The effect of mild traumatic brain injury on oscillatory brain activity

Mild traumatic brain injuries (mTBI) are common, and while most patients recover well, there is a minority of patients suffering from prolonged symptoms lasting over three months. Pathological processes provoke low-frequency (0.5 7 Hz) oscillatory brain activity, measurable with electroencephalography (EEG) and magnetoencephalography (MEG). After mTBI, low frequency activity (LFA) is hypothesized to arise from cortical neurons suffering from deafferentation after traumatic axonal injury. The natural evolution and prognostic value of low-frequency activity (LFA) measured with MEG, however, is not yet firmly established and reliable biomarkers for cognitive complaints after mTBI are lacking. The aim of this thesis was to examine the occurrence and natural evolution of low frequency activity (LFA) after mild traumatic brain injury (mTBI), and to assess its prognostic value in predicting those with prolonged symptoms. Additionally, we wanted to examine the effect of mTBI to brain oscillatory activity during cognitive tasks and find indicators for altered processing. The existence of LFA in healthy subjects might, however, hamper its’ diagnostic value. Therefore, in Study I we created a reference database of resting-state oscillatory brain activity and observed LFA in only 1,4% of healthy subjects’ MEG recordings. The Study II assessed the occurrence and evolution of LFA in resting-state MEG recordings of mTBI patients. At a singlesubject level, 7/26 patients presented aberrant 4–7 Hz (theta) band activity; 3/7 patients with abnormal theta activity were without any detectable lesions in MRI. Of the twelve patients with follow-up measurements, five showed abnormal theta activity in the first recording, but only two in the second measurement, implying the importance of early measurements in clinical settings. The presence of LFA was not, however, correlated with the prevalence of self-reported symptoms. The Study III concentrated on the modulation of oscillatory activity during cognitive tasks, Paced Auditory Serial Addition Test (PASAT) and a vigilance test. Attenuation of cortical activity at alpha band (8 – 14 Hz) during PASAT compared with rest was stronger in patients than in controls (p 0.05, corrected). Furthermore, the patients presented significant attenuation of oscillatory activity also in the left superior frontal gyrus and right prefrontal cortices which was not detected in controls. Spectral peak amplitudes of areal mean oscillatory activity at the alpha band were negatively correlated with the patients’ neuropsychological performance (p<0.01, uncorrected). Areal alpha frequency modulation during PASAT compared with rest was altered in patients: While the alpha peak frequency increased occipitally and remained stable at other areas in controls, it was stable occipitally and decreased at other areas in mTBI patients (p=0.012). According to our studies, LFA, especially theta-band oscillatory activity can provide an early objective sign of brain dysfunction after mTBI, and cortical oscillatory activity during a demanding cognitive task (PASAT) is altered after mTBI. Our observations suggest that both aberrant theta-band activity and the altered alpha activity during cognitive tasks may offer clinically relevant indicators of changes in neural processing after mTBI.