1.28 - The Visual Thalamus

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The thalamus has largely been ignored as an interesting brain structure, seen as a simple, machine-like relay of information to the cortex. This is now dramatically changed, largely from work on the visual thalamic relays – the lateral geniculate nucleus and the pulvinar. Two recent observations helped to bring about this new appreciation. First, ∼95% of input to geniculate relay cells is nonretinal and modulates the relay dynamically related to behavioral state. Much of this is related to the control of response mode – relay or burst. Second, the lateral geniculate nucleus and pulvinar are examples of two different types of relays: the lateral geniculate nucleus 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. Higher-order relays seem to play a key role in cortical function via a corticothalamocortical route. This view challenges the conventional dogma that such communication is based solely on direct corticocortical connections. Other examples of first- and higher-order relays also exist, and higher-order relays represent the majority of thalamic volume. Thus, the thalamus not only provides a behaviorally relevant, dynamic control over the nature of information relayed but also plays a key role in basic corticocortical communication.

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