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Sunday, February 21, 2010

Psst!… Pass it on! Cortical communication via the thalamus

Many neuroscientists think of the thalamus – a compact structure lying right in the centre of the brain – simply as a relay station, where sensory information from the periphery converges and is then passed on to the cortex.  The cortex is thought to be the site of perception and cognition, with different cortical areas specialized to subserve different functions.  Communication between cortical areas can be mediated by axonal tracts running in the white matter of the cortex.  This leads readily to the view that once information reaches the cortex it is processed and integrated with other information about the external world and internal states entirely within the cortex, resulting in conscious perception or some kind of motor (or emotional) output.  In most neuroimaging studies, for example, the focus is solely on activity in the cortex and the thalamus (and other lower brain areas) are explicitly ignored.

A new study by S. Murray Sherman and colleagues demonstrates unequivocally that cortical areas can also pass information indirectly via the thalamus.  It has been known for some time that communication between thalamus and cortex is bidirectional.  In fact, the thalamus receives far more inputs from the cortex than it does from the periphery.  The circuits between thalamus and cortex can be broken down into two main types – those that drive the activity of their target neurons (whether in thalamus or cortex) and those that act more to modulate the activity of their targets, especially their temporal responsiveness.  These pathways can be distinguished based on their neurochemical profiles, the types of synapses that they form and, in the case of projections from thalamus to cortex, the layers which they innervate.  Driving connections from thalamus project with quite precise topography to layers 4 and 6, while modulatory connections project more diffusely within layers 1 and 5. 

These modulatory connections from the thalamus are essential mediators of communication between cortical areas, due to their crucial role in the synchronization of ongoing neuronal oscillations – rhythmic patterns of activity of local ensembles of neurons.  Areas where these rhythms are oscillating in phase with each other are more responsive to signals from each other – this is, in effect, a kind of frequency modulation – when one cortical area sends out a signal, it is picked up selectively by those areas that are tuned to the same frequency.  This tuning can be mediated by corticothalamocortical loops (where the corticothalamic connection is driving and the thalamocortical connection is modulatory).  In this scheme, however, the information itself is transferred via direct cortical connections. 

The new study shows that even if these cortical connections are severed, information can still be transferred from one cortical area to another if corticothalamocortical circuits remain intact (in this case both the corticothalamic and the thalamocortical connections are driving).  In a thalamocortical slice preparation from the mouse brain, the authors showed, using a new optical imaging technique, that activity in secondary somatosensory cortex (S2) could be elicited by activating primary somatosensory cortex (S1).  There is nothing remarkable in that – what was remarkable was that when the direct connections between these cortical areas were severed that the activation of S2 was almost as potent.  This activation depended on an intact corticothalamocortical loop – subsequent disruption of these circuits abolished activation of S2.  There are thus most likely two routes of communication between these cortical areas – one direct and one via the thalamus. 

These findings reinforce the important point that the function of the cortex cannot be divorced from that of the thalamus.  Perception is not simply a matter of passing information along a hierarchy of processing stations – this would leave the question of who is receiving the final information.  It can be envisaged instead as a process of reiterative comparison of top-down predictions with bottom-up information, much of which may be mediated by reverberating activity in corticothalamocortical circuits.

 

Theyel, B., Llano, D., & Sherman, S. (2009). The corticothalamocortical circuit drives higher-order cortex in the mouse Nature Neuroscience, 13 (1), 84-88 DOI: 10.1038/nn.2449

7 comments:

  1. Very interesting. I suppose the big question is, does the thalmus just relay information back and forth, or does it "do something with it", like the cortex presumably does?

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  2. That’s a great question and there are several models that attempt to describe the computational operations which the thalamus performs (see Grossberg ref below, for example, for a very detailed model). Most centre on the functions of thalamocortical circuits actually, but, as I argued above, the functions of thalamus and cortex cannot really be separated from each other. The most popular ideas about the operations performed by the intrathalamic circuitry itself (especially the role of the inhibitory reticular nucleus) are that this circuitry acts as a filter to regulate attention to various stimuli by coordinating the passage of information with the regulation of synchronous oscillatory behaviour, which determines the responsiveness to that information. (So less processing the information of a specific stimulus in the way that a cortical microcolumn might do – extracting partiocular features of interest, for example – and more gating the flow of that information based on context). The reiterative, Bayesian-style comparison between top-down predictions and bottom-up information is also likely mediated by reverberating activity in thalamocortical circuits, which many think of as a marker of and possibly a substrate for conscious experience.

    Grossberg S, Versace M. Spikes, synchrony, and attentive learning by laminar thalamocortical circuits. Brain Res. 2008 Jul 7;1218:278-312

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  3. A great constructive article will help to understand the issue.

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  4. This is a very good blog!!
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  5. I'm reading all your posts in reverse chornological order since discovering you on GNXP. To me, you're in a class by yourself when it comes to explaining advances in neuroscience to laymen.

    BTW, have you anything to say on "the problem of consciousness?" I think it is interesting that in Popper's definition of what constitutes a scientific "fact" -- things about which there can be inter-subjective agreement -- consciousness is not even a fact. We see our own but nobody else can. We infer others have by analogy with ourselves, not direct observation.

    I bring this up because of an interesting video on "quantum entanglement" on the question of whether sub-atomic particles have certain properties independent of our measuring them, as is the case in the classical, macrocosmic world of physical objects. In other words is there an independent "reality" behind all the phenomena we observe (not counting the laws of nature)? The physicists say not.

    Here is the video: http://www.youtube.com/watch?feature=player_embedded&v=ta09WXiUqcQ#at=4063

    Lubos Motl writes about it: http://motls.blogspot.com/2011/05/copenhagen-interpretation-of-quantum.html

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  6. Cheers Luke for your kind comments.

    In answer to your question, I have nothing particularly insightful to say on the problem of consciousness. I am, of course, very interested in it, but am certainly as mystified as everyone else. I do think there is something there that needs explaining, unlike some folks such as Daniel Dennett, for example. I do not think we will need to invoke quantum phenomena to explain it (that won't be a useful level at which to seek for an explanation). And I also don't think there is an independent reality that cannot be described by the laws of physics (except in a very metaphorical sense referring to our subjective experience - it is not that this is not a product of normal, natural physical phenomena, it's just that, again, that is not a very useful level at which to describe or explain it). I do think there will be some principles explaining the emergent property of consciousness that we have not yet even begun to grasp. But the tools to get there are getting more and more powerful...

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  7. There is so much that comes from this part of the brain here. So much to research on this part of the brain here. Glad to see that you are talking about here. So important to do.
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