Migraine is a neurological disorder manifested with periodic symptoms, most notably a throbbing headache on one side of the head. The periodicity reflects functional and morphological changes in the brain that fluctuate over time. Further, the intensity of headaches relates to lights, sounds, and smells. In migrainous brain, temporal processing of external stimuli and sequential recruitment of neuronal networks are often dysfunctional. Researchers assessed visual evoked potentials (VEPs) that can reveal altered patterns of the brain's electrophysiological activity and aid in understanding migraine pathophysiology. The non-pharmacologic measures such as meditation, yogic exercises, and deep breathing can be adjuncts to routine antimigraine therapy.
The present study compared P100 latencies and amplitude in visual evoked potentials of healthy controls and migraineurs. Researchers found delayed P100 latency and decrease P100 amplitude in migraine patients compared to controls. In addition, the study evaluated the effect of Rajyoga meditation on migraine patients and found early P100 latencies and higher P100 amplitude after training. Similarly, Kennard et al. found delayed P100 latency and higher P100 amplitude in migraine patients.24 Winter et al. found a decrease in P100 (ms) after practicing Rajyoga meditation in migraine patients25. However, in a study, Polich et al. obtained visual evoked potentials from 20 migraine patients and compared them with age-matched controls. They showed no substantial differences in N75, P100, and N145 (latencies and amplitudes), elicited using full- and half-field reversing checkerboard stimulus presentations.26 Mariani et al. compared twenty migraine patients with healthy controls and found significantly longer P100 latency in migraine patients.27
The present study showed a decrease in P100 amplitude in migraine patients compared to controls. However, many neurophysiological studies found higher P100 amplitude in migraineurs. The higher P100 amplitudes in visual evoked potentials resulted from a decrease in habituation between attacks. 28–29 This implies that cortical information processing gets modified in most migraine patients between attacks, which can be considered an endophenotypic biomarker of the disorder. The underlying neural mechanisms might involve lower preactivation levels of sensory cortices due to thalamocortical dysrhythmia and low serotonergic tone, but these mechanisms are still under research. 30–33 Theoretically, one explanation was seen in the ceiling theory 34–35, based on the assumption that for evoked potentials, after reaching a ceiling point, cortical reactivity is reduced, which initiates a habituation response. Also, a lower preactivation level in migraine patients would cause a delayed or missing habituation because the ceiling would be reached later than healthy people. In this context, migraine patients' higher amplitudes of evoked potentials were due to the missing or reduced habituation. However, some researchers could not reproduce these findings36–42, attributed to various methodological issues, including the absence of blinding33 or differences in migraine phenotypes43.
The study showed substantially reduced N70 and P100 latencies after performing Rajyoga meditation. The shifting autonomic balance in favor of parasympathetic instead of sympathetic might explain the above phenomenon. However, it requires further studies on Rajyoga meditation. In addition, there was a significant increase in post-intervention P100 amplitude. Similarly, results obtained by light flashes before, during, and after Qigong meditation showed an increase in VEP amplitudes (N80, P115, N150, P200, N280) in advanced practitioners not in novices and beginners. However, a significant decrease in the VEP amplitudes was obtained within the same study group of Qigong practitioners of other schools44. Contrarary, Schöne et al. (2018) observed that modulations in much higher latency waveforms (500–6,800 ms) resulted in reduced VEP amplitudes acquired during multiple object tracking paradigms in the group that performed training in mindful breath awareness, compared to an active control group, trained in muscular relaxation. The results were attributable to an increased ability to ignore irrelevant stimuli with lower attentional effort after meditation training45. The results of evoked potentials (EP) are highly dependent on the onset, the number of repetitions, and the type (e.g., auditory or visual) of stimuli and rely on signal averaging around an event. Most results suggest that meditation affects the processes underlying the generation of endogenous EP; it is still tricky to thoroughly compare these studies because of distinct EP components evaluated, stimuli diversity, and most importantly, the mental state of the participants.