  Metastable brain dynamics are characterized by abrupt, jump-like modulations
so that the neural activity in single trials appears to unfold as a sequence of
discrete, quasi-stationary states. Evidence that cortical neural activity
unfolds as a sequence of metastable states is accumulating at fast pace.
Metastable activity occurs both in response to an external stimulus and during
ongoing, self-generated activity. These spontaneous metastable states are
increasingly found to subserve internal representations that are not locked to
external triggers, including states of deliberations, attention and
expectation. Moreover, decoding stimuli or decisions via metastable states can
be carried out trial-by-trial. Focusing on metastability will allow us to shift
our perspective on neural coding from traditional concepts based on
trial-averaging to models based on dynamic ensemble representations. Recent
theoretical work has started to characterize the mechanistic origin and
potential roles of metastable representations. In this article we review recent
findings on metastable activity, how it may arise in biologically realistic
models, and its potential role for representing internal states as well as
relevant task variables.
