In major depressive disorder (MDD), event-related potentials that are involved in auditory cortex function (i.e. N100 and P300) often have greater latencies and decreased amplitudes. The auditory mismatch negativity (MMN) is thought to be produced by generators in the auditory cortex, as well as the frontal lobes. Reports on differences in MMN in those with MDD have been varied. It was hypothesized that the wide range of results in the literature may be due to the use of different deviant types in eliciting the MMN. To attempt and explain these inconsistencies, the current study employed a multifeature MMN paradigm with 5 deviant tone types in community-dwelling participants with a diagnosis of MDD. We found those with MDD had higher MMN amplitudes following tones that deviated in intensity and location, but no difference in MMNs elicted by the other deivants (relative to unaffected controls). Location MMN deviants were negatively correlated with depression severity scores (i.e. larger MMN with greater severity). We also found longer MMN latencies following the pitch deviant. These results suggest the early auditory change detection process is altered in MDD, but only following certain types of auditory stimuli. Potential explanations for these findings, including high levels of anxiety and the influence of tryptophan are explored. Equally, the current report highlights the importance of using various deviant types when examining the MMN in clinical populations.

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Auditory hallucinations (AHs) are among the cardinal symptoms of schizophrenia (SZ). During the presence of AHs aberrant activity of auditory cortices have been observed, including hyperactivation during AHs alone and hypoactivation when AHs are accompanied by a concurrent external auditory competitor. Mismatch negativity (MMN) and P3a are common ERPs of interest within the study of SZ as they are robustly reduced in the chronic phase of the illness. The present study aimed to explore whether background noise altered the auditory MMN and P3a in those with SZ and treatmentresistant AHs. Methods: MMN and P3a were assessed in 12 hallucinating patients (HPs), 11 nonhallucinating patients (NPs) and 9 healthy controls (HCs) within an auditory oddball paradigm. Standard (P = 0.85) and deviant (P = 0.15) stimuli were presented during three noise conditions: silence (SL), traffic noise (TN), and wide-band white noise (WN). Results: HPs showed significantly greater deficits in MMN amplitude relative to NPs in all background noise conditions, though predominantly at central electrodes. Conversely, both NPs and HPs exhibited significant deficits in P3a amplitude relative to HCs under the SL condition only. Significance: These findings suggest that the presence of AHs may specifically impair the MMN, while the P3a appears to be more generally impaired in SZ. That MMN amplitudes are specifically reduced for HPs during background noise conditions suggests HPs may have a harder time detecting changes in phonemic sounds during situations with external traffic or "real-world" noise compared to NPs.

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