On the cerebral origin of EEG responses to TMS : insights from severe cortical lesions

Background: Transcranial magnetic stimulation combined with electroencephalography (TMS/EEG) represents a valuable tool to probe cortical excitability and connectivity. Although several procedures have been devised to abolish TMS-related artifacts, direct evidence that it is possible to record TMS-evoked potentials (TEPs) that purely reflect cortical responses to TMS are still lacking. Objective: To demonstrate that when TMS is delivered on a human head with intact nerves, scalp and ocular muscles, TEPs are present only if a functional portion of cortex is targeted and is absent otherwise. Methods: We performed extensive navigated TMS/EEG mappings in three vegetative state patients and in eight healthy controls. Patients were selected based on the extension of their cortical lesions as revealed by structural/functional imaging: the cerebral cortex was globally damaged in Patient 1 due to cerebral anoxia, Patient 2 showed a traumatic damage affecting one cerebral hemisphere, while Patient 3 was characterized by one left sided and one right-sided focal ischemic lesion. Results: In Patient 1, TMS performed at any targeted cortical site did not elicit statistically significant TEPs. In Patient 2, TEPs were absent when the damaged hemisphere was targeted, while were present over the healthy side. In Patient 3, significant TEPs were absent when cortical lesions were targeted and present otherwise. Significant TEPs were always present in healthy controls. Conclusions: These findings suggest that, provided that appropriate experimental procedures are employed, TEPs are genuine cortical responses detectable only when preserved cortical tissue is stimulated. Hence, a dependable assessment of cortical excitability and connectivity in brain-injured patients requires the use of neuronavigated TMS.