The cerebral cortex receives almost all of its sensory input from the thalamus. With the exception of olfaction, sensory information is delivered to cortical neurons through excitatory connections made by thalamic cells known as relay neurons. Although the name relay neuron might suggest that these cells simply pass the baton of sensory activity from the periphery to the cortex, it has become increasingly clear that these neurons are members of a complex circuit that involves ascending, descending, and recurrent sets of neuronal connections.
The major source of descending input to thalamic relay neurons comes from neurons with cell bodies located in layer 6 of the cerebral cortex. These corticothalamic neurons exert both an excitatory and an inhibitory influence on relay neurons, and it is the balance of this excitation and inhibition that is thought to influence many of the activity patterns and sensory response properties of relay neurons. The excitatory influence of the cortex is achieved by monosynaptic connections that are markedly robust in number. Indeed, the number of corticothalamic synapses made onto a relay neuron is much greater than the number of synapses made from any other single source, including the ascending pathways from the periphery. The inhibitory influence of the cortex, on the other hand, is achieved by polysynaptic connections either with intrinsic g-amino butyric acid (GABA)ergic interneurons within the relay nuclei or with GABAergic neurons with cell bodies located in the reticular nucleus (RTN) of the thalamus.
Given the number of inputs provided to thalamic relay cells by corticothalamic neurons, it has been tempting to speculate what functional role these corticothalamic pathways could serve. Despite the certain importance of the corticothalamic pathway for thalamic processing, a consensus about its function has been elusive. Proposed roles for cortical feedback fall into two general categories: first, to effect sensory responses and receptive field properties, and second, to effect firing mode and/or activity state. Several excellent reviews discussing this second category of proposed roles have recently been published. In this review, we focus our discussion on the first category — the effects of cortical feedback on sensory responses and receptive field properties. As the anatomical properties of cortical feedback are so similar for the visual, auditory, and somatosensory systems, it seems reasonable to suggest that the role(s) of feedback should generalize across systems. By identifying the effects of corticothalamic input that are shared by multiple sensory systems, we hope to present results that will foster a consensus in thinking about corticothalamic function. Thus, our approach will be to identify and describe effects of feedback that are shared by more than one sensory system. In particular, we focus our discussion of the role of corticothalamic feedback for visual, auditory, and somatosensory processing by examining results from studies of sensory responses in the lateral geniculate nucleus (LGN), medial geniculate body (MGB), and ventrobasal complex (VB).