ERPs were elicited by auditory stimuli that were presented through a small speaker centered approximately 20 cm above the rat’s head. EEG signals were recorded on a Nihon Kohden polygraph with sensitivity set at 75 microvolts/mm, using a bandpass of 0.3–35 Hz. ERPs were elicited by an acoustic “oddball” plus noise paradigm identical to that described above for the human subjects. Signals were transferred to an Apple computer and digitized at a rate of 256 Hz. The EEG amplifier input range corresponding to the full range of the 12-bit analog-to-digital converter was about +/- 250 microvolts. Periodic calibration results were used to scale the digitized EEG to microvolts. Trials containing excessive movement artifact were eliminated prior to averaging (<5% of the trials). Following the study, the rats that survived all procedures were decapitated and their brains immediately frozen on dry ice and stored at -80°C for verification of electrode locations. Of the 54 rats 46 had available data, free of artifacts, and with electrode locations verified in the correct positions.
