency TLM at or above 100 Hz, which participants experienced as steady light, to test whether non‑perceptible modulation still produces measurable visual cortex activation.

As expected, 50 Hz stimulation produced extensive activation throughout visual areas, with pronounced responses in both medial and lateral occipital cortex and the strongest signals in lateral occipital regions. At the same time, several regions showed deactivation relative to constant light, including medial posterior cortices (precuneus, posterior cingulate, retrosplenial cortex) and frontal and mediotemporal areas such as insula, central operculum, supplementary motor area, and frontal pole. The authors interpret this pattern as a possible shift of resources and attentional focus from internally oriented or default‑mode processing toward externally driven visual input during flicker exposure.

Critically, 100 Hz TLM, despite being subjectively non‑flickering, also elicited clear but weaker activation in the visual cortex. Activity appeared in primary and secondary visual areas (V1/V2) and in the inferior division of the left lateral occipital cortex, with spatial clusters largely overlapping those seen at 50 Hz and again showing peak responses in lateral occipital regions. This demonstrates that visual cortical neurons continue to register high‑frequency modulation beyond conscious perception, implying that the critical flicker fusion threshold reflects a limit of awareness rather than a limit of retinal or cortical sensitivity.

Deactivation patterns also accompanied high‑frequency stimulation, with several regions exhibiting higher activity during the constant‑light baseline than during TLM, though the precise cognitive meaning of these changes remains uncertain. The authors suggest these deactivations may reflect attentional reallocation or altered ongoing brain dynamics, but they call for further work to clarify these mechanisms and to link them more directly to behavior and symptom reports. They also discuss the relationship between individual critical flicker fusion thresholds and visual cortex excitability, noting that such measures may index trait‑like neural characteristics relevant to TLM sensitivity.

The study provides high‑field fMRI evidence that both visible (50 Hz) and invisible (≥100 Hz) TLM drive visual cortex activation, with non‑perceptible flicker producing qualitatively similar but reduced responses. These findings reinforce the idea that lighting systems with high‑frequency modulation can influence brain activity even when users experience the light as steady, which has implications for cognitive performance, comfort, and health, especially in individuals sensitive to visual stress or temporal light artifacts. The authors argue that lighting practice and standards should account not only for consciously perceived flicker and temporal artifacts, but also for these nonvisual neural effects of TLM when designing and specifying LED‑based installations.

Study citation:

Lindén, J., Hemphälä, H., Markenroth Bloch, K., Edvinsson, L., Surova, Y., Mauritsson, J., … Björkstrand, J. (2025). Temporal Light Modulation Activation in Visual Cortex – A 7T fMRI Study on Healthy Subjects. LEUKOS, 1–17. https://doi.org/10.1080/15502724.2025.2583961

More information is available here.

Image above: Pixabay.com

David Shiller

David Shiller is the Publisher of LightNOW, and President of Lighting Solution Development, a North American consulting firm providing business development services to advanced lighting manufacturers. The ALA awarded David the Pillar of the Industry Award. David has co-chaired ALA’s Engineering Committee since 2010. David established MaxLite’s OEM component sales into a multi-million dollar division. He invented GU24 lamps while leading ENERGY STAR lighting programs for the US EPA. David has been published in leading lighting publications, including LD+A, enLIGHTenment Magazine, LEDs Magazine, and more.

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