Reduced mismatch negativity in posttraumatic stress disorder: a compensatory mechanism for chronic hyperarousal?

Mismatch Negativity (MMN) is yet poorly understood in the context of Posttraumatic Stress Disorder (PTSD, e.g. [Morgan 3rd, C.A., Grillon, C., 1999. Abnormal mismatch negativity in women with sexual assault-related posttraumatic stress disorder. Biol. Psychiatry 45, 827-832.]). PTSD symptoms like hyperarousal, emotional pressure and avoidance may interfere with pre-attentive sensory processing. We tested this in an optimized MMN design [Näätänen, R., Pakarinen, S., Rinne, T., Takegata, R. (2004) The mismatch negativity (MMN): towards the optimal paradigm. Clin. Neurophysiol. 115: 140-144.] with PTSD victims and a control group without PTSD. A group of PTSD subjects was compared with gender and age-matched, healthy comparison subjects without PTSD. A "memory trace" was elicited by frequently presented "standard" auditory stimuli (50% occurrence) of 1 kHz, 75 ms duration, intermittently with 8 rare "deviants", which differed in frequency (higher/lower), intensity (louder/softer), duration (shorter), direction (left/right) or by the presence of a gap in the sound. During presentation of tones a silent film was shown. Psychometric data were collected by SCID, BSI, Attentiveness Inventory, Edinburgh Handedness Questionnaire, and the PTSD Screening Scale by Breslau et al. [Breslau, N., Peterson, E.L., Kessler, R.C., Schultz, L.R. (1999) Short screening scale for DSM-IV posttraumatic stress disorder. Am. J. Psychiatry 156: 908-911.]. Group comparisons of the MMN were performed for left/right-frontal/temporal, and for midline electrode sites. A good differentiation of both groups was found in psychometric and electrophysiological data. The PTSD group revealed on most BSI scales enhanced values of psychic aberration. The amplitude of the MMN was significantly reduced in the PTSD compared to non-PTSD subjects. MMN was significantly correlated with the total PTSD score. The data suggest a reduction in pre-attentive auditory sensory memory in PTSD due to specific symptom variables such as hyperarousal, sleeplessness, impaired concentration and a general enhanced excitation of the nervous system. This protective inhibition is thought to be a fine-tuning process in PTSD in order to prevent arousal overload.