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

Abstract

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.

Keywords: depression; electroencephalography; magnetoencephalography.

Fig. 1: (Top) auditory evoked magnetic field response to standard tone in healthy subjects (solid lines) and depressive patients (dotted lines) measured with a magnetometer. The stimuli were delivered to the left ear. The enlarged response presents the largest responses over the right temporal areas (contralateral to the ear stimulated). (Bottom) Contour maps at the peak of the P1m. Solid lines indicate where magnetic flux where it enters outside the head, and dotted lines indicate where magnetic flux exits within the head. Contour line separation is 10 ft. The arrows depict the strength and location of single equivalent current dipoles of P1m applied to data.

Fig. 2: Relation between P1m (left) and P1 latencies (right) and severity of depression, as measured by the Hamilton Depression Rating Scale (HDRS).

Fig. 3: Grand-averaged potential maps at the peak latency of the P1, N1, mismatch negativity (MMN) and P3a. Uppermost positions = frontal electrodes; lowermost positions = occipital electrodes. Blue dots indicate the locations of electrodes.