| Comprehension of how the neurons of the brain work to create the mind may require a comprehension of how groups of those neurons work together as ensembles. In order to observe potential ensemble activity in the cortex we used calcium imaging of large neuronal populations. Further, we use thalamocortical slices in order to allow us to monitor cortical population responses to anatomically realistic inputs from the thalamus. In these slices we demonstrate that stimulation of an anatomically specific and behaviorally relevant thalamocortical projections generates cortical network dynamics that are remarkably similar to those that arise in the cortex spontaneously. In fact, statistically indistinguishable networks of individual neurons are activated under both circumstances, displaying the same UP states and the same temporal sequence of activation. Our data indicate that thalamic inputs serve to release, or awaken, circuit dynamics that are intrinsic to the cortex, suggesting the existence of pre-existing cortical ensembles. We go on to demonstrate that these responses cannot be interrupted by further thalamic inputs. We show that neither patterned activations observed using calcium imaging, nor membrane potential dynamics using patch clamp, were altered by impinging inputs in the great majority of neurons. Finally as a potential mechanism for this insensitivity, we find that neocortical neurons have lower input resistance during UP states than during DOWN states, meaning that inputs may be shunted during UP states.; We have therefore found evidence for ensembles embedded in cortical circuits, and describe their features. Despite our observations that these ensemble activations are quite robust in our slice preparation, their role in the waking brain is not yet clear. Evidence from experiments performed in vivo points to their role as being either a prolonged version of the normal waking sensory response or as pattern replay units for memory consolidation during sleep. |
