Inter-hemispheric asymmetry of motor corticospinal excitability in major depression studied by transcranial magnetic stimulation

Background: Imaging and electroencephalographic studies have reported inter-hemispheric asymmetries in frontal cortical regions associated with depression. This study aimed at comparing motor corticospinal excitability assessed by methods of transcranial magnetic stimulation (TMS) between the right and left hemispheres in patients with major depression and healthy controls. Method: Patients with major depression (n = 35) and healthy controls (n = 35) underwent a bilateral study of various motor corticospinal excitability parameters, including rest motor threshold (RMT), corticospinal silent period (CSP) duration and intra-cortical inhibition (ICI) and facilitation (ICF). Indexes of asymmetry were calculated, and the relationships between excitability parameters and clinical scores of depression were statistically analyzed. Results: Depressed patients showed a reduced excitability of both excitatory (RMT, ICF) and inhibitory (CSP, ICI) processes in the left hemisphere, compared to the right hemisphere and to healthy controls. Conclusion: The present results confirmed the existence of inter-hemispheric asymmetries in frontal cortex activities of depressed patients in favor of a left-sided reduced excitability. This neurophysiological approach may help to guide repetitive TMS procedures in the treatment of depressive disorders.