Event-related potentials
schizophrenia-related abnormalities
The event-related potentials (ERPs) measured (via scalp EEG) included MMN during the oddball paradigm, the ASSR task, and the P300 wave during an active oddball task assessing amplitude to target tone. ERPs were recorded using a 64-channel system (SynAmp 2; Compumedics Neuroscan) with sintered Ag/AgCl electrodes at 1 kHz sampling rate with bandpass at 0.1 to 200 Hz. Impedance was kept below 5 kΩ. Participants sat in a semi-reclining chair inside a sound-attenuated chamber. A midline frontal electrode (FZ) was used for MMN measurement because this location typically shows the largest patient-control differences on MMN [ASSRs were recorded in the same setting while participants listened to click trains, delivered by headphones at 2.5, 5, 10, 20, 40, and 80 Hz. Based on previous frequency-specific findings of ketamine effects and schizophrenia-related abnormalities [For P300 assessments, 600 auditory stimuli were presented, of which 80% were standard tones (60 ms, 1000 Hz) and 20% were target tones (60 ms, 1400 Hz) presented at 75 dB. All tones had a 5 ms rise/fall time, with a stimulus onset asynchrony of 1300 ms. Participants were instructed to press a response button as fast as they could whenever they heard the target tone. No response was required for the standard tone. P300 responses were measured at a midline parietal electrode (PZ) following the same eyeblink correction routine used for MMN. Records were filtered at 0.1 to 30 Hz in 24 dB/octave, epoched, baseline-corrected, and threshold-filtered at ±75 µV for artifact rejection. Standard and target trials were averaged separately, followed by peak detection within a 250 to 400 ms post-stimulus window and followed by visual inspection to verify correct placement of each peak. Scoring was blinded.