EROs were elicited with acoustic oddball paradigms, these paradigms are similar to those demonstrated to successfully generate ERPs in human, monkey, rat, and mouse studies (Ehlers, 1988; Kaneko et al., 1996; Ehlers et al., 1999; Ehlers & Somes, 2002). The tones were generated by a programmable multiple-tone generator, the characteristics of which have been described previously (Polich et al., 1983). Three different stimulus change paradigms were used. In these paradigms the loudness of the tones, the frequency of the tones, and the probability of their occurrence, were varied. This allowed for the assessment of how changes in characteristics of the auditory stimulus would modify EROs at the two brain sites. The stimulus characteristics for the three paradigms were: paradigm (1) standard tone: 80% probability, 1kHz frequency, 70 dBSPL loudness, rare tone: 20% probability, 2kHz frequency 70 dBSPL loudness; paradigm (2) standard tone: 50% probability, 1kHz frequency, 70 dBSPL loudness, rare tone: 50% probability, 2kHz frequency 70 dBSPL loudness; paradigm (3) standard tone: 80% probability, 1kHz frequency, 70 dBSPL loudness, rare tone: 20% probability, 2kHz frequency 85 dBSPL loudness.
