Consciousness is a dynamic and complex process that coordinates the activities of different brain regions, particularly the brainstem, thalamus and cortex.

There are two levels of consciousness:

• Arousal, which only activates very deep brain structures within the brainstem.
• Awareness, which allows the conscious perception of information.

In the latter case, conscious "content" is encoded by the simultaneous activation of groups of neurons distributed in different areas of the cortex – a folded outer "shell" composed of six layers of neurons that line both cerebral hemispheres.
 
The loss of consciousness has been linked to a strong disruption in communication between the different areas of the cerebral cortex and also between the cortex and the thalamus (located halfway between the brainstem and the cortex). What would happen if these communications were reestablished through electrical stimulation?
 
Deep brain stimulation specifically targeting the thalamus can modulate arousal in anesthetized rodents or non-human primates and improve the condition of patients with impaired consciousness by restoring global cortical activity. However, a clear demonstration that this technique can restore both arousal and conscious access is currently lacking.

 To address this, researchers at the CEA-Joliot and the Brain Institute (ICM) studied non-human primates under general anesthesia. Using a deep brain stimulation electrode similar to the device used in patients with Parkinson's disease, the researchers were able to awaken the anesthetized animals. Electrical stimulation immediately caused the animals to open their eyes, and to resume spontaneous breathing and limb movements. Conversely, stopping the stimulation promptly returned the primates to a state of deep sedation. As expected, this experiment demonstrates that deep brain stimulation can restore the first level of consciousness.

 Using functional MRI and electroencephalography, the researchers succeeded for the first time in finely measuring the two levels of consciousness during thalamic stimulation.

 They observed the brain activations of animal subjects during anesthesia, with and without deep brain stimulation. Headphones were used to make primates listen to a series of different sounds. Whereas the anesthetized brain was unable to integrate the complexity of the sound composition, it recovered this ability as soon as brain stimulation began.

 Furthermore, using functional MRI of the brain at rest, the researchers demonstrated (by means of an algorithmic analysis applied to the MRI signal) that brain stimulation allowed the brain to recover a great number of activities that were lost under general anesthesia.
 
The result of more than five years of work in collaboration with the Hôpital Foch, the University of Versailles Saint-Quentin, Inserm and the Collège de France, these findings provide strong support for future clinical trials in patients suffering from chronic disorders of consciousness.