Thalamic Mechanisms in Vision

Abstract

The thalamus has long had bad press, seen as a simple, machine-like relay of information to the cortex. Work on the visual thalamic relays, the lateral geniculate nucleus (LGN), and the pulvinar, has dramatically changed this view, primarily in two different ways. First, ∼95% of input to geniculate relay cells is nonretinal and modulates the relay in dynamic and important ways related to behavioral state, including attention. Much of this is related to control of a voltage-gated, low-threshold Ca2+ conductance based on T-type Ca2+ channels. This conductance determines whether relay cells respond in tonic or burst mode, two very different response modes that affect the very nature of information relayed. Second, the LGN and pulvinar (a massive but generally mysterious and ignored thalamic relay) are examples of two different types of relay: the LGN is a first-order relay, transmitting information from a subcortical source (retina), while the pulvinar is mostly a higher-order relay, transmitting information from layer 5 of one cortical area to another area. Higher-order relays seem especially important to general corticocortical communication, and this view challenges the conventional dogma that such communication is based on direct corticocortical connections. In this sense, any new information reaching a cortical area, whether from a subcortical source or another cortical area, benefits from a thalamic relay. Other examples of first- and higher-order relays also exist. Thus, the thalamus not only provides a behaviorally relevant dynamic control over the nature of information relayed, but it also plays a key role in basic corticocortical communication.