Thalamic relays and cortical functioning
Introduction
Virtually all information reaching cortex, and thus conscious perception, must first pass through thalamus. It thus follows that the thalamus sits in a strategically vital position for brain functioning. One would think this enough to ensure that the thalamus was constantly a major focus of neuroscience research, but that has not been so. Indeed, we have recently emerged from the dark ages of thinking about thalamus: the prevalent idea being that its main purpose during normal waking behavior was simply to relay information from the periphery to cortex, a relay function that was machine-like, unvarying, and rather boring. According to this view, the thalamus only behaved in an interesting fashion during sleep or certain pathological conditions, such as epilepsy (Steriade and Llinaás, 1988; Steriade et al., 1993; McCormick and Bal, 1997), but this aspect of thalamic functioning, while interesting and still viable, is beyond the scope of the present study. Rather, the focus here is on the more recent finding that the thalamus plays an interesting and dynamic role during normal, waking behavior of the animal, and there are three aspects to this. First, it is considered that the thalamus provides a changeable relay of information to cortex, the purpose of which is to adjust the nature of relayed information to varying behavioral demands. Second, the thalamus serves not only to relay peripheral information to cortex, but it continues to play a vital role in further cortical processing of this information by acting as a central link in various corticothalamocortical routes of information processing. Third, most or all inputs to thalamus that are relayed to cortex carry information about ongoing motor instructions, so that the main role of thalamic relays is to provide a copy to cortex of these instructions. This last point has enormous implications for cortical functioning, and has been discussed in detail in Chapter 17 of this book (Guillery, 2005).
The vast majority of detailed information we have about the cell and circuit properties of the thalamus comes from studies of the lateral geniculate nucleus, which is the thalamic relay of retinal input to cortex. Studies of the lateral geniculate nucleus derive mostly from carnivores, rodents, and primates. Fortunately, this nucleus has served as an excellent model for thalamus, and all of the major concepts learned from study of this relay that are described below apply widely to thalamus.
Section snippets
Relay functions of the thalamus
One question that remains relevant and profound is: Why does information destined for cortex need to pass through a thalamic relay? Why, for instance, does retinal information pass through the lateral geniculate nucleus instead of projecting directly to cortex? If one looks at information processing in the visual system, it is clear that as one progresses up the hierarchical ladder across the various synaptic zones in retina, the receptive fields of cells become richer and more complex, and
Role of thalamus in corticocortical communication
The discussion in the previous section offers some functions for the thalamus to perform in relaying information to cortex, and other functions will doubtless be added as we learn more about this topic. This section examines the case that thalamus does more than just relay peripheral information to cortex; instead, it continues to play a role in how cortex processes such information. The logic underlying these arguments begins with a consideration of inputs to thalamic relay cells.
Conclusions
It should now be clear that the thalamus actually plays a central and dynamic role in cortical functioning. Thalamus controls the flow of virtually all information to cortex, and does so in interesting ways that we are just beginning to resolve; it not only relays peripheral information to cortex in the first place but also plays a continuing role in further corticocortical processing; and the nature of the information relayed to cortex in many and perhaps all cases seems to be a copy of motor
Acknowledgments
This research has been supported by funding from the National Eye Institute of the National Institutes of Health. I would like to thank R.W.Guillery for many helpful discussions and comments on this manuscript.
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