Cerebral metabolism and consciousness

In vivo 13C magnetic resonance spectroscopy studies of the brain have measured rates of glutamate-glutamine cycle (Vcyc) and glucose oxidation (CMRglc(ox)) by detecting 13C label turnover from glucose to glutamate and glutamine. In both the awake human and in the anesthetized rat brains Vcyc and CMRglc(ox) are stoichiometrically related, and form a major pathway in which approximately 80% of the energy from glucose oxidation supports events associated with glutamate neurotransmission. The high energy consumption of the brain at rest and its quantitative usage for neurotransmission reflect a high level of neuronal activity for the non-stimulated brain. This high activity supports a reinterpretation of functional imaging data, e.g., where the large baseline signal has commonly been discarded. Independent measurements of energy consumption (delta CMRO2%) obtained from calibrated fMRI equaled percentage changes in neuronal spiking rate (delta nu %) measured by electrodes during sensory stimulation at two depths of anesthesia. These quantitative biophysical relationships between energy consumption and neuronal activity provide novel insights into the nature of brain function. The high resting brain activity is proposed to include the global interactions constituting the subjective aspects of consciousness. Anesthesia by lowering the total firing rates correlates with the loss of consciousness. These results, which measure the localized neuronal response and distinguish inputs of peripheral neurons from inputs of neurons from other brain regions, fit comfortably into the neuronal scheme of a global workspace proposed by Dehaene and Changeux.