Stimulating the central lateral thalamus produces consciousness

(Warning: neuroscience weeds)

The thalamus.
Image credit: Wikipedia

A couple of people have asked me about this study, described in numerous popular science articles (such as this one).  A monkey had electrodes installed in its brain that allowed scientists to stimulate parts of its thalamus, the region at the center of the brain which links the cortex to the brainstem and other systems, as well as serves as a relay station for some inter-cortical communication.

Stimulating the monkey, while it was anesthetized, in the central lateral thalamus region caused it to wake up, look around, and reach for things.  Ceasing the stimulation caused the monkey to immediately lose consciousness.  Notably, this region is heavily interconnected with frontal and parietal regions.

Diagram showing the various regions of the thalamus
Image credit: Madhero88 via Wikipedia

Interestingly, stimulating the medial dorsal thalamus, which is heavily connected to the prefrontal cortex, “proved less effective”, and stimulating the central medial thalamus, which projects to the striatum, was also less effective.

In other words, consciousness seemed to be associated with the central lateral thalamus region and its projections to layers in the frontoparietal network.

Diagram showing the regions of the brain
Lobes of the brain
Image credit: BruceBlaus via Wikipedia

One interesting point about this study, is it seems to contradict another study from a year or two ago which ruled out the thalamus as having a role in wakefulness (favoring the basal ganglia instead, if I recall correctly), a reminder that it’s not a good idea to hang too tightly on the results of individual studies.  Another point is the demonstration that the frontoparietal network overall, not just the prefrontal cortex, seemed to be most important for stimulating consciousness.

What does it all mean?  Well, it seems like a dramatic experiment.  And it seems to re-establish the role of the thalamus in wakefulness.  The part about stimulating the regions projecting to the prefrontal cortex not being effective makes me wonder about implications for higher order theories that focus on that region.

All that said, I think we have to bear in mind the distinction between the state of consciousness, that is wakefulness or vigilance, and awareness.  A lot of the information in this experiment seems to be about the state more than awareness.  In that sense, some of the anatomical details are new, but the overall macroscopic picture doesn’t seem to be much affected.

But this is a technical paper and there are probably implications I’m missing.  In particular, the implications for anesthesiology  and other clinical situations may be very significant.

16 thoughts on “Stimulating the central lateral thalamus produces consciousness

  1. And we use words like awake when sleep is not involved, anesthetics were. The action of the anesthetic needs to be taken into account. The experiment showed that such stimulation can offset the action of the anesthetic and it may have little or nothing to do with sleep and wakefulness.

    Liked by 3 people

    1. Well, yes and no. If the anesthetics caused a lack of feeling, but still left the subject in some kind of awake but vegetative state, that might be true. But if their eyes are closed, their muscles slack, with no motor activity whatsoever, then the causal factors are different, but the relationships between the states seem much closer. Admittedly, this is far from a simple matter.

      Liked by 2 people

  2. Very interesting post, Mike. I wonder if this research could have positive implications for waking comatose patients, who otherwise would have remained in a vegetative state? Another question that arose from reading your post: how should we differentiate states of consciousness from sates of alertness and/or awareness? Are they merely other terms for the same state or are they qualitatively different? What is your take on my question?

    Liked by 2 people

    1. Thanks Mike! The paper in the discussion section does talk about the effects for clinical cases like disorders of consciousness.

      On awakeness vs awareness, they’re not the same. They almost always coincide in a healthy person. But it’s possible for someone in a persistent vegetative state to have sleep / wake cycles, where they respond reflexively and display spontaneous movements, but show no signs of actually being aware of their environment or themselves.

      Teri Schiavo was a grim example of this scenario. Her family couldn’t imagine she wasn’t still in there somewhere. However, a postmortem on her brain showed massive damage, including that her pyramidal cells were all gone, making any actual awareness impossible.

      (Note, I replaced “alertness” with “awakeness” because the word “alertness” might imply some sort of discriminatory ability. If you specifically meant it in that manner, then the lines are blurrier.)

