Non-parametric time-series analysis of cortical gamma functional connectivity under general anesthesia
Abstract
I seek to understand the mechanism of anesthetic ablation of consciousness. Gamma (20-60 Hz) oscillations in cortical neuronal activity have been associated with neural processes supporting conscious perception and action (Engel et al. 2001. Trends Cogn Sci ). In my dissertation, I examined the effect of volatile anesthetics halothane, isoflurane, and desflurane on spontaneous and visually evoked gamma activity in rat primary visual cortex. I also investigated the question whether these anesthetics at doses that produce unconsciousness may interfere with the exchange of information encoded in gamma oscillations between frontal and posterior regions. Spontaneous and event-related local field potentials to sixty light flashes presented every five seconds, were recorded with chronically implanted bipolar intracortical electrodes from rat primary visual, parietal association, and frontal cortical regions in the waking state and at increasing concentrations of halothane (0-2%), isoflurane (0-2%), and desflurane (0-10%). Also, the loss of righting reflex--a generally accepted behavioral index of the loss of consciousness in rat--was assessed as a function of steady-state graded anesthetic concentrations. The local field potentials were band-pass filtered at gamma frequencies. Thomson's Multitaper power spectral analysis and wavelet decomposition were used to estimate gamma power and capture its temporal and spectral variations at the time scale of neuronal communication. Transfer entropy was calculated from wavelet-transformed flash-induced gamma oscillations to quantify feedforward and feedback information exchange among primary visual, parietal association, and frontal cortices. I showed that the three anesthetic agents studied augmented gamma power at the anesthetic concentrations producing unconsciousness. Since gamma power was not reduced even at surgical anesthetic levels, this finding suggested that gamma power alone is not a correlate of volatile anesthetic-induced loss of consciousness. On the other hand, I showed that halothane, isoflurane, and desflurane selectively decreased information transfer of the feedback projection at 50 Hz more than that of the feedforward projection between frontal and posterior cortical regions at similar anesthetic concentrations. This finding supports the role of cortical feedback projection in consciousness, and suggests that the disruption of information transfer at 50 Hz along the feedback projection is a likely neurophysiological correlate of the anesthetic-induced unconsciousness.
This paper has been withdrawn.