Spectral and temporal stimulus characteristics in the processing of abstract auditory features.

The processing of abstract stimulus features in the human brain was studied by presenting the subjects with frequent standard tone pairs and infrequent deviant tone pairs. Both pairs varied randomly over a wide frequency and/or intensity range, there being no physically constant standard stimulus. The common feature of the standard pairs was the direction of change within the pair, e.g. the second tone was louder in intensity and/or higher in frequency than the first tone. Deviant pairs, having opposite feature-change direction, elicited the mismatch-negativity (MMN) event-related potential component. MMN was similar to deviations in the direction of frequency and intensity changes and showed no additivity for simultaneous changes in both feature directions. Moreover, MMN was elicited even when the within-pair interval exceeded the 200 ms limit of auditory temporal integration. Results demonstrate that extraction of abstract features is not limited to frequency-based rules, nor is it dependent on temporal integration mechanisms. The lack of MMN additivity between violations of multiple abstract rules suggests that the processing of higher-order invariances differs from that of simple physical features.