Previous studies of repetitive transcranial magnetic stimulation suggest a hemispheric imbalance in patients with major depression. These studies report an antidepressant effect by activation of the left prefrontal cortex or inhibition of the right prefrontal cortex. The aim of this study is to investigate interhemispheric differences in cortical excitability in a large sample of patients with major depression.
Methods: The resting motor threshold (RMT) was measured on 91 patients with treatment-resistant major depression. We controlled for current medication use, gender, age, handedness, and study site.
Results: There was no significant difference between the left RMT (55.96 [10.356]) and the right (57.74 [11.359]) (P = 0.131, Wilcoxon matched-pairs test). A multivariate analysis found no significant association between depression scores and right or left RMT. After adjusting for important cofactors, benzodiazepine use was found to be a significant predictor of left RMT (P = 0.017, linear regression) and right RMT (P = 0.007, linear regression).
Conclusion: Our results do not support the existence of an interhemispheric imbalance of cortical excitability in depressed patients. Benzodiazepine use was found to raise both the left and right RMT.