The thalamocortical pathway is the main route of sensory information to the cerebral cortex. Vision, touch, hearing, taste, and balance all depend on the integrity of this pathway that connects the thalamic structures receiving sensory input with the cortical areas specialized in each sensory modality. Only the ancient sense of smell is independent of the thalamus, gaining access to cortex through more anterior routes. While the thalamocortical pathway targets different layers of the cerebral cortex, its main stream projects to the middle layers and has axon terminals that are dense, spatially restricted, and highly specific in their connections. The remarkable specificity of these thalamocortical connections allows for a precise reconstruction of the sensory dimensions that need to be most finely sampled, such as spatial acuity in vision and sound frequency in hearing. The thalamic axon terminals also segregate topographically according to their stimulus preferences, providing a simple principle to build cortical sensory maps: neighboring values in sensory space are represented by neighboring points within the cortex.
Thalamocortical processing is not static. It is continuously modulated by the brain stem and corticothalamic feedback based on the level of attention and alertness, and during sleep or general anesthesia. When alert, visual thalamic responses become stronger, more reliable, more sustained, more effective at sampling fast changes in the scene, and more linearly related to the stimulus. The high firing rates of the alert state make thalamocortical synapses chronically depressed and excitatory synaptic potentials less dependent on temporal history, improving even further the linear relation between stimulus and response. In turn, when alertness wanes, the thalamus reduces its firing rate, and starts generating spike bursts that drive large postsynaptic responses and keep the cortex responsive to sudden stimulus changes.