FUNCTIONAL-ORGANIZATION OF THALAMOCORTICAL RELAYS

1. The thalamus has long been seen as responsible for relaying informa tion 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 attract ed significant attention. Whereas earlier views tended to relegate tha lamic function to a simple, machine-like relay process, recent researc h, 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 func tion. Suggestions that the thalamic circuits and cell properties only come into play during certain phases of sleep to effectively disconnec t the relay are correct as far as they go, but they are incomplete, be cause they fail to take into account interesting and variable properti es of the relay that, we argue, occur during normal waking behavior. A lthough 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 t hat the thalamus is not used just to relay peripheral information from , for example, visual, auditory, or cerebellar inputs, but that some t halamic nuclei are arranged instead to relay information from one cort ical area to another. The second is that the thalamus is not a simple, passive relay of information to cortex but instead is involved in man y dynamic processes that significantly alter the nature of the informa tion relayed to cortex.