Inhibition causing action potentials

I would like to get past my difficulty with inhibition causing action potentials. It appears to be due to the differences between ion channel gates in timing and it can result in waves of a particular frequency. It is the key to the thalamocortical system. Here are some sources for understanding the phenomenon.

Paul King (here) hints at the importance of inhibition:

In the cerebral cortex, it has been proposed that gamma frequency synchronization (40 – 80 Hz) comes about partially due to the push-pull between excitatory (E) and inhibitory (I) cells. The E cells excite the I cells, which inhibit the E cells. When inhibition wears off, the E cells that spike are more likely to do so at around the same time.

G Lindsay on the Neurdiness site (here) gives this more detailed but non-mathematical explanation:

The more complicated a system is, and the more its component parts counteract each other, the less likely it is that simply “thinking through” a conceptual model will provide the correct results. This is especially true in the nervous system, where all the moving parts can interact with each other in frequently nonlinear ways, providing some unintuitive results. For example, the Hodgkin-Huxley model demonstrates a peculiar ability of some neurons: the post-inhibitory rebound spike. This is when a cell fires (counterintuitively) after the application of an inhibitory input. It occurs due to the reliance of the sodium channels on two different mechanisms for opening, and the fact that these mechanisms respond to voltage changes on a different timescale. This phenomenon would not be understandable without a model that had the appropriate complexity (multiple sodium channel gates) and precision (exact timescales for each). So, building models is not a fundamentally different approach to science; we do it every time we infer some kind of functional explanation for a process. However, formalizing our models in terms of mathematics allows us to see and understand more minute and complex processes.

There is a mathematical explanation and model in the Intro to CHS part 1 (here) of Hodgkin-Huxley. It is probably somewhat simpler than the actual thalamocortical loop but is a stepwise explanation with graphs of the parameters. It builds to a finely timed train of impulses. Follow the link if you want to look at the detail.

The thalamus rules

There appears to be a consensus that the thalamus is important to consciousness. But it is usually considered an equal partner or even as a lesser one to the neocortex is far as consciousness goes. I asked you to suspend disbelief for a short while, you can resume it later, and consider that the thalamus may be the seat and center of consciousness. Why might this be so?

First, the neocortex is too new to be the center of consciousness. It seems reasonable that an animal that purposefully moves must have some level of consciousness. Awareness and a model of the organism-in-its-environment seems required for successful, useful movement. An animal has to know where it is and where it is going even if that knowledge is very rudimentary. The thalamus is at least as old as the earliest vertebrates. Invertebrates may have a different path to whatever amount of consciousness they have (and I assume they have some).

Second, the thalamus is the central place where things come together. For most types of information it is the first integration of different sources of information. The thalamus supplies the neocortex with almost all its input.

Third, the thalamus is the source of control of consciousness for the neocortex. Signals from the brain stem to the thalamus and from the thalamus on to the neocortex are essential to initiate and maintain consciousness. Remove the thalamus and there is no consciousness. Remove great pieces of the neocortex and consciousness remains. Remove the whole neocortex and we don’t know what level of consciousness may exist; it might be a very rudimentary one that cannot be reported (so who knows).

Fourth, the thalamus develops before the neocortex and it controls the migration of neurons to take their places in the cortex. This results in a mapping of neurons in the neocortex with neurons in the thalamus. And this results in giving the functions that concern the thalamus areas to the matching neocortex areas. Large parts of the thalamus are effectively mirrored on the neocortex.

Fifth, the thalamus and neocortex communicate through feedback loops. Where the thalamus neuron sends an axon to a few neocortex neurons, the neocortex neurons send an axon back to the same thalamus neuron. The thalamo-cortical loop is a correlate of consciousness. The thalamus appears to control the behaviour of this massive feedback system. It drives the system and controls what parts of the cortex participate at any time. It appears that any part of the cortex only contributes its content to consciousness if it is driven into synchrony with other parts of the cortex and the thalamus via the thalamo-cortical loops.

Sixth, the thalamus also control the level of activity of parts of the cortex by loops that are not specific to a particular information source. This seems to give the thalamus control over which network/types of cognition the neocortex does at any time. It also appears to control attention and possibly the use of working memory.

Taken together this looks like the thalamus creates, drives, feeds with input and micro-manages the neo-cortex. It is as if the thalamus has an online cognition engine in the way it uses the cortex to detail and refine its inputs into a very useful model, a model that the thalamus then uses to construct a conscious experience and the very short-term memory of that experience.

The suspension of disbelieve can now be removed.

Here is an abstract from a Radolfo Llinas paper, Consciousness and the thalamocortical loop, International Congress Series (2003).

Attempting to understand how the brain might be organized seems, for the first time, to be a serious topic of inquiry. One aspect of its neuronal organization that seems particularly central to global function is the rich thalamocortical interconnectivity and most particularly the reciprocal nature of the thalamocortical neuronal loop function. Moreover, the interaction between the specific and nonspecific thalamic loops suggests that rather than a gate into the brain, the thalamus represents a hub from which any site in the cortex can communicate with any other such site or sites. The goal of this paper is to explore the basic assumption that large-scale, temporal coincidence of specific and nonspecific thalamic activity generates the functional states that characterize human cognition.


The question that has to been asked is exactly how the thalamocortical system works.


The thalamus as conductor

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.”


Looking at the thalamic reticular nucleus

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

Controlling focus of attention

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.


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?


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.

Prinz view of consciousness

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?

  1. 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.

  2. 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? …

  1. 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.

Super MRI

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

The cortex is not the hub

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.

Include the thalamus

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.