23/04/2012 by admin.

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.

Posted in attention, thalamus | 1 Comment »

04/09/2011 by admin.

ScienceDaily (here) reports research by Huguenard and others, ‘A new mode of corticothalamic transmission revealed in the Gria4-/- model of absence epilepsy’.

Absence or petit-mal seizures are a sudden loss of consciousness for a short period which may or may not be noticed by onlookers but is not noticed by the person having the seizure. “It’s like pushing a pause button.”

A bioengineered strain of mice, without the GluA4 receptor, is prone to these seizures and were used to investigate the cause of absence seizures. During seizures (human and mouse) there is an unusual, strong oscillation involving the cortex and thalamus. What causes this rhythm?

Normally:

To keep from being constantly bombarded by distracting sensory information from other parts of the body and from the outside world, the cortex flags its activity level by sending a steady stream of signals down to the thalamus, where nearly all sensory signals related to the outside world are processed for the last time before heading up to the cortex. In turn, the thalamus acts like an executive assistant, sifting through sensory inputs from the eyes, ears and skin, and translating their insistent patter into messages relayed up to the cortex. The thalamus carefully manages those messages in response to signals from the cortex.

These upward- and downward-bound signals are conveyed through two separate nerve tracts that each stimulate activity in the other tract. In a vacuum, this would soon lead to out-of-control mutual excitement, similar to a microphone being placed too close to a P.A. speaker. But there is a third component to the circuit: an inhibitory nerve tract that brain scientists refer to as the nRT. This tract monitors signals from both of the other two, and responds by damping activity. The overall result is a stable, self-modulating system that reliably delivers precise packets of relevant sensory information but neither veers into a chaotic state nor completely shuts itself down.

The bioengineered mice lack the GluA4 receptor which is critical to the stimulation of nRT cells.

This leaves nRT receiving signals from one tract, but not the other, which upsets the equilibrium usually maintained by the circuit. As a result, one of its components — the thalamocortical tract — is thrown into overdrive. Its constituent nerve cells begin firing en masse, rather than faithfully obeying the carefully orchestrated signals from the cortex. This in turn activates the nRT to an extraordinary degree, because its contact with the thalamocortical tract is not affected in these mice…. In the face of over-amped signaling from the thalamocortical tract, however, the fraction of excited nRT nerve cells rose much higher, perhaps as much as 50 percent — enough to effectively silence all signaling from the thalamus to the cortex — a key first step in a seizure….But the shutdown was transitory. A property of thalamic cells (like other nerve cells) is that when they’ve been inhibited they tend to overreact and respond even more strongly than if they had been left alone. After a burst of nRT firing, this tract’s overall inhibition of the thalamocortical tract all but halted activity there for about one-third of a second. Like boisterous schoolchildren who can shut up only until the librarian leaves the room, the thalamocortical cells resumed shouting in unison as soon as the inhibition stopped, and a strong volley of signaling activity headed for the cortex. Then the nRT’s inhibitory signaling recommenced, and the stream of signals from the thalamus to the cortex ceased once again. This three-Hertz cycle of oscillations consisting of alternating quiet and exuberant periods repeated over the course of 10 or 15 seconds was the electrophysiology of a seizure.

The group is now looking for triggers that could produce a similar malfunction in humans, that would allow the cortico-thalamo-cortical transmission system to escape the control of the nRT (reticular thalamic nucleus).

Here is the abstract:

Cortico-thalamo-cortical circuits mediate sensation and generate neural network oscillations associated with slow-wave sleep and various epilepsies. Cortical input to sensory thalamus is thought to mainly evoke feed-forward synaptic inhibition of thalamocortical (TC) cells via reticular thalamic nucleus (nRT) neurons, especially during oscillations. This relies on a stronger synaptic strength in the cortico-nRT pathway than in the cortico-TC pathway, allowing the feed-forward inhibition of TC cells to overcome direct cortico-TC excitation. We found a systemic and specific reduction in strength in GluA4-deficient (Gria4^−/−) mice of one excitatory synapse of the rhythmogenic cortico-thalamo-cortical system, the cortico-nRT projection, and observed that the oscillations could still be initiated by cortical inputs via the cortico-TC-nRT-TC pathway. These results reveal a previously unknown mode of cortico-thalamo-cortical transmission, bypassing direct cortico-nRT excitation, and describe a mechanism for pathological oscillation generation. This mode could be active under other circumstances, representing a previously unknown channel of cortico-thalamo-cortical information processing.

