The thalamus revisited

For a few decades, I have had the opinion that to understand how the brain works it is important to look at more than the neocortex, but also look to the other areas of the brain that may modify, control or even drive the activity of the cortex. Because of my special interest in consciousness, the thalamus was always interesting in this respect. Metaphorically the cortex seemed to be the big on-line computer run by the thalamus.

A recent paper makes another connection between the cortex and the thalamus, to add to many others – (F. Alcaraz, A. R. Marchand, E. Vidal, A. Guillou, A. Faugere, E. Coutureau, M. Wolff. Flexible Use of Predictive Cues beyond the Orbitofrontal Cortex: Role of the Submedius Thalamic Nucleus. Journal of Neuroscience, 2015; 35 (38): 13183 DOI: 10.1523/JNEUROSCI.1237-15.2015).

The various parts of the thalamus are connected to incoming sensory signals, all parts of the cortex, the hippocampus, the mid-brain areas, the spinal cord and the brain stem. It is one of the ‘hubs’ of the brain and its activity is essential for consciousness. However, the particular bit of the thalamus that is implicated in this particular function (adaptive decision making flexibility) appears to have been mainly studied in relationship to pain and control of pain. There is a lot to learn about the thalamus!

Here is the abstract: “The orbitofrontal cortex (OFC) is known to play a crucial role in learning the consequences of specific events. However, the contribution of OFC thalamic inputs to these processes is largely unknown. Using a tract-tracing approach, we first demonstrated that the submedius nucleus (Sub) shares extensive reciprocal connections with the OFC. We then compared the effects of excitotoxic lesions of the Sub or the OFC on the ability of rats to use outcome identity to direct responding. We found that neither OFC nor Sub lesions interfered with the basic differential outcomes effect. However, more specific tests revealed that OFC rats, but not Sub rats, were disproportionally relying on the outcome, rather than on the discriminative stimulus, to guide behavior, which is consistent with the view that the OFC integrates information about predictive cues. In subsequent experiments using a Pavlovian contingency degradation procedure, we found that both OFC and Sub lesions produced a severe deficit in the ability to update Pavlovian associations. Altogether, the submedius therefore appears as a functionally relevant thalamic component in a circuit dedicated to the integration of predictive cues to guide behavior, previously conceived as essentially dependent on orbitofrontal functions.

SIGNIFICANCE STATEMENT: In the present study, we identify a largely unknown thalamic region, the submedius nucleus, as a new functionally relevant component in a circuit supporting the flexible use of predictive cues. Such abilities were previously conceived as largely dependent on the orbitofrontal cortex. Interestingly, this echoes recent findings in the field showing, in research involving an instrumental setup, an additional involvement of another thalamic nuclei, the parafascicular nucleus, when correct responding requires an element of flexibility (Bradfield et al., 2013a). Therefore, the present contribution supports the emerging view that limbic thalamic nuclei may contribute critically to adaptive responding when an element of flexibility is required after the establishment of initial learning.

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