  The scope of human consciousness includes states departing from what most of
us experience as ordinary wakefulness. These altered states of consciousness
constitute a prime opportunity to study how global changes in brain activity
relate to different varieties of subjective experience. We consider the problem
of explaining how global signatures of altered consciousness arise from the
interplay between large-scale connectivity and local dynamical rules that can
be traced to known properties of neural tissue. For this purpose, we advocate a
research program aimed at bridging the gap between bottom-up generative models
of whole-brain activity and the top-down signatures proposed by theories of
consciousness. Throughout this paper, we define altered states of
consciousness, discuss relevant signatures of consciousness observed in brain
activity, and introduce whole-brain models to explore the mechanisms of altered
consciousness from the bottom-up. We discuss the potential of our proposal in
view of the current state of the art, give specific examples of how this
research agenda might play out, and emphasise how a systematic investigation of
altered states of consciousness via bottom-up modelling may help us better
understand the biophysical, informational, and dynamical underpinnings of
consciousness.