      Liked by 2 people

    1. Based on what I could find in the paper, the two used in the experiments didn’t suffer.

      From the paper:

      We acquired data from two male monkeys (Macaca mulatta, 4.3-5.5 years old, 7.63-10.30 kg body weight). Animal daily needs maintained by experimenters and husbandry staff at the Wisconsin National Primate Research Center (WNPRC), where animals were housed. Animal health was monitored by veterinarians at the WNPRC. The University of Wisconsin-Madison Institutional Animal Care and Use Committee approved all procedures, which conformed to the National Institutes of Health Guide for the Care and Use of Laboratory Animals.

      Liked by 1 person

  3. “… a reminder that it’s not a good idea to hang too tightly on the results of individual studies.”

    That is a very important point to me. Way too often, the popular press will pull a single study out of context and cite it as the final, official word of science. This sort of research needs to be seen as part of an ongoing conversation.

    Liked by 2 people

    1. I thought you might appreciate that point since you read a lot of papers yourself. With recent papers increasingly being available in preprint form, which is awesome, but not peer reviewed yet, it’s become even more important to remember.

      Liked by 2 people

        1. Believe it or not, this paper actually cites one of the preprints I highlighted a few weeks ago, so even the papers aren’t waiting anymore. I’m starting to suspect that peer review is in trouble. A lot of people detest it due to the abuse anonymity enables. The scientific community may have to just rely on public feedback in the future.

          Liked by 1 person

          1. It makes me nervous too. But widespread peer review isn’t that old, arising only in the 20th century. And the politics of peer review can sometimes warp published content. I remember as a grad student learning that you needed to “defensively” cite as many active researchers in the field as you could in order to avoid offending potential reviewers.

            The nice thing about the preprint system is that the pre-review content is out there, so if the reviewers are simply biased, it’s more obvious what happened.

            Liked by 1 person

  4. Rodolfo R Llinás (the discover of how myelinated action potentials work) advocated for a theory of consciousness in which the 40-50 Hz activity seen in the thalmic relays connecting spatially distance cortical populations was necessary but not sufficient. It’s nice to see the evidence still piling up. Check out his book, “I of the Vortex: From Neurons to Self”. It’s a great read.

    Click to access I%20of%20the%20Vortex_%20From%20Neurons%20to%20Self_%20Rodolfo%20R%20Llinas.pdf

    Liked by 2 people

    1. There was a paper recently that worked to further develop thalamo-cortical loop theory that you might find interesting, if you haven’t seen it yet.

      Liked by 1 person

      1. This study, which I have pointed to previously, naturally was what I thought of also.

        It seems like consciousness requires some level of sensory input so the thalamus necessarily is going to be engaged somewhere in the process. This seems to apply to anesthesia and deep sleep. Where dreams fit I’m not sure.

        Of course, I’m not a fan of the higher-order theories but I don’t know that this study really does great damage to them. If sensory input flowing to cortex is required, it may just be what they stimulated nullified the effects of the anesthesia and reestablishes the flow of sensory input to the cortex. Worse case would be the theories might need a small amendment.

        Solms has always been of the view that consciousness comes from the RAS but, when you see what he is actually talking about, it is that wakefulness comes from it. It could be what the RAS does is actually nothing more than stimulate this part of the thalamus exactly as they did in the experiment electrically. For Solms the representations of consciousness form in the cortex. So is consciousness caused by one necessary thing that is required to create it (RAS? thalamus?) or where it actually forms (the cortex). Maybe the answer is both.

        Liked by 2 people

        1. I agree on the higher order theories probably not being that affected by it. I just mentioned it as a possibility, although as you mention, the whole system is designed to work together, so suppressing the whole thing and stimulating parts has limitations, particularly using non-humans.

          From what I’ve read, the RAS enables consciousness, powers it through excitatory activation up through the midbrain, thalamus, and basal ganglia. But it’s not where awareness happens. I haven’t read Solms directly, but his views are usually cited with Bjorn Merker and Jaak Panksepp, who situate awareness, or at least some awareness, in the midbrain region. But most of the field sees awareness happening in the thalamo-cortical system, and I think the evidence for that is pretty strong, at least for healthy people. But regardless, the RAS is crucial.

          Liked by 1 person

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