A late negativity – evoked by the auditory oddball paradigm: mismatch negativity, sustained field, or distinct component?

A relationship between the mismatch negativity (MMN) and selective adaptation of the N1m has been suggested by several studies. Here, magnetoencephalography was used to investigate the relationship between selective adaptation and auditory sustained fields (SF) in the context of the auditory oddball paradigm. Auditory evoked fields were recorded for pure tones of 1000 and 1050Hz presented to 19 listeners, while they watched a silent movie of their own choice. The tone duration was 50, 150 or 600ms in separate sets. The 1000Hz- and the 1050Hz tones were either used as standards or deviants in subsequent replications. If selective adaptation of the sustained field contributed to the MMN, we expected that the response would increase with tone duration.

A dipole model was generated consisting of two symmetrical pairs of dipoles in the left and right auditory cortex; one pair fitted to the MMN of the difference condition, the second pair fitted to the sustained fields evoked by the standard condition. The sources fitted to the MMN were located anterior to the sources fitted to the sustained fields evoked by standard tones. The source waveforms revealed a late negative wave evoked by deviants in the more anterior source, which occurred subsequent to, but with a similar source configuration as the main MMN. This late negativity increased with tone duration, but was already evoked by 50-ms tones, which was in contrast to the sustained fields evoked by standard tones. The sustained fields in the posterior source were not different between standard and deviant conditions. However, a second transient MMN component with a longer peak latency than the main, anterior MMN was also imaged by the posterior sustained-field source. In summary, source analysis of the differential response evoked by deviant tones indicates at least two separate generators of the MMN in auditory cortex. A late negativity follows the MMN in its main source, whose functional relationship with the MMN and sustained fields remains to be explored in more detail. Alternatively, the late negativity may be an additional component of the auditory response evoked by deviants in the auditory oddball paradigm.

Research supported by BMBF grant 01EV0712 (Junior Research Group Bioimaging).