I see this result somewhat differently. More from the bottom up then the to down. Consciousness is driven by waves of activity - waves from the brain stem up through the ascending reticular formation into the thalamus (the nRT part) and from the thalamus radiated to most of the cortex. It is the rhythm from below that drives the thalamus-cortex rhythm not vice versa. (Of course seizures are different and may be driven differently.) I do not have access to the original paper and so I am not sure whether the authors imply in it that the cortex controls the thalamus. I continue to view the close relationship between the thalamus and the cortex during consciousness as a partnership of equals. However it is closer to ‘the thalamus having the assistance of the cortex’ rather than ‘the thalamus acting as the executive assist to the cortex’ in my view. Perhaps further work on absence seizures will change my mind.

Posted in thalamus | 1 Comment »

05/12/2010 by admin.

The OnTheHuman site has an article by J. Prinz (here). I certainly like his approach and find his arguments very convincing.

We … ask which of our psychological states can be conscious. Answers to this question range from boney to bulgy. At one extreme, there are those who say consciousness is limited to sensations; in the case of vision, that would mean we consciously experience sensory features such as shapes, colors, and motion, but nothing else. This is called conservatism (Bayne), exclusivism (Siewert), or restrictivism (Prinz). On the other extreme, there are those who say that cognitive states, such as concepts and thoughts, can be consciously experienced, and that such experiences cannot be reduced to associated sensory qualities; there is “cognitive phenomenology.” This is called liberalism, inclusivism, or expansionism. If defenders of these bulgy theories are right, we might expect to find neural correlates of consciousness in the most advanced parts of our brain. …

Not only do I think consciousness is restricted to the senses; I think it arises at a relatively early level of sensory processing. Consider vision. According to mainstream models in neuroscience, vision is hierarchically organized. Let’s consider where in that hierarchy consciousness arises. … I think consciousness arises at the intermediate level. We experience the world as a collection of bounded objects from a particular point of view, not as disconnected, edged, or viewpoint invariant abstractions. … I think this is true in other senses as well. For example, when we listen to a sentence, the words and phrases bind together as coherent wholes (unlike low-level hearing), and we retain specific information such as accent, pitch, gender, and volume (unlike high-level hearing). Across the senses, the intermediate-level is the only level at which perception is conscious. …

Expansionists say we can be conscious of concepts and thoughts, and that such experiences outstrip anything going on at the intermediate-level of perception. … Associative visual agnosia … cannot recognize objects, but they seem to see them. When presented with an object, they can accurately describe or even draw its shape, but they can’t say what it is. Bayne thinks their experiences are incomplete. He thinks knowing the identity of an object changes our experience of it. This is intuitively plausible. … Instead, we can suppose that our top-down knowledge of the meaning changes how we parse the image. … imaginatively impose a new orientation; we segment figure and ground; and we generate emotions and verbal labels, which we experience consciously along with the image; these are just further sensory states—bodily feelings in the case of emotions, and auditory images in the case of words. I think features of this kind can also explain what is missing in agnosia. Without meaning, images can be hard to parse, and associated images and behaviors do not come to mind.

Another argument comes from Charles Siewert. He focuses on our experience of language. Sometimes, when hearing sentences, we undergo a change in phenomenology, and that change occurs as a result of a change in our cognitive interpretation of the meanings of the words. … Phenomenology also changes when we repeat a word until it becomes meaningless, or when we learn the meaning of a word in a foreign language. In all these cases, we experience the same words across two different conditions, but our experience shifts, suggesting that assignment of meaning is adding something above and beyond the sound of the words. … But there are many sensory changes that take place as a result of sentence comprehension. First, we form sensory imagery. … Second, comprehension effects parsing. … Third, comprehension entails knowing how to go on in a conversation … Fourth, meaning effect emotions. …

The third argument I will consider comes from David Pitt. He begins with the observation that we often know what we are thinking, and we can distinguish one thought from another. This knowledge seems to be immediate, not inferential, which suggests we know what we are thinking by directly experiencing the cognitive phenomenology of our thoughts. The most obvious reply is that knowledge of what we are thinking is based on verbal imagery. … I think this is a kind of illusion. We erroneously believe that we are directly aware of the contents of our thoughts when we hear sentences in the mind’s ear. This belief stems from two things. First, we often use verbal imagery as a vehicle for thinking …Second, when contemplating a word that we understand, we can effortlessly call up related words or imagery, which gives us the impression that we have a direct apprehension of the meaning of that word. Our fluency makes us mistake awareness of a word for awareness of what it represents. …

Putting these points together, I think restrictivsts should admit that thinking has an impact on phenomenology, but that impact can be captured by appeal to sensory imagery including images of words, emotions, and visual images of what our thoughts represent. Expansionists must find a case where cognition has an impact on experience, without causing a concomitant change in our sensory states. That’s a tall order.

