During a recording session, the contrast polarity of each stimulus check will be temporally modulated at a reversal frequency of 1 Hz (2 pattern reversals equates to 1 reversal cycle); therefore, each reversal occurs at 2 Hz or twice per second. This stimulus is termed a pattern reversal stimulus and has a duty cycle of 50%. The 15% and 85% contrast stimuli will be presented for each eye (the fellow eye was covered) for 15 seconds. The right eye will be the first one to be tested for all patients.