A fundamental problem in neuroscience and medicine is to understand the mechanisms for loss of consciousness under general anesthesia. Studying loss of consciousness under general anesthesia is a complex problem. The phenomenon of general anesthesia has five components: loss of consciousness, amnesia, loss of pain perception, and immobility, while maintaining hemodynamic stability. Induction and different planes of general anesthesia produce highly stereotyped changes in electroencephalogram (EEG) patterns. Loss of consciousness under general anesthesia is measured clinically by observing the loss of response to external stimuli. The failure to respond to external stimuli could be a result of disruption of afferent sensory pathways, central processing, efferent motor pathways, or some combination of these three. We wish to conduct a series of studies using concurrent EEG and functional MRI recordings that will characterize changes of function under general anesthesia in a number of afferent, efferent, and central processing pathways. The specific aims of this study are to:
a. Specific Aim 1: Test the hypothesis that we can characterize the relationship between changes of function in the somatosensory system and loss of consciousness during induction of general anesthesia under propofol using concurrent EEG and fMRI imaging.
b. Specific Aim 2: Test the hypothesis that we can characterize the relationship between changes of function in the auditory system and loss of consciousness during induction of general anesthesia under propofol using concurrent EEG and fMRI imaging.
c. Specific Aim 3: Test the hypothesis that we can characterize the relationship between changes of function in cognitive systems subserving implicit memory formation and loss of consciousness during induction of general anesthesia under propofol using concurrent EEG and fMRI imaging.
By observing functional changes in somatosensory, auditory, and cognitive systems relative to the time point of loss of consciousness and changes in stereotyped EEG patterns, we wish to begin developing neurophysiological correlates that can describe the state of unconsciousness under general anesthesia.
General anesthesia is complex phenomenon characterized by changes in clinical signs, EEG patterns, and brain metabolism. Loss of consciousness under general anesthesia, measured in terms of the loss of response to verbal or tactile stimuli, could be mediated by changes of function in afferent, central processing, or efferent pathways. Animal studies of general anesthesia have revealed pathway-specific effects in the pain and somatosensory systems, but analogous studies in humans using functional imaging are in their infancy. We postulate that it is possible to characterize changes in auditory, somatosensory, and cognitive pathways in relation to loss of consciousness and stereotyped EEG patterns under anesthesia, and we have designed a series of combined EEG/fMRI studies to investigate the influence of propofol general anesthesia upon these pathways. Using these studies as a starting point, it may be possible to formulate a set of neurophysiological correlates that can describe the state of general anesthesia. These neurophysiological correlates could provide a basis for development of site-specific anesthetic drugs with fewer side-effects, as well as functionally-specific approaches to EEG-based anesthetic monitoring.