Causes and Triggers
Simplified explanation of the cause
Visual stress occurs when certain images trigger an overreaction in the brain. It can lead to physical discomfort and visual perceptual difficulties.
In simple terms, visual stress happens when lights, patterns, movement or colours cause the brain to overreact. It can lead to nausea, headaches, fatigue or sensitivity to light, as well as difficulties with reading, attention and sensory processing.
Scientific theory of the cause
The most widely supported theory of visual stress explains that it is the result of a general overexcitation of the visual cortex due to hypersensitivity to contrast or pattern glare. A neural mechanism for visual stress has found support in studies showing that the visual stimuli that induce discomfort also induce a large haemodynamic response, both in absolute terms, and relative to the response to comfortable stimuli (Wilkins A., A physiological basis for visual discomfort: application in lighting design. Lighting Res Technol. 2016; 48:44---54).
This large haemodynamic response triggers neurons that signal movement or colours, which are consequently experienced as illusions or hallucinations. In other words, the visual cortex works normally until stimulation is too strong, and then cortical processing becomes inefficient (Wilkins et al., 2004b).
For a detailed description of how the initial triggers progress into symptoms, see the Visual Stress Flowchart.
Triggers include lights, glare, patterns, contrast and colours. To reduce triggers and their effect on symptoms, it is important to understand them to then be able to make the required changes.
Potentially, any stimulus that creates on-off signals in the visual cortex can trigger visual stress. The most obvious examples are high contrast, rapidly flashing or flickering illumination such as:
Fluorescent and some LED lighting
CRT computer monitors with low refresh rate
Television shows and movies
Bright sunlight viewed through trees when moving in a vehicle
All these stimuli cause headaches in many people, especially those who suffer from migraines, and they also trigger seizures in people with photosensitive epilepsy.
Glare from reflection can sometimes be avoided by moving the angle of the source, however, other sources of glare are not so easily reduced, for example:
Vehicle lights at night
Reflection of lights on wet surfaces
Shine on desks
Reflections from walls, posters and windows in classrooms
Geometric repetitive patterns, such as stripes, create on-off signals similar to those caused by flashing lights, which explains why such patterns can cause unpleasant somatic and perceptual side effects.
A proportion of people who suffer from photosensitive epilepsy also report that stationary gratings, stripes or checkered patterns, can trigger seizures, especially when there is a strong light/dark contrast in the pattern. Research has found that about 30 per cent of photosensitive patients were also sensitive to patterns.
Text is a pattern:
Since text can resemble a pattern of stripes with visually stressful characteristics, this explains why it can provoke perceptual distortions and cause headaches. The visual grating created by moving the eyes across lines of print, especially where the pattern is glaring, can generate similar physiological effects to those created by flashing lights.
Visual Stress symptoms fall on a continuum:
These findings suggest a continuum of photosensitivity for people suffering from photosensitive epilepsy, migraine and visual stress. Individuals who suffer from visual stress (but not photosensitive epilepsy or migraine) would be regarded as ‘moderately photosensitive’, so that their symptoms are not as extreme as those of individuals who suffer from photosensitive epilepsy or migraine, and these symptoms are less easily triggered.
What can be done about it?
Research suggests that because the wavelength of light is known to affect neuronal sensitivity, the use of colour could reduce over-excitation, redistributing cortical hyperexcitability and thus reducing perceptual distortion and headaches.