To provide exactly the same recording conditions throughout the longitudinal study, a PDP-11/23 has been continuously maintained for stimulus presentation. Electrophysiological data were amplified by 20k using a Grass Model 12 Neurodata system set to a bandpass of .01 to 30 Hz. Each trial was sampled for 1200 ms at 8 ms intervals beginning with a 200 ms prestimulus baseline. Each participant performed an auditory (Choice Reaction Time) task with Ag/AgCl electrodes placed at frontal, vertex, parietal, and occipital locations (Fz, Cz, Pz, Oz, P3, P4). An additional electrode was placed under the left eye and referred to linked ears for recording eye movement and blink artifacts. All active electrodes were referred to linked ears with a forehead ground. Eye blinks were tracked online using an oscilloscope. Any trial affected by eye artifact (blinks or eye movements greater than 50μV) were excluded online. Average ERPs were computed from artifact-free trials.
