INVESTIGATIONS OF HEADACHES: VISUAL EVOKED POTENTIALS
The visual evoked response is a more recent technique that is also based on recording electrical activity from the brain. It is still only a research tool as far as migraine is concerned. When an object is seen, an electrical discharge passes along the optic pathway to cause a specific but tiny response in the brain. When the same impulse is presented repeatedly the responses can be added up by an EEG machine to give a much bigger response, shown on a screen as a wave-form. Using this technique, the time taken for the impulse to travel along the op tic pathway scan is measured, by taking the time from seeing the object (the stimulus) to the peak of the wave. Diseases such as inflammation or pressure on the optic nerve will slow the response, whilst damage to the brain can alter the shape of the wave-form.Because it has been suggested that repeated attacks of severe migraine could possibly damage the brain, VERs have been done in some cases for the purposes of research. Patients with migraine had VERs measured from each side of the head. Migraine patients have a delay in the time interval from the stimulus to the wave, indicating that the nerve impulse travels more slowly through the brain than in non-migrainous subjects. The wave responses, rather than being smaller, were in fact larger, and this did not seem to make sense nor be easily explicable. Those whose headaches were on the left side of the head had a much bigger wave-form than those whose headaches were on both sides or on the right side, another observation for which there is as yet no certain explanation.The prolonged latency can be explained more easily: this is not due to stroke-like damage since in strokes the latencies are normal (only the height of the waves is less). In certain situations changing the neurotransmitters in the brain can alter the latency, so the most likely explanation is that migraine patients have some difference with respect to neurotransmitters. Which neurotransmitter is involved is not yet known, but it is unlikely to be due to noradrenalin or adrenalin since stress can actually speed up the response.These differences between migraine subjects and non-sufferers are still circumstantial and to a large extent contradictory, suggesting again that migraine is possibly not a single disorder.
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