Electrophysiological studies of stimulus-evoked brain activation suggest that sensory processing in migraine patients is abnormal between attacks. The main findings are increased amplitudes and decreased habituation of cortical evoked potentials. Recent findings in healthy individuals showed that evoked potentials result mainly from phase resetting of background electroencephalographic activity. We recorded single trial visual evoked potentials during repetitive visual stimulation in migraine patients and healthy controls and analyzed these in the frequency domain for amplitude and phase. Increases in visual evoked potential amplitudes in migraine patients are explained almost entirely by increases in local amplitude, rather than increases in phase synchrony across trials. As amplitude modulation is generally considered more energy demanding than phase synchronization, this may explain the increased vulnerability of migraine patients to sensory stressors and the effectiveness of drugs that reduce evoked potential amplitudes or enhance aerobic energy metabolism.