I have long thought of the thalamus as the ‘grand central station’ of the brain. An extension of the spinal cord (the reticular formation) comes through the lower brain and ends in the thalamus. It is the ascending reticular formation that controls consciousness when it is active, we are aware and when it is quiet, we are not aware. The signals that keep us awake come from the brain stem up the reticular formation into the thalamus, at the thalamic reticular nucleus. The parts of the thalamus seems to be connected to everything else too. It sends signals and receives signals from every part of the cortex and these signals are essential for consciousness. It has input from all the senses which it feeds on to the cortex (bar smell which mostly goes straight to the cortex and reaches the thalamus via the cortex). The thalamus communicates with the basal gangia and receives information on motor commands through them. And on it goes; there seems to be little that does not involve the thalamus directly or indirectly.
Basilis Zikopoulos and Helen Barbas have a series of papers on attention that put the gate to attention in the thalamic reticular nucleus. We have attention that is top-down and centered on the current task, bottom-up and centered on novel sensory input. They imply that there is also attention centered on strong emotional inputs. The thalamic reticular nucleus inhibits contributions to attention. It receives input from the amygdala (the emotional center) and if this is intense, other potential objects of attention are inhibited. The frontal cortex gives input to the same area and may trigger the inhibition of other potential objects of attention to give top-down attention. Input in the same area from the thalamic mediodorsal nucleus may serve the same purpose for bottom-up attention. The strength and priority of these signals would be used by the thalamic reticular nucleus to drive the spotlight of attention.
Here is the abstract from Zikopoulos and Barbas’ recent paper, Pathways for Emotions and Attention Converge on the Thalamic Reticular Nucleus in Primates, in the Journal of Neuroscience:
How do emotional events readily capture our attention? To address this question we used neural tracers to label pathways linking areas involved in emotional and attentional processes in the primate brain (Macaca mulatta). We report that a novel pathway from the amygdala, the brain’s emotional center, targets the inhibitory thalamic reticular nucleus (TRN), a key node in the brain’s attentional network. The amygdalar pathway formed unusual synapses close to cell bodies of TRN neurons and had more large and efficient terminals than pathways from the orbitofrontal cortex and the thalamic mediodorsal nucleus, which similarly innervated extensive TRN sites. The robust amygdalar pathway provides a mechanism for rapid shifting of attention to emotional stimuli. Acting synergistically, pathways from the amygdala and orbitofrontal cortex provide a circuit for purposeful assessment of emotional stimuli. The different pathways to TRN suggest distinct mechanisms of attention to external and internal stimuli that may be differentially disrupted in anxiety and mood disorders and may be selectively targeted for therapeutic interventions.
The thalamus is indeed a fascinating and vital part of the brain, and it’s great that you give it attention! But at least the dorsal thalamus (the part that is most developed in mammals) is perhaps best viewed as part of the neocortex - I call it “layer 0”, since the cortex itself has 6 layers, with layer 1 at the top of the cortical sheet. As some of the earlier posts point out, the thalamus receives massive feedback from the cortex, and almost all the information entering the cortex arrives via the thalamus, much of it information that the cortex itself has already partly processed (in a different area).
The cortex’s job is widely regarded as to figure out the “meaning” of what one experiences, that is to say to discover their underlying causes. It does this in large part by learning, mediated in large part by changes in synaptic connections. Interestingly, the main transformation that any given area of the cortex (eg the primary visual cortex) achieves seems to be accomplished by the pattern of the synaptic connections from thalamus to cortex, rather than by subsequent purely intracortical processing. This moves the thalamus even more to center stage.
In my opinion, as well as needing more knowledge about the neuroscience of the thalamus and cortex, we also need much better understanding of the general principles that could solve the experience-to-meaning problem. This is being actively, and at last rather succesfully, tackled by the “New AI” movement. The key to understanding animal and human inteliigence is to combine the neuroscience and AI approaches.
