Remember the thalamus

 

What are we fairly sure about in regards to consciousness?

  • It only happens when the thalamus and cerebrum are in communication;

  • it seems to be associated with a positive signal 300 ms after an event (the P300);

  • it seems to be connected with working memory and the start of further stages of memory;

  • it seems to be connected to the focus of attention;

  • it is probably discontinuous (like frames of a movie);

  • it seems to be involved in communication of its contents to many parts of the brain;

  • gamma waves (about 40 a second), start in the thalamus and move front to back in the cortex, create synchrony in their path, and are essential for consciousness.

There are theories that put these observations together, such as the global workspace model. I am not convinced that these theories are complete because they appear to by-pass the importance of the thalamus. For years there has been a concentration on the cerebral cortex as if it was the “brain”.

Where is the model that puts the thalamus in the center of the action?

Let us assume for a while that the thalamus is the seat of consciousness. When we wake up a group of neurons in the brain stem “wake up” the thalamus, it “wakes up” the cortex and establishes the thalamo-cortical loops, then we are conscious. When we go to sleep the brain stem puts the thalamus “to sleep” and it puts the cortex “to sleep”. Signals from the thalamus control the activity of most regions of the cortex: the input from outside, activity levels, wave synchrony. Signals from the thalamus coordinate attention and the use of working memory – but, they also are the source of the cycle that produces a “frame” of consciousness and feeds it to the hippocampus to become part of a memory. The gathering of the contents of a frame of consciousness through synchrony or short-term memory in the hippocampus allows a certain type of global access to relevant information across the cortex.

Of course the cortex also affects the activity in the thalamus – this is a two way street. But it does seem to me that the thalamus drives the mechanics of consciousness. Something like attention probably is controlled by the cortical products of cognitive processes, acting through cortical executive processes which feed back to the thalamus to be implemented by its control of cortical activity. There would be all sorts of complex interactions like this where the control was in effect circular. But – the thalamus would be the time keeper and the trigger for each stage of the cycle that produces consciousness.

I cannot see a model of consciousness that ignores the thalamus as any more acceptable than one that ignores the cortex.

 

7 thoughts on “Remember the thalamus

  1. Quentin

    Thank you for these very interesting points.
    By curiosity, what is the evidential support for the assumption that consciousness is discontinuousdiscontinuous and frame-like?
    Another question: I know very little in neuroscience and it might be naive but could we interpret your statements as the idea that the cortex corresponds somehow to the “object” of consciousness, where the content is located, while the thalamus plays the role of a coordinating “subject” acting on this content? Or do you think this metaphor is misguided? I admit it seems a bit reminiscent of dualism, homonculus and the like…

    Reply
    1. JKwasniak Post author

      Stuff on discrete consciousness is in a post (May 29). Other question: The metaphor cortex=object/thalamus=subject may have some merit. Personally I would not find it useful even if I got it untangled from dualism and homonculus or even the Cartesian theatre. I tend to see it as the thalamus “having its own computer called the cortex” which it uses/drives in order to produce a perceptual moment which it uses as a basis for consciousness, but the conscious experience is more like a memory than much else and therefore is displayed in the neo-cortex and hippocampus as well as the thalamus. Take vision: the retina has neurons that pre-process the information from the rods and cones; is goes via the optic nerve to a map of the retina in the thalamus where it is further processed; from there it is copied to a map of the retina in the cortex primary visual field; the information is transferred through a series of maps in the cortex until it becomes objects (still or moving) in 3D space. All the time it is being processed in the cortex there is 2-way traffic with the thalamus. Finally this visual picture is coordinated with the pictures from the other senses, the previous perception and the prediction of what should be the current picture. This coordination is always in 2-way traffic with the thalamus. The thalamus/cortex add other non-sense information to the perception (motor, emotional etc) and the result is stored as a moment in memory. Series of moments become clumped and eventually the series of clumps become an event to be semi-permanently stored in the hippocampus. We cannot introspect our consciousness in real-time, and have to examining memories in the process of early storage. I see that the mechanism and the contents can be thought of as separate but I have trouble with an observer. The observer (self, subject or whatever) seems to be everywhere and nowhere. So I assume that there isn’t an observer/self/subject in the sense that we usually use those words.

      Reply
      1. Quentin

        Thank you for your response. This is very interesting. As far as I can see the coordination processes involved in the thalamus are unconscious and do not correspond to what we would intuitively call “conscious processes” (such as reasonning, making decisions and the like–whether these are genuine “conscious processes” or merely consciously perceived processes) and indeed the metaphor I proposed seem to be misguided.

        Reply
  2. Bonnie Rae Van Ausdal (Hall)

    Hi.. I am a long distant cousin,…I find your article fascinating … Thank you for your time, your blog has been invaluable…..Bonnie Hall

    Reply
    1. JKwasniak Post author

      Thank you for the comment. I will cut out parts of this comment here and will send you a private email soon. Janet

      Reply
  3. bolko

    What is the initial, ancestral role for the thalamus? I read somewhere that in fish and amphibians it is the main center for sensory integration. If true, is it the functional equivalent of the neocortex, which later became just a gateway and controler of the neocortex?

    Reply
    1. JKwasniak Post author

      I believe it was also that area controlling attention, memory and consciousness, such as these things were – just an opinion with no evidence.

      Reply

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