Dysfunction in early auditory processing in major depressive disorder revealed by combined MEG and EEG.

BACKGROUND: Patients with major depressive disorder (MDD) show impairments in cognitive functions. However, neural mechanisms underlying these disturbances are poorly understood. We investigated whether MDD affects neural mechanisms of involuntary attention studied by auditory evoked potentials (AEPs) and auditory evoked magnetic fields (AEFs).
METHODS: AEPs and AEFs were studied in a passive odd-ball paradigm in 13 drug-free patients with unipolar MDD during an acute episode and 12 age-and sex-matched healthy subjects. Auditory responses to monaurally presented frequent "standard" tones, infrequent "deviant" tones (10% and 20% frequency change) and occasional "novel" sounds (complex sounds) were simultaneously recorded with whole-head magnetoencephalography (MEG) and electroencephalography (EEG).
RESULTS: P1 and P1m latencies were decreased in patients with MDD, compared with those in healthy subjects. Further, the mismatch negativity amplitude to the 10% frequency deviance in the EEG, but not in the MEG, was increased in MDD. We observed no differences in N1/N1m and P3a responses in the MEG and EEG. The magnitude of decrease in P1/P1m latency correlated negatively with the severity of depression in the patients.
CONCLUSIONS: Early auditory processing is impaired in patients with MDD during an acute episode, probably reflecting dysfunctional frontotemporal neural circuits. These dysfunctions may contribute to the cognitive disturbances observed in people with MDD.