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.

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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.

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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.

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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.

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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.

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