In our study, the presentation of an auditory infrequently presented (rare/target) stimulus produced a robust and highly significant increase in phase locking of EROs, as compared to the frequently presented stimulus, both within and between brain areas in all frequency bands. Higher phase-locking to target vs. non target tones in a number of ERO frequency ranges has been reported previously in human subjects [101-104]. In most cases the authors ascribed the differences in phase locking to the response requirements, and or salience of the stimulus. In the present study we compared responses to infrequent tones that required response (active task) to those that did not (passive task). We found that the effect of the tone characteristics (amplitude, frequency) produced more significant and robust differences in EROs than the effect of the task. We suggest that in this simple sensory paradigm the most likely explanation of this finding is that the results observed represent a change in neural state associated with attending to a more novel, possibly environmentally relevant noise rather than to any task requirements or salience. These findings are
