  A major shift is happening within neurophysiology: a population doctrine is
drawing level with the single-neuron doctrine that has long dominated the
field. Population-level ideas have so far had their greatest impact in motor
neurophysiology, but they hold incredible promise for resolving open questions
in cognition. Here, we codify the population doctrine and survey recent work
that leverages this view to probe cognition. Our discussion is organized around
five core concepts that provide a foundation for population-level thinking: (1)
state spaces, (2) manifolds, (3) coding dimensions, (4) subspaces, and (5)
dynamics. The work we review illustrates the progress and promise that
population neurophysiology holds for cognitive neuroscience$-$for delivering
new insight into attention, working memory, decision-making, executive
function, and learning.