At this point the dispute between restrictivists and expansionists often collapses into a clash on introspective intuitions. … By way of conclusion, I will try to break this stalemate by sketching five reasons for thinking restrictivism is preferable even if introspection does not settle the debate.

Next comes the arguments for excluding cognitive phenomenology.

1. To make a convincing case for cognitive phenomenology, expansionists should find a case where the only difference between two phenomenologically distinct cases is a cognitive difference. But so far, no clear, uncontroversial case has been identified.

2. The second argument points to the fact that alleged cognitive qualities differ profoundly from sensory qualities in that the latter can be isolated in imagination. … If other qualia can be isolated, why not cognitive qualia?

3. Third, it is nearly axiomatic in psychology that we have poor access to cognitive processes. … The only processes we ever seem to experience consciously are those that we have translated, with great distortion, into verbal narratives.

4. A fourth argument follows on this one. The incessant use of inner speech is puzzling if we have conscious access to our thoughts. Why bother putting all this into words when thinking to ourselves without any plans for communication? …

5. Finally, expansionism seems to dash hopes for a unified theory of consciousness. … But there is little reason to think a single mechanism could explain how both perception and thought can be conscious, if cognitive phenomenology is not reducible to perception. This is especially clear if the mechanism is attention. There is no empirical evidence for the view that we can attend to our thoughts. There are no clear cognitive analogues of pop-out, cuing, resolution enhancement, fading, multi-object monitoring, or inhibition of return. Thoughts can direct attention, but we can’t attend to them. Or rather, thoughts become objects of attention only when they are converted into images, words, and emotions. Expansionists might say that thought and sensations attain consciousness in different ways, but, if so, why think that the term “consciousness” has the same meaning when talking about thoughts, if it does not refer to the same mechanism?

This fits with the idea that only what enters the cortex through the thalamus, can be involved in the thalamo-cortical loops that synchronize their firing during the conscious experience. This category is sensory input (except the bulk of smell) and input about movement and emotion input via the basal ganglia.

Posted in thalamus, qualia | 1 Comment »

02/12/2010 by admin.

One of the reasons that the neo-cortex has center stage in our view of the brain is that it is big, very big; another is that it is relatively bigger in humans than in animals; and finally is the fact that we can examine it more easily than other parts of the brain. So, hey, it just must be the center of thought. A trick to correct this habit of thought for a few moments now and then is to envision the neo-cortex as the computer used by the thalamus, archeocortex and basal ganglia to do the donkey work in sorting out the detail in perception, motor programming etc. You may not want to get too fond of this picture but it is a good antidote for the continuous emphasis of the neo-cortex.

Things may change. It seems that a more powerful fMRI is now available and it can actually show specific parts the the thalamus etc. ‘lighting up’. Here is the abstract of a recent paper from Otto-von-Guericke University:

Thalamocortical loops, connecting functionally segregated, higher order cortical regions, and basal ganglia, have been proposed not only for well described motor and sensory regions, but also for limbic and prefrontal areas relevant for affective and cognitive processes. These functions are, however, more specific to humans, rendering most invasive neuroanatomical approaches impossible and interspecies translations difficult. In contrast, non-invasive imaging of functional neuroanatomy using fMRI allows for the development of elaborate task paradigms capable of testing the specific functionalities proposed for these circuits. Until recently, spatial resolution largely limited the anatomical definition of functional clusters at the level of distinct thalamic nuclei. Since their anatomical distinction seems crucial not only for the segregation of cognitive and limbic loops but also for the detection of their functional interaction during cognitive-emotional integration, we applied high resolution fMRI on 7 Tesla. Using an event-related design, we could isolate thalamic effects for preceding attention as well as experience of erotic stimuli. We could demonstrate specific thalamic effects of general emotional arousal in mediodorsal nucleus and effects specific to preceding attention and expectancy in intralaminar centromedian/parafascicular complex. These thalamic effects were paralleled by specific coactivations in the head of caudate nucleus as well as segregated portions of rostral or caudal cingulate cortex and anterior insula supporting distinct thalamo-striato-cortical loops. In addition to predescribed effects of sexual arousal in hypothalamus and ventral striatum, high resolution fMRI could extent this network to paraventricular thalamus encompassing laterodorsal and parataenial nuclei. We could lend evidence to segregated subcortical loops which integrate cognitive and emotional aspects of basic human behavior such as sexual processing.

