The thalamus as conductor

16/02/2013 by admin.

We might think that the thalamus controls the input to the cortex and then is involved in consciousness, attention and working memory, but, it is the cortex alone that does the cognitive work. Not so fast. Two 2009 studies by M Sherman’s group in Chicago (here) show that the thalamus stays involved.

The first paper:

One set of experiments, conducted by Brian Theyel and Daniel Llano in Sherman’s laboratory and published online December 6 (2009) in Nature Neuroscience … The flavoprotein autofluorescence imaging technique, developed with University of Chicago assistant professor of neurobiology Naoum Issa, allowed the researchers to observe neuronal activity in a specially-prepared mouse brain slice that preserved connections between thalamus and somatosensory cortex. …Once sensory information reaches the cortex, it is thought to remain segregated there as it moves from primary cortex to secondary cortex and higher-order areas. But when Theyel severed the direct connection between primary and secondary cortical regions, stimulating primary somatosensory cortex still activated secondary cortex as well as the thalamus, suggesting a robust pathway from cortex to thalamus and back. Only when the thalamus itself is interrupted does the activation of secondary cortex fail. … The observation that at least a portion of sensory information passes back through the thalamus on its travels between cortical areas refutes the notion of the thalamus as a passive, one-time relay station, Theyel and Sherman said. … “The ultimate reality is that without thalamus, the cortex is useless… The somatosensory pathway finding demonstrates for the first time that this corticothalamocortical loop, which is also present in the auditory and visual systems, significantly activates cortex. Keeping the thalamus “in the loop” may help the brain coordinate sensory information with motor systems to direct attention or coordinate multiple cortical areas to accomplish different tasks, Sherman said. “The thalamus is a remarkable bottleneck,” Sherman said. “But that may be because as a bottleneck, it provides a convenient way to control the flow of information. It is a very strategically organized structure.”

The second paper:

In the PNAS paper, published online on December 7, postdoctoral researcher Charles Lee mapped two auditory pathways entering different parts of the thalamus to see whether they carried the same or different information….Lee recorded from neurons in different areas of the thalamus while stimulating different areas of the inferior colliculus, another brain region of the auditory pathway. When the central nucleus of the inferior colliculus was stimulated it excited an area in the thalamus known to project to primary auditory cortex, suggesting that this is the direct route for auditory information through the brain. …By contrast, stimulating the surrounding “shell” region of the inferior colliculus provokes a different response, sending a mixed combination of excitatory and inhibitory input to a different region of the thalamus in contact with higher-order cortex. “These are two parallel streams serving different functions,” Lee said. “The thalamus is also the central hub for transferring information between cortical areas. Rather than carrying information, this second pathway winds up modulating information being sent between cortical areas.”

Summing up:

(The thalamus is) not a crossroads, but a conductor. “These experiments not only give you a new way of looking at how cortex functions, but also answers a question about what most of thalamus is doing,” Sherman said. “People who study how the cortex functions now have to take the thalamus into account. This can’t be ignored.”

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Looking at the thalamic reticular nucleus

13/02/2013 by admin.

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.

In summary:

The TRN-modulation hypotheses for consciousness, attention, and awareness can be summarized as follows:

[1] ‘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.

[2] ‘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.

[3] 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

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Music and movement

10/02/2013 by admin.

Mark Changizi has an interesting way of looking at things. The brain has functions and facilities that have evolved very long ago for the situations that an ape would need to deal with. He puts forward the idea that language and music adapted to what the brain can do, and not, that the brain adapted to do what was needed for language and music. This is a main idea in his book Harnessed: How Language and Music Mimicked Nature and Transformed Ape to Man. From its blurb:

“In particular, language and music came to have the structures of the sounds in nature, just the sorts of sounds our brain had evolved to process. It is this “nature-harnessing” that explains who we are today. For speech, Changizi provides a barrage of evidence that speech across human languages mimics the fundamental sounds of physical events in the world. By mimicking the sounds that solid objects make when they hit, slide and ring, speech harnesses our ancient event-recognition powers that were never intended for language. And, for music, Changizi lays out his case that music mimics another equally important category of sound in the world: the sounds of human movement. Just as we possess brains specially designed to recognize facial expressions, our brains evolved to recognize what people are doing in our midst from the sounds they make. Music harnesses that ancient brain capability, turning a human action recognition system into a music appreciation machine.”

