Abstract
The thalamocortical system plays a key role in the breakdown or emergence of consciousness, providing bottom-up information delivery from sensory afferents and integrating top-down intracortical and thalamocortical reciprocal signaling. A fundamental and so far unanswered question for cognitive neuroscience remains whether the thalamocortical switch for consciousness works in a discontinuous manner or not. To unveil the nature of thalamocortical system phase transition in conjunction with consciousness transition, ketamine/xylazine was administered unobtrusively to ten mice under a forced working test with motion tracker, and field potentials in the sensory and motor-related cortex and thalamic nuclei were concomitantly collected. Sensory and motor-related thalamocortical networks were found to behave continuously at anesthesia induction and emergence, as evidenced by a sigmoidal response function with respect to anesthetic concentration. Hyperpolarizing and depolarizing susceptibility diverged, and a non-discrete change of transitional probability occurred at transitional regimes, which are hallmarks of continuous phase transition. The hyperpolarization curve as a function of anesthetic concentration demonstrated a hysteresis loop, with a significantly higher anesthetic level for transition to the down state compared to transition to the up state. Together, our findings concerning the nature of phase transition in the thalamocortical system during consciousness transition further elucidate the underlying basis for the ambiguous borderlines between conscious and unconscious brains. Moreover, our novel analysis method can be applied to systematic and quantitative handling of subjective concepts in cognitive neuroscience.
Highlights
Many studies have characterized electrophysiological differences between conscious and unconscious states such as vegetative state and coma [1], anesthesia-induced unconsciousness [2]; the dynamic processes of electrophysiological activity during consciousness transitions have been poorly characterized
Steyn-Ross et al [5] formulated a theoretical model for the cerebral cortex response to general anesthesia and predicted a discontinuous phase transition of electrical fluctuations in the cortex at the point of transition into unconsciousness, which was characterized by a divergence in cortical synchrony and hysteresis in transitional paths
Assuming susceptibility to anesthesia depends on the concentration of the anesthetic in the cerebral circulatory system [13], the concentration of ketamine in the cerebral circulatory system was considered the reference parameter reflecting the endogenous state of the brain
Summary
Many studies have characterized electrophysiological differences between conscious and unconscious states such as vegetative state and coma [1], anesthesia-induced unconsciousness [2]; the dynamic processes of electrophysiological activity during consciousness transitions have been poorly characterized. Liley and Bojak [6] later adopted mean-field theory to correlate neuronal population behaviors with physiologically observable parameters (e.g., electroencephalogram (EEG)), and Molaee-Ardekani et al [7] added voltage-dependent slow motion of ions to the above models, predicting the existence of a broad regime of fluctuation between two states, which is a property of continuous phase transition. These models describe the mechanisms underlying why and how anesthesia drives cortical neurons from the up state (depolarized mode) to the down state (hyperpolarized mode). The dynamic behavior of the sensorimotor thalamocortical network was characterized according to external conditions such as drug induction time and anesthetic concentration in the brain
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