A commenter to this blog a couple of months back, Boris, got me thinking and looking at the detail of thalamus activity. Particularly the inhibitory signals seemed to be a bit of a mystery.
In the next few postings, I am going to look at a couple of papers that shed light on this aspect of the thalamocortical loop system. The first paper by Byoung-Kyong Min (citation below) examines activity of the thalamic reticular nucleus.
The position of the thalamic reticular nucleus (TRN) is interesting. It forms a thin covering over much of the thalamus so that axons entering the thalamus from the cortex pass through the TRN and branches of these axons make contact with TRN neurons on their way to neurons elsewhere in the thalamus. The formation has input from the reticular formation which is like an extension of the spinal cord, through the brain stem carrying input from areas in the lower brain. The TRN neurons can inhibit the neurons within the thalamus that are active. In other words the TRN monitors the thalamus traffic and controls level of activity.
The abstract reads:
[Background]: It is reasonable to consider the thalamus a primary candidate for the location of consciousness, given that the thalamus has been referred to as the gateway of nearly all sensory inputs to the corresponding cortical areas. Interestingly, in an early stage of brain development, communicative innervations between the dorsal thalamus and telencephalon must pass through the ventral thalamus, the major derivative of which is the thalamic reticular nucleus (TRN). The TRN occupies a striking control position in the brain, sending inhibitory axons back to the thalamus, roughly to the same region where they receive afferents.
[Hypotheses]: The present study hypothesizes that the TRN plays a pivotal role in dynamic attention by controlling thalamocortical synchronization. The TRN is thus viewed as a functional networking filter to regulate conscious perception, which is possibly embedded in thalamocortical networks. Based on the anatomical structures and connections, modality-specific sectors of the TRN and the thalamus appear to be responsible for modality-specific perceptual representation. Furthermore, the coarsely overlapped topographic maps of the TRN appear to be associated with cross-modal or unitary conscious awareness. Throughout the latticework structure of the TRN, conscious perception could be accomplished and elaborated through accumulating intercommunicative processing across the first-order input signal and the higher-order signals from its functionally associated cortices. As the higher-order relay signals run cumulatively through the relevant thalamocortical loops, conscious awareness becomes more refined and sophisticated.
[Conclusions]: I propose that the thalamocortical integrative communication across first- and higher-order information circuits and repeated feedback looping may account for our conscious awareness. This TRN-modulation hypothesis for conscious awareness provides a comprehensive rationale regarding previously reported psychological phenomena and neurological symptoms such as blindsight, neglect, the priming effect, the threshold/duration problem, and TRN-impairment resembling coma. This hypothesis can be tested by neurosurgical investigations of thalamocortical loops via the TRN, while simultaneously evaluating the degree to which conscious perception depends on the severity of impairment in a TRN-modulated network.
Synchrony is critical in forming consciousness and so Min is looking for a neural control system that can bring chaotic activity into a unitary synchronization. In the conscious state, the experiences of the internal and external milieu merge into a temporally and spatially unitary experience. The inhibitory GABA neurons of the TRN may act is pacemakers as GABA neurons do in some other places in the brain.
McCormick suggested the possibility of a cyclical thalamocortical interaction whose key feature is the strong activation of GABAergic neurons within the thalamus. Taken together, the findings … lead me to hypothesize that the inhibitory TRN cells play a key role in coordinating our conscious perception through the inhibitory feedback network across both the thalamus and the cortex.
TRN cells demonstrate several frequencies of rhythmic oscillations.
it was found that a large proportion of TRN cells (about 34%) discharged like clocks within a 25-60 Hz frequency bandwidth (i.e., gamma activity). … When a GABAergic network induces synchronization of neural activity, coherent gamma oscillations are observed. The gamma-range (more than about 30 Hz) synchronization is occasionally considered a key mechanism of information processing in neural networks. Again, the TRN is located in a particularly suitable position for controlling the entire cerebral network. Therefore, TRN-mediated synchronization in the thalamocortical network may result in gamma-band oscillations related to the binding of the stimulus features into a whole. Moreover, cortical gamma activity is concurrent with thalamic gamma activity at discrete conscious events, most likely, neural synchronization.