There is some independent experimental evidence of a connection between music and movement. Derck Bownds (here) has a posting on a paper by B. Sievers and others, Music and movement share a dynamic structure that supports universal expressions of emotion, PNAS Jan 2013. Here is the abstract:

Music moves us. Its kinetic power is the foundation of human behaviors as diverse as dance, romance, lullabies, and the military march. Despite its significance, the music-movement relationship is poorly understood. We present an empirical method for testing whether music and movement share a common structure that affords equivalent and universal emotional expressions. Our method uses a computer program that can generate matching examples of music and movement from a single set of features: rate, jitter (regularity of rate), direction, step size, and dissonance/visual spikiness. We applied our method in two experiments, one in the United States and another in an isolated tribal village in Cambodia. These experiments revealed three things: (i) each emotion was represented by a unique combination of features, (ii) each combination expressed the same emotion in both music and movement, and (iii) this common structure between music and movement was evident within and across cultures.

Bownd’s description of the computer program is clearer.

They designed an ingenious computer program that used slider bars to adjust a music player or a bouncing ball with varying rate, jitter (regularity of rate), direction, step size, and dissonance/visual spikiness. Participants were instructed to take as much time as needed to set the sliders in the program to express five emotions: “angry,” “happy,” “peaceful,” “sad,” and “scared.” One set of participants was instructed to move sliders to express the emotion with the moving ball, then other set told to move the sliders to use music to express the emotion. U.S. college students were one experimental group, the other was a culturally isolated Kreug ethnic minority in northern Cambodia with music formally dissimilar to Western music.

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Changizi on colour

07/02/2013 by admin.

I really appreciate how Mark Changizi approaches the subject of why we think the way we do. In a posting (here) he looks at colour.

I noticed that whenever the ‘hard question’ is discussed, the example that is used to illustrate its nature is colour. How do we explain colour and how do we tell if others see the same colours as we do? Changizi has an answer.

One of the reasons may be that the world can seem arbitrarily labeled in color, as if a painter dabbed over everything in order to make it beautiful… and that naturally makes us wonder what a different artist might do…. It’s an unfortunate intuition, one that seeps its way not only into the minds of laymen, but into our “enhancement” products and even the hallowed halls of philosophy. In trying to explain what’s wrong with the intuition, let me begin with a thought experiment concerning a product that gives the wearer “shape enhancement” vision…. But few of us would be interested in using them for everyday wear. We want to see the world roughly as it is, not geometrically warped for no reason… Why should it be acceptable to warp colors but not shapes? I’ll suggest here that it’s not acceptable - that once we appreciate the meaning of color it becomes apparent that we shouldn’t arbitrarily engage in color distortion…. colors are just as steeped in meaning as are shapes, pitches, and all the other non-invertible dimensions of our experience. I’ve argued in my research and in my book The Vision Revolution that our primate-variety color vision is optimized for sensing the spectral signals on skin when we blush, flush, blanch and signal other emotions. Our peculiar variety of color vision is just the needed peculiarity to sense oxygenation and concentration modulations in the blood under the skin, the physiological dimensions undergirding the colors we signal. …But I don’t believe that the fundamental appeal of color is due to this arbitrary-splashes basis at all. Instead, it seems more likely that our love of color comes from the meaning of color, namely, that color vision for us primates is a deeply human and emotional sense. Color is evocative and aesthetic because its subject-matter concerns the most evocative states of the most important objects in our lives: other people. That’s why we find color so captivating. It’s not because color floats above the world ungrounded, but, rather, because it is so deeply rooted in our psyche.

And, have you noticed that there is sometimes an assumption that our perception of colour is less mechanical than other senses. ScienceDaily (here) has a report from U of Rochester, Color Perception Is Not In The Eye Of The Beholder: It’s In The Brain. Williams and Hofer found large differences in people’s retinas but very small differences in their perception of colour. I am not sure that their results should have surprised them. There is a well known phenomena called colour consistency which ensures that the perceived color of objects remains relatively constant under varying illumination conditions.

But really, our development puts a lot of biological cost into vision – eyes, the crossover of the optic nerve, a lot of processing prior to the optical cortex and then the size and complexity of the optic cortex. Can anyone think that this system is constructed by just slapping it together so that each person’s sight is a question of chance? Your red can be my green is nonsense. What is important is what the colour means to us.

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After the event

04/02/2013 by admin.