Inhibition causing action potentials

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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.

Chemical imbalance

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The medical profession has often embraced unscientific theories when dealing with mental illness. Some of their treatments have worked and some haven’t – that is true of medicine in general – but their theories have a tendency to be simply unbelievable.

Take Freudian psychoanalytic theory which is comprised a lot of entities and processes that are like just-so stories. I was there in the 50s; I had a young open mind; and I tried to take it seriously but failed. It was just not believable. Many ordinary people I knew felt the same. There was no evidence that the brain worked in that way – no experiments, no statistical tests, only interpretations of individual cases. Some patients found psychoanalysis helpful but there was no proof that other ‘talk’ methods were not better and no evidence that the talk’s success, when it was successful, was connected to the theory.

Now we have another silly theory, chemical imbalance. This time we assume that the brain is something like a vat of chemicals; illness is too much or too little of some particular chemical; and the condition can be corrected by adding or subtracting that chemical. The medicines work somewhat some of the time, but usually their testing leaves a lot to be desired. We do not know if they ever even reach the brain, and we do not know what they do if they get there. The whole theory is based on assumptions. Biochemistry is complex and effects are not straightforward and a lot of effects are counterintuitive.

Where is the science behind the theory? Neuroscience is slowly building a picture of how the brain works and so far it has all most nothing in common with either the Freudian scheme or imbalance of chemicals in a ‘vat’. Those theories are simply fairy stories made out of whole cloth. That does not mean that there are no correlations or effective treatments just that a better theory is needed than chemical imbalance.

What is involved in causality?

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A recent paper looked at when the cause and effect of an event is identified: M. Rolfs, M. Dambacher, and P. Cavanagh; Visual Adaptation of the Perception of Causality; Current Biology, 2013. Here is the abstract:

We easily recover the causal properties of visual events, enabling us to understand and predict changes in the physical world. We see a tennis racket hitting a ball and sense that it caused the ball to fly over the net; we may also have an eerie but equally compelling experience of causality if the streetlights turn on just as we slam our car’s door. Both perceptual and cognitive processes have been proposed to explain these spontaneous inferences, but without decisive evidence one way or the other, the question remains wide open. Here, we address this long-standing debate using visual adaptation—a powerful tool to uncover neural populations that specialize in the analysis of specific visual features. After prolonged viewing of causal collision events called “launches”, subsequently viewed events were judged more often as noncausal. These negative aftereffects of exposure to collisions are spatially localized in retinotopic coordinates, the reference frame shared by the retina and visual cortex. They are not explained by adaptation to other stimulus features and reveal visual routines in retinotopic cortex that detect and adapt to cause and effect in simple collision stimuli.

In other words, this is evidence supporting the idea that at least some types of causality are identify very early in perception, perhaps all. This is not that surprising, perception is for this sort of clarification: identifying objects, movements and producing a meaningful world. It is nice to have the conformation. Very young babies identify causal event (and aminacy and intention) long before they understand all that much about the world. They use these automatic perceptions in order to learn about the world.

What has bothered me is a quote in ScienceDaily by one of the authors. I am unable to read the original paper so I do not know if this is quote is typical of the framework of the paper. It does not seem to be found in the abstract or the paper’s figures which I can access. The underlining is mine.

Rolfs, who conducted much of the research as a post-doctoral fellow in NYU’s Department of Psychology. “This finding ends a long-standing debate over how some visual events are processed: we show that our eyes can quickly make assessments about cause-and-effect — without the help of our cognitive systems.” We frequently make rapid judgments of causality (“The ball knocked the glass off the table”), animacy (“Look out, that thing is alive!”), or intention (“He meant to help her”). These judgments are complex enough that many believe that substantial cognitive reasoning is required — we need our brains to tell us what our eyes have seen. However, some judgments are so rapid and effortless that they “feel” perceptual — we can make them using only our visual systems, with no thinking required. It is not yet clear which judgments require significant cognitive processing and which may be mediated solely by our visual system. In the Current Biology study, the researchers investigated one of these — causality judgments — in an effort to better understand the division of labor between visual and cognitive processes.

I seems to me that it is not possible to draw any sort of firm line between perception and cognition. Perception is sort-of-automatic and cognition ranges from purely automatic to not-so-much so. Whether you include perception in cognition (as I do) or you separate them using some criteria, that criteria cannot be whether the process in automatic or not. The parts of the brain work together, the functions are complex and mixed. Thought is not as it appears – much more is unconscious then we ‘feel’ is.

