The lateral geniculate nucleus is the best understood thalamic relay and serves as a model for all thalamic relays. Only 5-10% of the input to geniculate relay cells derives from the retina, which is the driving input. The rest is modulatory and derives from local inhibitory inputs, descending inputs from layer 6 of the visual cortex, and ascending inputs from the brainstem. These modulatory inputs control many features of retinogeniculate transmission. One such feature is the response mode, burst or tonic, of relay cells, which relates to the attentional demands at the moment. This response mode depends on membrane potential, which is controlled effectively by the modulator inputs. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to the cortex for the first time. By contrast, the other main thalamic relay of visual information, the pulvinar region, is largely a higher-order relay, since much of it relays information from layer 5 of one cortical area to another. All thalamic relays receive a layer-6 modulatory input from cortex, but higher-order relays in addition receive a layer-5 driver input. Corticocortical processing may involve these corticothalamocortical 're-entry' routes to a far greater extent than previously appreciated. If so, the thalamus sits at an indispensable position for the modulation of messages involved in corticocortical processing.

Examines the two-way relationships between the thalamus and the cerebral cortex; with updated material and a new chapter on the link between perception and action. The thalamus plays a critical role in perceptual processing, but many questions remain about what thalamic activities contribute to sensory and motor functions. In this book, two pioneers in research on the thalamus examine the close two-way relationships between thalamus and cerebral cortex and look at the distinctive functions of the links between the thalamus and the rest of the brain. Countering the dominant "corticocentric" approach to understanding the cerebral cortex—which does not recognize that all neocortical areas receive important inputs from the thalamus and send outputs to lower motor centers—S. Murray Sherman and R.W. Guillery argue for a reappraisal of the way we think about the cortex and its interactions with the rest of the brain. The book defines some of the functional categories critical to understanding thalamic functions, including the distinctions between drivers (pathways that carry messages to the cortex) and modulators (which can change the pattern of transmission) and between first-order and higher-order thalamic relays—the former receiving ascending drivers and the latter receiving cortical drivers. This second edition further develops these distinctions with expanded emphasis throughout the book on the role of the thalamus in cortical function. An important new chapter suggests a structural basis for linking perception and action, supplying supporting evidence for a link often overlooked in current views of perceptual processing.

When one nerve cell acts on another, its postsynaptic effect can vary greatly. In sensory systems, inputs from ''drivers'' can be differentiated from those of ''modulators.'' The driver can be identified as the transmitter of receptive field properties; the modulator can be identified as altering the probability of certain aspects of that transmission. Where receptive fields are not available, the distinction is more difficult and currently is undefined. We use the visual pathways, particularly the thalamic geniculate relay for which much relevant evidence is available, to explore ways in which drivers can be distinguished from modulators. The extent to which the distinction may apply first to other parts of the thalamus and then, possibly, to other parts of the brain is considered. We suggest the following distinctions: Crosscorrelograms from driver inputs have sharper peaks than those from modulators; there are likely to be few drivers but many modulators for any one cell; and drivers are likely to act only through ionotropic receptors having a fast postsynaptic effect whereas modulators also are likely to activate metabotropic receptors having a slow and prolonged postsynaptic effect.

The thalamus has long been seen as responsible for relaying information on the way to the cerebral cortex, but it has not been until the last decade or so that the functional nature of this relay has attracted significant attention. Whereas earlier views tended to relegate thalamic function to a simple, machine-like relay process, recent research, reviewed in this article, demonstrates complicated circuitry and a rich array of membrane properties underlying the thalamic relay. It is now clear that the thalamic relay does not have merely a trivial function. Suggestions that the thalamic circuits and cell properties only come into play during certain phases of sleep to effectively disconnect the relay are correct as far as they go, but they are incomplete, because they fail to take into account interesting and variable properties of the relay that, we argue, occur during normal waking behavior. Although the specific function of the circuits and cellular properties of the thalamic relay for waking behavior is far from clear, we offer two related hypotheses based on recent experimental evidence. One is that the thalamus is not used just to relay peripheral information from, for example, visual, auditory, or cerebellar inputs, but that some thalamic nuclei are arranged instead to relay information from one cortical area to another. The second is that the thalamus is not a simple, passive relay of information to cortex but instead is involved in many dynamic processes that significantly alter the nature of the information relayed to cortex.