The stimulus will be presented on a gamma corrected Acer V173BM 17-inch LCD monitor, having a refresh rate of 75 frames / second. The CSF pattern stimulus will consist of black/white horizontal gratings (24° and 16° circular field, 98% contrast and 102.28 candelas/m2 mean luminance), reversing at 15 reversals/second) with a duration of 25 seconds. A red spinning ring will be used as a fixation target with a diameter of 1 cm and 1 mm of thickness. This target ring will be at the corner of four checks, centered on the stimulus field.