What we can take home is that the judgment of cause and effect can be done earlier and faster than we had suspected. There is no evidence that it is not modified later as the experimental events were very simple and there would be nothing to correct. Nor is there evidence that a causal judgment could be made later for some events that are too complex for an early, fast determination. We need the rough and ready, early and fast, judgment when it is possible but that is not the whole story. What is the difference in complexity between a judgment about causality and all the other magic of perception: colour consistency is complex, constructing a 3D world is complex – the whole thing is amazingly difficult. Identifying simple causal events may be one of the easier things that perception is involved with. That there is a causal connection when a motion switches the object carrying it seems to require less ‘computation’ then constructing a 3D world. The idea that “no thinking is required” in perception is in danger of taking the meaning out of thinking. If no thinking is involved in perception then, by that logic, we will find that none is required in moving our muscles, and then none is required for memory, none is required for learning and finally that we don’t think at all. This is not a useful path to go down.

The thalamus rules

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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.

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The thalamus as conductor

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

 

The first paper:

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

 

The second paper:

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

 

Summing up:

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

 

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Looking at the thalamic reticular nucleus

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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.

 

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Min, B. (2010). A thalamic reticular networking model of consciousness Theoretical Biology and Medical Modelling, 7 (1) DOI: 10.1186/1742-4682-7-10

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Music and movement

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

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

 

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

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

 

Bownd’s description of the computer program is clearer.

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

 

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Changizi on colour

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

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

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

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

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

After the event

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A recent paper by C. Sergent and others has been commented on by R. Kentridge (citations below). They showed that attention to the visual space where a stimulus was, but is now gone, can bring that stimulus into consciousness. This retroperception effect can occur as late as 400 ms after stimulus presentation ends.

 

Here is the abstract of the Sergent paper:

Is our perceptual experience of a stimulus entirely determined during the early buildup of the sensory representation, within 100 to 150 ms following stimulation? Or can later influences, such as sensory reactivation, still determine whether we become conscious of a stimulus? Late visual reactivation can be experimentally induced by postcueing attention after visual stimulus offset. In a contrary approach from previous work on postcued attention and visual short-term memory, which used multiple item displays, we tested the influence of postcued attention on perception, using a single visual stimulus (Gabor patch) at threshold contrast. We showed that attracting attention to the stimulus location 100 to 400 ms after presentation still drastically improved the viewers’ objective capacity to detect its presence and to discriminate its orientation, along with drastic increase in subjective visibility. This retroperception effect demonstrates that postcued attention can retrospectively trigger the conscious perception of a stimulus that would otherwise have escaped consciousness. It was known that poststimulus events could either suppress consciousness, as in masking, or alter conscious content, as in the flash-lag illusion. Our results show that conscious perception can also be triggered by an external event several hundred ms after stimulus offset, underlining unsuspected temporal flexibility in conscious perception.

 

The Kentridge commentary concludes:

Although the new study does not directly address underlying mechanisms, the effect must depend on attention acting on some neural trace that persists after the offset of the target. We know that attention modifies the neural response elicited by targets so it is, perhaps, unsurprising that attention can affect neural responses that continue after target offset. Neural activity elicited by transient visual stimuli persists for long periods. What is surprising is that retro-active attention brings otherwise unseen stimulus into consciousness.

Attention plays a role in many theories of consciousness. Both Lamme and Dehaene et al. propose that attention can amplify the neural trace of a stimulus so that it has long-lasting effects spreading from sensory areas of cortex to frontal regions. They accommodate findings that attention can act on stimuli that do not elicit consciousness by suggesting that attention only promotes stimuli to conscious report whose sensory neuronal representation persists through feedback of signals between areas. When attention produces a behavioural effect in the absence of consciousness the strength of neural response is enhanced but no recurrent feedback takes place. … The neural traces that attention acts on in Sergent et al.’s experiments persist for so long that they are likely to depend on feedback of neural signals, so it appears that without attention these recurrent signals do not elicit consciousness, as Dehaene et al. suggest.

The philosopher Ned Block, however, distinguishes between two forms of consciousness: phenomenal consciousness, which corresponds to the experience elicited by a stimulus, and access consciousness, in which the properties of the stimulus become available to cognitive processes. … He explains that ‘‘the strong but still losing coalitions in the back of the head are the neural basis of phenomenal states (so long as they involve recurrent activity)’’. … The contrary position, for example, is that experience of the unreported items is incomplete and so there is no dissociation between experience and cognitive access. For this to occur we need to have, in Block’s own words, ‘‘unconscious representations that are specific enough to do the task with the observed accuracy. the cue is supposed to promote attentional amplification of the cued unconscious specific representation, which, when combined with the conscious generic representation, results in a conscious specific representation of the cued item.’’ That is, of course, exactly what Sergent et al. have found (except that their subjects do not even appear to report a generic representation of the unseen stimulus).