All the anatomical detail aside (not that it is not important) what we are finally coming close to seeing is the heart of the system – the interaction of the various parts of the brain, the important feedback loops, and not just the neo-cortex. I believe that we need to understand those loops before we can come close to understanding the mind. We have a new window – great.

Metzger CD, Eckert U, Steiner J, Sartorius A, Buchmann JE, Stadler J, Tempelmann C, Speck O, Bogerts B, Abler B, & Walter M (2010). High field FMRI reveals thalamocortical integration of segregated cognitive and emotional processing in mediodorsal and intralaminar thalamic nuclei. Frontiers in neuroanatomy, 4 PMID: 21088699

Posted in thalamus | 1 Comment »

12/01/2010 by admin.

An item in the Scientific American (here), Reviving Consciousness in Injured Brains by C. Koch, describes the effects of deep-brain stimulation. It is a reminder not to confuse the content of consciousness with its functional container.

Most scholars concerned with the material basis of consciousness are cortical chauvinists. They focus on the two cortical hemispheres that crown the brain. It is here that perception, action, memory, thought and consciousness are said to have their seat.

There is no question that the great specificity of any one conscious perceptual experience… is mediated by coalitions of synchronized cortical nerve cells and their associated targets in the satellites of the cortex, thalamus, amygdala, claustrum and basal ganglia. Groups of cortical neurons are the elements that construct the content of each particular rich and vivid experience. Yet content can be provided only if the basic infrastructure to represent and process this content is intact. And it is here that the less glamorous regions of the brain, down in the catacombs, come in… injury to large chunks of cortical tissue, particularly of the so-called silent frontal lobes, can lead to a loss of specific conscious content but without any massive impairment in the victim’s behavior. … But destruction of tissue the size of a sugar cube in the brain stem and in parts of the thalamus, especially if they occur simultaneously on the left and right sides, may leave the patient comatose, stuporous or otherwise unable to function… can cause consciousness to flee permanently…

pioneers are finding innovative ways to help. Their technology of choice is deep-brain stimulation (DBS). The method has been much in the public eye as a way to ameliorate the symptoms of Parkinson’s disease. Electrodes are implanted into a region just below the thalamus, the quail-egg-shaped structure in the center of the brain. When the electric current is turned on, the rigor and tremors of this movement disorder disappear instantly…Over the past 15 years neurosurgeon Takamitsu Yamamoto and his colleagues at the Nihon University School of Medicine in Tokyo stimulated parts of the intralaminar nuclei (ILN) of the thalamus in vegetative state and minimum conscious state patients. These regions were targeted because they are involved in producing arousal and in controlling widespread activity throughout the cortex. Indeed, according to the late neurosurgeon Joseph Bogen of the University of Southern California, the ILN is the one structure absolutely essential to consciousness.

The deep-brain stimulation is helpful to some patients, but it is early days. The research does show (again) that the cortex does not work without control from older parts of the brain.

Posted in thalamus | 1 Comment »

13/12/2009 by admin.

ScienceDaily has a report of two studies on the thalamus from M. Sherman’s lab by B. Theyel and D. Llano and by C. Lee. (here).

Two new studies show that the thalamus–the small central brain structure often characterized as a mere pit-stop for sensory information on its way to the cortex–is heavily involved in sensory processing, and is an important conductor of the brain’s complex orchestra. …”The thalamus really hasn’t been a part of people’s thinking of how cortex functions,” said Sherman, “It’s viewed as a way to get information to cortex in the first place and then its role is done. But the hope is these kinds of demonstrations will start putting the thalamus on the map.”… information makes a stopover in the thalamus before being sent to the visual cortex of the brain to be processed. Similarly, auditory and somatosensory (touch) information is routed through the thalamus before traveling to cortex for more complex processing. …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, it’s not receiving any information in the first place,” …. “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.” … “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.”… Both papers newly characterize the complexity of the thalamus and its role in shaping sensory information both before and after that information reaches higher cortical regions — not a crossroads, but a conductor. … “People who study how the cortex functions now have to take the thalamus into account. This can’t be ignored.”

