Abstract

Following the pragmatic practices of anesthesiologists an individual is defined to be in a state of consciousness empirically by the behavioral ability to respond to simple stimuli and the loss of consciousness is defined by the loss of that facility. Several brain activities are proposed as properties of the state of consciousness. Baseline brain energy consumption has been shown by 13C MRS to be almost completely used for neuronal signaling. PET measurements of glucose or oxygen consumption, from several laboratories, show a widespread ~45% reduction in cerebral energy during anesthesia-induced loss of consciousness. We propose that the high level of brain energy consumption of the awake state is a necessary property of the state of consciousness. Two additional neuronal properties are drastic changes in the patterns of fMRI activation and neuronal population activity at deep vs. light levels of anesthesia, which correspond to low vs. high energy states, respectively. The brain-wide fMRI activity patterns with rat sensory stimulation at the higher energy state collapses to contralateral somatosensory response at the lower energy state. Firing rates of an ensemble of neurons in the rat somatosensory cortex show that most of the energy in light anesthesia is consumed in the spiking frequency range of 20-40 Hz, which include γ-band electrical signals. In deep anesthesia the frequency range shifts to below 10 Hz. The high brain energy consumption, characteristic of the state of consciousness, supports elevated levels of high frequency neuronal firing observed in the resting, awake brain. In our model, necessary brain properties of the state of consciousness create our physical understanding of that state rather than the converse, popular in neuroimaging, in which subjective mental processes are identified to be necessary and sufficient correlates of the state of consciousness