Gamma rhythms may be a natural state for the TRN GABA neurons, an equilibrium state in the physiology the GABA cells. Although other rhythms are possible in various conditions. A group of cells in TRN are called the pacemaker for thalamic oscillation in the rat where they were located. They have two firing modes: burst-spike and tonic-spike which they can switch between.
From the viewpoint of a gate-keeping state of the thalamus, tonic mode firing in the thalamus may be responsible for a thalamic-gate passive mode (unconscious state), whereas burst firing may account for a thalamic-gate active mode (conscious state). In keeping with such a gate-keeping mechanism, I hypothesize that a conscious state would be established when a TRN-modulated thalamocortical network activates over a certain threshold to initiate overall synchronization. In contrast, in the sub-threshold state, sensory inputs may simply pass through the thalamus without the generation of conscious awareness.
Attention picks out a small subset of possible contents of consciousness for prominence, by strengthening the ‘foreground’, weakening the ‘background’ or both. The TRN’s selective inhibition of the thalamocortical loops is well-placed for this. As well as the strength of signals, their synchrony is important. Again this is TRN speciality.
the TRN seems to play a critical and supervising role in controlling the whole brain network. Attention is eventually accomplished through cooperatively integrating information from attention-related cortical regions (e.g., the dorsolateral prefrontal cortex, the parietal cortex, and the orbitofrontal cortex) and from other sub-cortical regions such as the superior colliculus. In this respect, the inhibitory feedback mechanism of the TRN on the thalamocortical network becomes a potential candidate for controlling and coordinating the orientation of attention. In accordance with this conception, TRN lesions effectively prevented perseverative behavior in rats, while lesions of the orbitofrontal cortex failed to do so. … it is likely that working memory can be thought of as temporal mental traces of
attended conscious awareness during a transient time range around the present.
There is a good deal of detailed argument for the TRN’s involvement in attention and working memory in the paper.
The theory also includes awareness.
the TRN can be said to act as an integrative junction of different but associated thalamocortical circuits. Sherman and Guillery suggested that the functional significance of such a gathering venue may be most important for the interactions among first-order and higher-order circuits that belong to the same modality grouping. …the first-order and higher-order relay circuits controlled by the TRN can yield more refined and thus higher cognitive information, as their circulating feedbacks run over and over again in an integrative reprocessing manner. In this sense, the compact latticework formation of the TRN is advantageous to coordinate the overall conscious experience.
The TRN-modulation hypotheses for consciousness, attention, and awareness can be summarized as follows:
 Consciousness is referred to as thalamocortical response modes controlled by the TRN and is embodied in the form of dynamically synchronized thalamocortical networks ready for upcoming attentional processes.
 Attention is neurophysiologically substantiated by a highlighted neural ensemble among a number of synchronized thalamocortical candidates, the topographical maps of which are projected onto the TRN.
 Thalamocortical looping via the TRN is necessary for the conscious awareness of an attended object.
I should inject a word of caution here. Feedback loops cannot be understood by figuratively starting in one place with a finger and tracing around the loop; this method will give a different picture depending on where you start. Feedback loops cannot be thought of sequentially. Instead a loop will become stable in a particular state or cycle of states which can be predicted mathematically (think op amp formula). But when large numbers of parallel loops overlap (as in thalamocortical feedback) it is practically impossible to predict their behavior by the ‘finger method’ or even the ‘equation method’. This does not mean that the thalamocortical system will not be understood but it will take some effort.
Min, B. (2010). A thalamic reticular networking model of consciousness Theoretical Biology and Medical Modelling, 7 (1) DOI: 10.1186/1742-4682-7-10