JK: Yes, the thalamus never seems to get the attention it deserves. Thanks for your comments.
<p>In private conversation with me, Paul stated that TRN is largely silent in awake brain. If that’s true, then it can’t be effective in modulating attention. That doesn’t sound right to me, do you have an opinion? Thanks!</p>
JK: I am thinking about your question. The TRN is not like other parts of the thalamus and I want to be sure I have re-thought this and read some recent papers. I will answer later. Thanks for the comment.
JK:
Boris, I do not want to be too firm on this answer to your question because it is not an area that I have been studying. Given that, I can tell you what my best guess is. As I understand it, sleep and waking does not originate in the thalamus but in the brain stem. Activity travels up the reticular formation when we wake. That activity affects the thalamus and that results in communication being re-established between the thalamus and the cortex. That gives awake consciousness. It is not that the cortex or the thalamus has no activity during sleep they both do. It is the coordinated communication between the two that is missing.
The thalamus is not a single nucleus but a number of them. It has input from all the senses except smell and feeds this input to the cortex. It has input from the whole cortex. It has input from the basal ganglia, cellebellum and also from the hind brain via the reticular formation. And there is much communication between the parts of the thalamus. It is ‘grand central station’.
But the thalamic reticular nucleus is not like the other parts of the thalamus. (It has no special relationship with the reticular formation although they have similar names.) The TRN is a thin covering over a large part of the thalamus. It appears to have very few conventional inputs, but rather many nerves past through it and send out little spurs into the TRN. This looks like the TRN neurons monitor the traffic between the thalamus and the cortex going both in and out. It also seems to sample traffic from the reticular formation.
The TRN neurons are inhibitory ones. When they fire they inhibit the neurons in other parts of the thalamus that they synapse with. In other words the TRN monitors the cortical-thalamic loops and affects which are active at any time. It could be likened to the spot light of attention or to the gathering of the content of consciousness that is simplistic but something like that.
It is hard to say that ‘largely quiet’ would be. I would have thought that the TRN would have more to inhibit than to leave alone or there would be too much at high priority at once. I assume that the TRN neurons would be busy during awake states but the effect would be to keep the cortical-thalamic loops from being too active all at the same time.
I can see another layer of neurons making a change in this. It the TRN neurons synapse with neurons in the rest of the thalamus that do not drive the cortical-thalamic loops but gate them, than the numbers would be the other way around. Inhibiting an inhibitor is to be an activator.
If you or Paul find (or have found) experimental results that make the situation clearer, I hope you will send me a link. I will let you know if I find something. Thanks again for raising this question.
Janet
Thanks for looking into this, Janet!
>I would have thought that the TRN would have more to inhibit than to leave alone or there would be too much at high priority at once…
That was my assumption too: TRN mediates competitive inhibition among thalamo-cortical loops. Although I don’t about the range of this inhibition: spurs | side-branches from within nuclei, regions, or cross-region?
But, this is from Paul’s message (hashcash didn’t let me post it all):
” TRN firing hyperpolarizes the relays, removing the inactivation of calcium current, which shifts the relay to “burst” mode (i.e. driving input eg from the retina will cause the relay to fire a very brief high frequency burst of spikes. If TRN cells are quiet, the relay would instead fire in single-spike, tonic, mode.
Note however that this effect of TRN is rather different in sleep or waking. Roughly, many TRN cells are firing during slow wave sleep, so many relays are in burst mode, while in the awake state, TRN activity is much sparser, so only a few relays are in burst mode at any given moment…
”
So, if “only a few relays are in burst mode at any given moment”,
then TRN / thalamus doesn’t modulate attention. Although, I suspect this might be some experimental artifact, it’s hard to believe that the whole burst mode exists mostly to maintain slow-wave sleep.
I’ve raised this with Paul, but he didn’t respond yet.
Appreciate you researching this!
Boris.
This is the paper he attached: http://rstb.royalsocietypublishing.org/content/357/1428/1767.full.pdf+html