During non-rapid eye movement sleep, the brain's response to a direct cortical stimulus is stronger but fails to propagate beyond the stimulation site, unlike during quiet wakefulness when the activation spreads to distant connected areas. This breakdown in cortical effective connectivity may explain why consciousness fades during certain sleep stages despite ongoing brain activity.
During the first REM sleep episode of the night, transcranial magnetic stimulation (TMS) triggered widespread and differentiated patterns of cortical activation on electroencephalography (EEG), similar to those observed during wakefulness. In contrast, during NREM sleep, cortical activations became more local and stereotypical, indicating impaired intracortical dialogue. These findings suggest that TMS combined with high-density EEG can probe the internal dialogue of the thalamocortical system, potentially offering a method to assess brain function in patients who are unable to move or communicate.