I like to think of the neo-cortex as the thalamus’ on-line computer. The thalamus-cortex loop is certainly part of the neurological basis of consciousness.

Posted in thalamus | 1 Comment »

21/12/2008 by admin.

Here is more from that article in the New York Times about Rodolfo Llinas’ by Sandra Blakeslee (here). This part is about conditions that appear to involve the loss of thalamus driven brain waves.

“When the brain is awake, neurons in the cortex and thalamus oscillate at the same high frequency, called gamma…. .Such coherent activity allows you to see and hear, to be alert and able to think…But at day’s end, cells in the thalamus naturally enter a low-frequency oscillation… .You fall asleep. Your brain is still tapping out slow rhythms, but consciousness is suspended.

So if a small part of the thalamus gets permanently stuck at a low frequency, or part of the cortex fails to respond to the wake-up call, Dr. Llinás said, an abnormal rhythm is generated, a so-called thalamocortical dysrhythmia….a maintained, abnormal low frequency in a part of the brain can generate what is called an attractor. Think about a tornado. It’s just wind that is turning on itself. In doing so, it becomes a thing that, while made out of air, has a life of its own.

“A thalamocortical dysrhythmia also has a structure. It is a thing. And it leads to the symptoms seen in a wide variety of brain diseases.”

Dr. Llinás believes that these disrupted rhythms can be set off by a variety of causes — faulty genes, brain injury, chemical imbalance…. dysrhythmias can be treated with deep brain stimulation, drugs or tiny surgical lesions that return brain oscillations to normal, he said. The goal is to wake up parts of the brain that have fallen into low-frequency sleep mode.

In Parkinson’s, chemical changes send bits of the thalamus into a low-frequency mode. If the affected part of the thalamus connects to the brain’s primary motor center, a slow tremor, at four cycles per second, appears. The patients shake at the same frequency as the oscillating motor thalamus.

If the abnormal bit of thalamus connects to a region that plans movements, the patients cannot initiate movement.

And if the piece of thalamus is involved in making smooth movements, the patients experience increased muscle tone. They become rigid.

Dr. Llinás says a patient can experience several of these symptoms or only one, depending on the site of the abnormal rhythm. By the same token, he says, normal function can be restored by acting on the right spot.

Deep brain stimulation, in which slender electrodes are implanted directly into the cortex or thalamus, has been used in 40,000 patients around the world, mostly for movement disorders, and is now being tried for schizophrenia, epilepsy, Tourette’s syndrome, dystonia, chronic pain, depression, phantom pain and traumatic brain injury.”

The interesting thing here, for our concern with understanding consciousness, is that the thalamus appears to control the existence of consciousness. It may also control the focus and nature of the conscious experience.

Posted in thalamus | 1 Comment »

06/12/2008 by admin.

There has been an article in the New York Times about Rodolfo Llinas’ ideas (here). I have blogged before about his insights – they are worth many visits. Here is part of the NYT piece by Sandra Blakeslee.

“Dr. Llinás, the chairman of neuroscience and physiology at the N.Y.U. School of Medicine, believes that abnormal brain rhythms help account for a variety of serious disorders, including Parkinson’s disease, schizophrenia, tinnitus and depression. His theory may explain why the technique called deep brain stimulation — implanting electrodes into particular regions of the brain — often alleviates the symptoms of movement disorders like Parkinson’s.

…Unlike neuroscientists who study the brain’s outer layer, or cortex, he has focused his attention on the thalamus, a paired structure in the midbrain. He has found that each walnut-size thalamus has 30 or more nuclei, each of which specializes in one type of information collected from the senses — sights, sounds, movements, external touches, internal feelings and so on.

Each nucleus sends its message to a specific area of the cortex for initial processing. But then the information is shuttled back down to the thalamus, where it is associated with other senses. And then it is returned to the cortex in a richer, multisensory form that is constantly elaborated, reverberating into a symphony of life experiences.

The thalamus and cortex work dynamically by passing loops of information back and forth, Dr. Llinás said. “If you think of the brain as an orchestra, the thalamus is the conductor. The players are in the cortex. When the conductor makes a move, the players follow. The conductor then hears their sounds and makes new moves, resulting in a continuous dialogue.”

Cells in the thalamus and cortex rely on intrinsic electrical properties to keep the music going. “Groups of neurons, millions strong, act like little hearts beating all their own,” Dr. Llinás said. They can oscillate at multiple frequencies, depending on what is happening in the outside world.

