Changes in total volume in the frontal cortex were estimated for regions of interest in the three frequencies bands in three stimulus paradigms where the frequency of the tone, the loudness of the tone and the probability of the tone occurring were varied. Results of these analyses are presented in figure 1. In the first stimulus paradigm, the rare tone occurred 20% of the time and the standard (Freq=1 kHz) and rare (Freq=2 kHz) tones were of equivalent loudness In this paradigm there was a significant volume difference found between the standard and rare tones within a specific frequency range and region of interest. Following the rare tone a reduction in total volume of the 50-350 ms region of interest in the 0.75-3.5 Hz frequencies was found (rare tone= 320 ± 130, standard tone= 338 ± 125; F=7.2, df=1,64, p<0.009). This reduction in total volume may be related to the N1a component as previously published data has demonstrated that the N1a component of the ERP recorded at this site in this latency range is significantly larger following the standard tone
