Bottom-Up and Top-Down Mechanisms of General Anesthetics Modulate Different Dimensions of Consciousness

General anesthetics may suppress consciousness through two complementary neural pathways: a bottom-up mechanism that alters arousal by acting on brainstem and diencephalon sleep-wake nuclei, and a top-down mechanism that disrupts cortical and thalamocortical circuits responsible for integrating information. This article synthesizes these approaches by mapping them to two interrelated dimensions of consciousness—level and content. The framework explains why different anesthetic drugs produce diverse effects on subjective experience. The authors argue that level and content interact to generate consciousness, and understanding this interaction is key to explaining how anesthetics induce unconsciousness.

Study at a glance

Abstract

There has been controversy regarding the precise mechanisms of anesthetic-induced unconsciousness, with two salient approaches that have emerged within systems neuroscience. One prominent approach is the "bottom up" paradigm, which argues that anesthetics suppress consciousness by modulating sleep-wake nuclei and neural circuits in the brainstem and diencephalon that have evolved to control arousal states. Another approach is the "top-down" paradigm, which argues that anesthetics suppress consciousness by modulating the cortical and thalamocortical circuits involved in the integration of neural information. In this article, we synthesize these approaches by mapping bottom-up and top-down mechanisms of general anesthetics to two distinct but inter-related dimensions of consciousness: level and content. We show how this explains certain empirical observations regarding the diversity of anesthetic drug effects. We conclude with a more nuanced discussion of how levels and contents of consciousness interact to generate subjective experience and what this implies for the mechanisms of anesthetic-induced unconsciousness.