Electroencephalographic (EEG) recording

The participants were seated on a comfortable chair in a Faraday chamber, with attenuated levels of light and noise, and were instructed to move as little as possible during the recording. Visual stimuli were presented with a subtended visual angle of 1.7° × 3.3° of arc, on a 19″ flat screen monitor with a vertical refresh rate of 120 Hz. The monitor was located at a distance of one meter from the participant. The EEG was recorded from 49 ring electrodes placed in an elastic cap (Easycap, GmbH), according to the International 10–10 system. All electrodes were referenced to an electrode attached to the tip of the nose, and an electrode positioned at Fpz served as ground. The horizontal electro-oculogram (EOG) was recorded via two electrodes placed at the outer canthi of both eyes, whereas the vertical EOG was recorded via two electrodes placed supra- and infra-orbitally on the right eye. The EEG was continuously digitized at a rate of 500 Hz (bandpass 0.01–100 Hz), and electrode impedances were maintained below 10 kΩ.

Once the signal was stored, ocular artifacts were corrected and the EEG was segmented by extraction of −150 to 1300 ms epochs, synchronized with each auditory stimuli. The epochs were then classified a posteriori as Standard, Deviant, and Novel conditions, depending on the type of auditory stimulus. Only epochs related to the auditory stimuli-Go visual stimuli were evaluated. Thus, three conditions were obtained: Standard (standard auditory stimulus-Go visual stimulus), Deviant (deviant auditory stimulus-Go visual stimulus), and Novel (novel auditory stimulus-Go visual stimulus). ERP waveforms for the three age groups (Young, Middle-aged, Old) in the three conditions are shown in Figure ​Figure11.

Grand-average event-related potential waveforms at Fz, Cz, and Pz, in the Standard, Deviant, and Novel conditions, for the three age groups (Young, Middle-aged, and Old). In all three age groups, a P3b component elicited by the visual target stimulus was observed in each condition, although this component was not evaluated in this study. A, auditory stimulus; V, visual stimulus. Digital bandpass filter: 0.1–20 Hz.

The signal was passed through a digital 0.1–30 Hz (24 dB/octave slope) bandpass filter and epochs were corrected to the mean voltage of the 150-ms pre-stimulus recording period. EEG segments exceeding ±100 μV, as well as the first five epochs of each block, were automatically excluded from the averages. Finally, for identification and measurement of MMN, P3a, and RON, the Deviant minus Standard (D-S) and Novel minus Standard (N-S) difference waveforms were obtained.