Objectives – Reduced habituation of visual evoked potentials (VEP) has been reported in migraine. We aimed to study if preattack excitability changes were related to check size using a paired longitudinal design. Materials and Methods – Magnocellular and parvocellular functions were studied with monocular 31´ and 62´ checks in 33 adult migraine patients without aura (MwoA), 8 with aura (MA) and 31 controls. VEP was recorded in four blocks of 50 stimuli. N1P1 and P1N2 amplitudes were measured. Sessions were classified as preattack or interictal. Results – MA patients had significantly higher P1N2 and N1P1 amplitude than the controls and MwoA. VEP amplitude habituation was not found in controls. Migraine patients had significantly higher P1N2 amplitude before the attack compared with a paired interictal recording for large checks. Conclusions – Cortical excitability is high in MA. Headache severity affects visual excitability. Increased P1N2 VEP amplitude before the attack suggests a cyclic decreased intracortical inhibition in extrastriate magnocellular pathways in migraine.