Although migraine is one of the most prevalent neurological conditions in the world, little is known regarding its cause and neural mechanism. Apart from intermittent headache attacks, migraineurs in previous research have reported strong visual sensations to light during the headache-free period, with striped patterns having been shown to induce strong visual illusions and discomforts. Other studies suggest that such abnormal experiences originate from the hyperactive, or hyperexcitable, visual cortex of migraineurs.
Testing a Hypothesis
For a study published in NeuroImage: Clinical, Terence Fong, PhD, and colleagues aimed to uncover neurological evidence supporting this hypothesis by comparing the visual-evoked potentials (VEPs) elicited by striped patterns of specific spatial frequencies (0.5, 3, and 13 cycles per degree [cpd]) between female migraineurs and non-migraineurs (controls). VEPs to the same patterns were also compared among non-migraineurs between those classified as hyper-excitable and non-hyperexcitable using a previously established behavioral pattern glare task. “Participants were asked to view certain striped patterns presented by a computer screen while their brain activities were recorded by electroencephalography (EEG),” explains Fong. They were instructed to gaze on a fixation point at the center of the grating.
Following the stimuli presentation, study participants ranked the intensity of the associated visual distortions (AVDs; visual pain, physical eye strain, unease, nausea, headache, dizziness, light-headedness, faint, shadowy shape, illusory stripes, shimmering, flickering, jitter, zooming, blur, bending of lines, and color distortions: red, green, blue, yellow) on a 7-point Likert scale (0 = not at all, 6 = extremely), with responses added together for a total AVD score for that grating (high, medium, or low frequency) and scores for each grating based on the average from the three repetitions for that spatial frequency.
Significantly Different Responses
“We found that migraineurs had a significantly different neural response compared with controls when gratings in high spatial frequency were presented,” says Dr. Fong. Indeed, migraineurs were found to have significantly increased amplitude of N2—an electrophysiological response influenced by the sensory processing of the visual cortex—for stimuli with 13 cpd gratings (Table). N2 is a negative component generally peaked around 200 ms post-stimulus presentation. “In other words, migraineurs have a much stronger activation at their visual cortex compared to the normal population when they look at patterns in higher spatial frequency. Such abnormal neural responses could be associated with their visual discomfort and other visual experiences.”
Non-migraineurs in the study who had stronger visual sensitivity to gratings appeared to have similar neural response patterns to migraineurs, albeit in an attenuated form, explains Dr. Fong.
Putting the Pieces Together
Based on the findings, Dr. Fong and colleagues believe migraineurs and some non-migraineurs both have a hyperexcitable visual cortex that lead to their abnormal visual sensations in their everyday life. “The ‘hyperexcitability’ appears to have shaped some people into being more neurologically vulnerable than others,” Dr. Fong says. “These patients would be more likely to have abnormal visual sensations or even migraine headaches, which can be triggered by various environmental or physiological factors, including strong visual stimulations, stress, sleep deprivations or hormonal change.”
Dr. Fong notes a desire for future research focused more on the “sub-clinical group”—those without migraine but with migraine-like experiences—identified in this study, as comparing this population with actual migraineurs both psychologically and physiologically could provide a better understanding of the factors that trigger or prevent migraine. “A better understanding of the brain’s physical structure would certainly help migraine research,” he adds.
Differences in early and late pattern-onset visual-evoked potentials between self-reported migraineurs and controls