When the brain is awake, neurons in the cortex and thalamus oscillate at the same high frequency, called gamma. “It’s like a Riverdance performance,” Dr. Llinás continued. “Some cells are tapping in harmony and some are silent, creating myriads of patterns that represent the properties of the external world. Cells with the same rhythm form circuits to bind information in time. Such coherent activity allows you to see and hear, to be alert and able to think.”

But at day’s end, cells in the thalamus naturally enter a low-frequency oscillation. They burst slowly instead of firing rapidly. With the thalamus thrumming at a slower rhythm, the cortex follows along. You fall asleep. Your brain is still tapping out slow rhythms, but consciousness is suspended….” 

This is a very convincing description to me. This is probably because it seems close to the MPOFBL idea – massively parallel over-lapping feedback loops.

Posted in thalamus | 1 Comment »

11/09/2008 by admin.

In the center of the brain is a little structure called the thalamus. It seems to be a center of activity and one of the places where four systems cross: sensory, activation, motor, limbic. Taber, Wen, Khan and Hurley start their paper, The Limbic Thalamus, with the following statement.

“The thalamus has been referred to as the “Grand Central Station” of the brain because virtually all incoming information relays through it en route to the cortex. In turn, virtually all areas of the cortex project to divisions of the thalamus. Thus, knowledge of thalamic anatomy and connections is critical in understanding thalamic influence on cortical function and in the interpretation of functional brain imaging studies.”

The first system is the sensory one. All the sensory information, such as that carried by the optic nerves, enters the thalamus (with the exception of smells). In the thalamus is a map of the retinas that receives the sight information. There is a map of the Corti membranes that receives the hearing information and a map of the body to receive touch information. Pain, visceral feelings and taste come to the thalamus. The sensory information that the cortex receives and processes comes via the thalamus. The axons that run from the thalamus to the cortex are matched by axons running in the opposite direction. Each small area of cortex appears to receive input from the thalamus and also to send its output to the thalamus. This is also true of the associative areas where two senses mix. The associative areas of the cortex are in two way communication with the associative areas of the thalamus. The thalamus also appears to have control over how parts of the cortex communicate with other parts of the cortex.

The second system is the activating one. The nervous system as a whole is the spinal cord and its extension into what is called the brain stem which has a number of structures attached to it, that we call the brain. In the brain stem is a structure called the activating reticular formation. This is an extremely ancient part of the brain. It seems to control the level of alertness: sleep, dreaming, wakefulness, alertness, fatigue, motivation. Waves of activation from the reticular formation seem to keep consciousness (or dreaming) going. When it is quiet, there is a deep sleep. If it is damaged so that it cannot maintain activation, a deep permanent coma results. This structure ends where it merges into the thalamus.

The third system is the motor one. Planning, initiating and controlling action is done by a system that includes the frontal cortex especially the pre-frontal, pre-motor and motor cortex, the basal ganglia and the cerebellum. The thalamus has a motor portion in communication with areas of the frontal cortex. This is two-way traffic as with sensory communication. The cortex can directly send signals to muscles but without modification these result in jerky movements. The cortex works with the basal ganglia and the cerebellum for fine control to give smooth movements. Cortex signals can go directly to the basal ganglia and the cerebellum but the return path goes through the thalamus. The thalamus is therefore one of the important elements of motor control.

Finally there is the limbic system which is primarily concerned with emotion, smell and memory.  It is not easy to list its components because different people have different borders to the area, but it at least contains the amygdala (chiefly associated with fear and anger), the hippocampus (associated by memory), the mammillary bodies (associated with memory), the hypothalamus (associated by drives and regulation of the internal body), the entorhinal cortex (associated with smell) and the limbic areas of the thalamus.

Input to the limbic thalamus comes from the amygdale, entorhinal cortex, septal nuclei and mammillary bodies. It is in mutually reciprocal communication with the parietal cortex, prefrontal cortex and cingulated cortex. The thalamus is also important to sensing and responding to pain.

It look very much to me, in fanciful moments, that the thalamus runs the cortex. I think of it as an ancient part of the brain that once was state-of-the-art in sensory perception and cooperated with other ancient parts of the brain to insure that responses were appropriate to situations. Then the thalamus got a brand new PC to use called the neo-cortex. The interaction between the thalamus and the cortex is probably the seat of consciousness.

Posted in thalamus | 1 Comment »