A study using transcranial magnetic stimulation to investigate motor mechanisms in psychomotor retardation in depression

The pathophysiological basis of psychomotor retardation in depression is unclear. In this study, transcranial magnetic stimulation (TMS) was used to examine the functioning of the motor cortical system in 19 depressed patients and 10 healthy control subjects. Motor-evoked potentials were measured in the biceps brachii muscle during a series of tests with the muscle at rest and during voluntary elbow flexion contractions. Maximal voluntary force, as well as force and electromyographic responses to TMS were also measured during fatiguing maximal contractions. Depressed psychomotor-retarded subjects were less able to produce output from the motor cortex than non-psychomotor-retarded, depressed subjects and healthy controls during maximal exertion and fatigue. This finding was independent of depression severity. In contrast, responses to TMS elicited during relaxation or weak contractions did not differ between healthy and psychomotor-retarded subjects. Our study suggests that although the motor pathway from the motor cortex to the muscle is unimpaired, psychomotor retardation in depression is characterized by a reduced ability to drive the motor cortex.