The second ERP stimulus paradigm differed from the first in that the probability of the presentation of the rare and standard tone was equivalent; this meant that the only difference in stimuli characteristics between the standard and rare tones was the frequency of the tone. Never-the-less a significant reduction was found in the total volume of the 0-50 ms region of interest in the 0.75-3.5 Hz frequencies in response to the rare tone as compared to the standard (rare tone=1004 ± 528, standard tone 1107 ± 526; F=28.8, df=1,53, p<0.0001). A comparison of the total volume of ERO regions of interest between the rare tone presented in paradigm 1 (20% probability) and that presented in paradigm 2 (50% probability), both of which had the same frequency and loudness, did not produce any statistically significant results.
