Neural correlates of consciousness during general anesthesia using functional magnetic resonance imaging (fMRI).

Hypnotic anesthetic agents alter consciousness by targeting specific brain networks in a dose-dependent manner. Higher-order networks involved in mental content and sub-cortical networks regulating thalamic activity are affected first as concentrations increase and inhibitory neurotransmission is enhanced. Lower-order sensory networks, including thalamo-cortical connectivity within them, are preserved even at concentrations that suppress responsiveness, though cross-modal sensory interactions are inhibited. Thalamo-cortical connectivity into consciousness networks decreases with higher concentrations and becomes anti-correlated between thalamus and cortex at the deepest sedation levels when the subject is non-responsive. Whether these changes occur with agents that inhibit excitatory neurotransmission remains unknown, and the link between fMRI observations and biochemical targets is still unidentified.

Study at a glance

Abstract

This paper reviews the current knowledge about the mechanisms of anesthesia-induced alteration of consciousness. It is now evident that hypnotic anesthetic agents have specific brain targets whose function is hierarchically altered in a dose-dependent manner. Higher order networks, thought to be involved in mental content generation, as well as sub-cortical networks involved in thalamic activity regulation seems to be affected first by increasing concentrations of hypnotic agents that enhance inhibitory neurotransmission. Lower order sensory networks are preserved, including thalamo-cortical connectivity into those networks, even at concentrations that suppress responsiveness, but cross-modal sensory interactions are inhibited. Thalamo-cortical connectivity into the consciousness networks decreases with increasing concentrations of those agents, and is transformed into an anti-correlated activity between the thalamus and the cortex for the deepest levels of sedation, when the subject is non responsive. Future will tell us whether these brain function alterations are also observed with hypnotic agents that mainly inhibit excitatory neurotransmission. The link between the observations made using fMRI and the identified biochemical targets of hypnotic anesthetic agents still remains to be identified.