Glossary of Technical Terms
5-${\mathit{HT}}_{\mathbf{2}\mathbf{A}}$ receptor	Serotonin receptor in which acute activation by serotonergic psychedelics produces a transient altered state of consciousness.
Attractor	Set of points in the phase space of a dynamical system towards which the system approaches during its temporal evolution.
Bifurcation	Phenomena in the field of dynamical systems that occurs when a small change in the parameter values causes a sharp qualitative change in the behavior of the system.
Bottom-up approach	Defines the local dynamics of interacting units (such as neurons or groups of neurons) in order to generate features as similar as possible to the ones observed in the brain during different experimental conditions.
Entropic brain hypothesis	An example of top-down approach. Postulates that the richness of conscious experience depends on the complexity of the underlying population-level neuronal activity, which determines the repertoire of states available for the brain to explore.
Functional connectivity (FC) and Functional Connectivity Dynamics (FCD)	Second order statistics summarizing the pair-wise dependence between the activity of brain regions. The FCD is obtained from computing the similarity between the FCs associated with different time windows.
Integrated information theory (IIT)	An example of top-down approach. Based on certain first-person qualities of subjective experience, which are accessed by introspection and can be taken as “postulates” or “axioms” for the theory. This theory strives to provide a quantitative characterization of consciousness analyzing the causal relationships of brain activity using multivariate information theory.
Hopf bifurcation	Example of a bifurcation of a nonlinear dynamical system where steady dynamics change their stability and a limit cycle emerges, giving rise to periodical solutions.
Lempel-Ziv complexity	Lossless compression algorithm that provides an effective tool to estimate the entropy rate of a signal.
Lyapunov exponent	An exponent that indicates how two trajectories with similar initial conditions diverge in their temporal evolution along each dimension. A positive value for the Lyapunov exponent is indicative of deterministic chaos.
Mind-brain problem	Dualistic perspective addressing the relationship between the mental and the embodied brain processes.
Neural correlates of consciousness (NCC)	Minimal set of neural events associated with a certain subjective experience.
Perturbational complexity index	Measure of the complexity of the cortical activity evoked by transcranial magnetic stimulation.
Phenomenal and access consciousness	The first represents the subjective experience of sensory perception, emotion, thoughts, etc. The second represents the global availability of conscious content for cognitive functions, such as speech, reasoning, and decision-making, enabling the capacity to issue first-person reports.
Psychedelic drugs	Psychoactive drugs in which the primary effect is to produce profound changes in perception, mood, and cognitive processes, triggering non-ordinary states of consciousness. There are two major types: serotonergic (e.g., LSD, DMT), which activate the serotonin 2A receptor (5-$H{T}_{2A}$ ), and glutamatergic dissociatives (e.g., ketamine, PCP), in which action blocks the PCP site of NMDA glutamate receptors.
Resting state networks	Represent specific patterns of synchronous activity between brain regions in whole-brain recordings. They are consistently found in healthy subjects in fMRI data when no explicit task is being performed.
Stuart-Landau oscillators	Non-linear oscillating system near a Hopf bifurcation.
Top-down approach	Focuses on the use of subjective signatures of consciousness as guiding principles to analyze brain signals in order to narrow down the possible biophysical mechanisms compatible with those signatures.
Whole-brain computational models	An implementation of bottom-up approach. Defines a set of differential equations ruling the dynamics and interactions between simulated brain regions in order to reproduce observables from neuroimaging data.