  Thalamus has traditionally been considered as only a relay source of cortical
inputs, with hierarchically organized cortical circuits serially transforming
thalamic signals to cognitively-relevant representations. Given the absence of
local excitatory connections within the thalamus, the notion of thalamic
`relay' seemed like a reasonable description over the last several decades.
Recent advances in experimental approaches and theory provide a broader
perspective on the role of the thalamus in cognitively-relevant cortical
computations, and suggest that only a subset of thalamic circuit motifs fit the
relay description. Here, we discuss this perspective and highlight the
potential role for the thalamus -- and specifically mediodorsal (MD) nucleus --
in dynamic selection of cortical representations through a combination of
intrinsic thalamic computations and output signals that change cortical network
functional parameters. We suggest that through the contextual modulation of
cortical computation, thalamus and cortex jointly optimize the information/cost
tradeoff in an emergent fashion. We emphasize that coordinated experimental and
theoretical efforts will provide a path to understanding the role of the
thalamus in cognition, along with an understanding to augment cognitive
capacity in health and disease.
