Within the brain of both humans and monkeys are extended systems that translate the visual input entering the brain into coordinated movements based on that input and the needs of the primate. A major step on this pathway in the brain is the thalamus. The traditional view of the thalamus is that of a relay station which receives sensory input and conveys this information to cortex. This sensory input determines most of the properties of first order thalamic neurons, and so is said to drive, rather than modulate, these neurons. This holds as a rule for first order thalamic nuclei, but in contrast, higher order thalamic nuclei receive much of their input back from cerebral cortex. In addition, higher order thalamic neurons receive inputs from subcortical movement-related centers. In the terminology popularized from studies of the sensory system, can we consider these ascending motor inputs to thalamus from subcortical structures to be modulators, subtly influencing the activity of their target neurons, or should we consider them to be drivers, dictating the activity of their target neurons? We have gathered evidence from a series of experiments that indicates that the ascending motor-related inputs appear to be drivers. One pathway extends from the superior colliculus (SC) to relay neurons at the lateral edge of the mediodorsal nucleus of the thalamus (MD), and then to the frontal eye field (FEF) in prefrontal cortex. We found multiple lines of evidence that the SC provides the primary driving input to the MD relay neurons. The most important SC signal transmitted was oculomotor, starting just before saccadic eye movements. We hypothesized that this ascending presaccadic activity supplied a corollary discharge ? information to the FEF about upcoming movement ? and indeed inactivation of the pathway caused deficits consistent with impeding corollary discharge, but left the generation of saccades per se intact. In other inactivation studies, we demonstrated that inactivating the MD relay neurons interrupts transmission of signals from SC to FEF and affects the activity of FEF neurons, providing direct confirmation of a driver input from SC onto MD. Another potential input from the SC to visual cortex has been shown to contribute to visual spatial attention, and we believe this also implies a driving influence of SC onto thalamic relay neurons, although the location of those thalamic neurons remains to be determined. Thus we conclude that the inputs to the higher order nuclei of the thalamus from subcortical oculomotor areas should be regarded as motor drivers analogous to the sensory drivers at the first order thalamic nuclei. We view these motor drivers at the thalamus as being at the top of a series of information processing loops that provide signals to cortex by way of the thalamus on what information is leaving the brain, just as the sensory drivers provide signals indicating what is coming into the brain. space, and opens the possibility of understanding the basic mechanisms in the brain that underlie visual spatial attention.
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