Thalamic relay functions.

The lateral geniculate nucleus is the best understood thalamic relay. Only 5-10% of the inputs to geniculate relay cells derive from retina, which is the driving input. The rest, being modulatory, derive from local inhibitory inputs, descending inputs from visual cortex, and ascending inputs from brainstem. The nonretinal, modulatory inputs, which form the vast majority, dynamically control the nature of the geniculate relay. Among other actions, these modulatory inputs regulate membrane properties of relay cells and thereby control their mode of response to retinal inputs, and this dramatically affects the nature of information relayed to cortex. Our studies of the lateral geniculate nucleus of the cat lead to the speculation that this dynamic control depends on the animal's behavioral state and represents the neuronal substrate for many forms of visual attention. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to cortex for the first time. In contrast, the other main thalamic relay of visual information, the pulvinar (and lateral posterior nucleus in carnivores), is largely a higher-order relay, since much of it seems to relay information from one cortical area to another. Much more corticocortical processing may involve these 're-entry' routes than has been hitherto appreciated. If so, the thalamus sits at an indispensable position for corticocortical processing.