Cortico-thalamic interplay and the security of operation of neural assemblies and temporal chains in the cerebral cortex

Much evidence suggests that the mammalian thalamus is not merely a set of nuclei relaying signals to the cerebral cortex, but is engaged in two-way interplay with it. Three important features constrain ideas about the nature of this interplay: (i) thalamic projection neurones lack local axon collaterals; (ii) most corticothalamic projections have very long axonal conduction time; (iii) in the waking state the membrane potential of thalamic projections cells appears to be poised just beneath threshold for firing. It is proposed that cortico-thalamo-cortical pathways represent connections between different cortical loci which have higher security than the direct cortico-cortical route. Thus each thalamo-cortical projection neurone can have a singular and pivotal role in the activation of one or more cortical neural assemblies. The long delays of cortico-thalamic conduction suggest that the cortico-thalamo-cortical loop also plays a crucial role in the operation of time-structured neural assemblies ('synfire chains': Abeles), by providing a high-security link from one active node of the chain to nodes activated at a later time in the sequence. It is predicted that, in the waking animal, thalamic projection cells should have a response selectivity to complex percepts and concepts, conferred on them by the cortical assemblies in whose activation they participate.