Sergent et al.’s result does not necessarily invalidate the distinction between access and phenomenal consciousness, but it does lend weight to the alternative, and perhaps simpler, position that consciousness is just consciousness.

 

I suspect that the thalamocortical traffic has a lot to do with sustaining perception of unattended stimuli for some time and the directing of attention to them after the original stimuli have disappeared. There is more going on than feedforward and feedback confined to the neocortex.

ResearchBlogging.org

Sergent, C., Wyart, V., Babo-Rebelo, M., Cohen, L., Naccache, L., & Tallon-Baudry, C. (2013). Cueing Attention after the Stimulus Is Gone Can Retrospectively Trigger Conscious Perception Current Biology, 23 (2), 150-155 DOI: 10.1016/j.cub.2012.11.047

Kentridge, R. (2013). Visual Attention: Bringing the Unseen Past into View Current Biology, 23 (2) DOI: 10.1016/j.cub.2012.11.056

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I am back with some bits and pieces noticed in January

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My holiday from posting is over (I did not get moved though – now end of March date). I did notice some interesting things in January and include five of them below. Also went through the million visits mark mid-Jan.

One:

Deric Bownds (here) had this: Nakano and others, Blink-related momentary activation of the default mode network while viewing videos, in Jan 2013 PNAS.

It remains unknown why we generate spontaneous eyeblinks every few seconds, more often than necessary for ocular lubrication. Because eyeblinks tend to occur at implicit breakpoints while viewing videos, we hypothesized that eyeblinks are actively involved in the release of attention. We show that while viewing videos, cortical activity momentarily decreases in the dorsal attention network after blink onset but increases in the default-mode network implicated in internal processing. In contrast, physical blackouts of the video do not elicit such reciprocal changes in brain networks. The results suggest that eyeblinks are actively involved in the process of attentional disengagement during a cognitive behavior by momentarily activating the default-mode network while deactivating the dorsal attention network.

This fits with the idea that blinks mark the small divisions of memory, the bits which are strung together in chronological order to make a memory. It also fits with the finding that people blink in unison when watching a film. As blinks can be plainly seen and can also be recorded with electrodes, they could be useful in studies to mark the divisions of attention, thought and memory.

Two:

I have never been a fan of personality types since in first encountered them seriously over 50 years ago. They just never rang true for explaning me or people I knew well. There are schemes that have seemed true and basic that have turned out to be just surface similarities (for example the elements of air, water, fire and earth). I have noticed that there is not much evidence for personality types in physiology or genetics. There is still some disagreement among believers on how many and what archetypes to use. But, for some reason I cannot understand, they are still held to be important universal parameters by many. So I note – surprise, surprise – they may not be universal. ScienceDaily has an item (here) on a paper by M. Gurven and others, How Universal Is the Big Five? Testing the Five-Factor Model of Personality Variation Among Forager–Farmers in the Bolivian Amazon, Journal of Personality and Social Psychology, 2012.

Five personality traits widely thought to be universal across cultures might not be, according to a study of an isolated Bolivian society. Researchers who spent two years looking at 1,062 members of the Tsimane culture found that they didn’t necessarily exhibit the five broad dimensions of personality — openness, conscientiousness, extraversion, agreeableness and neuroticism — also known as the “Big Five.” … these researchers discovered more evidence of a Tsimane “Big Two:” socially beneficial behavior, also known as prosociality, and industriousness. These Big Two combine elements of the traditional Big Five, and may represent unique aspects of highly social, subsistence societies. … Other recent research has shown the existence of Big Five personality traits may be lacking in some developing cultures, particularly in Asia and Africa. … Despite its popularity, there is no good theory that explains why the Big Five takes the form it does.

Three:

ScienceDaily has an item (here) on a paper about dopamine that might change the interpretation of a number of studies – J. Salamone and M. Correa, The Mysterious Motivational Functions of Mesolimbic Dopamine. Neuron, 2012.

The widespread belief that dopamine regulates pleasure could go down in history with the latest research results on the role of this neurotransmitter. Researchers have proved that it regulates motivation, causing individuals to initiate and persevere to obtain something either positive or negative. … (This) poses a major paradigm shift with applications in diseases related to lack of motivation and mental fatigue and depression, Parkinson’s, multiple sclerosis, fibromyalgia, etc. and diseases where there is excessive motivation and persistence as in the case of addictions. … It was believed that dopamine regulated pleasure and reward and that we release it when we obtain something that satisfies us, but in fact the latest scientific evidence shows that this neurotransmitter acts before that, it actually encourages us to act. In other words, dopamine is released in order to achieve something good or to avoid something evil. … Studies had shown that dopamine is released by pleasurable sensations but also by stress, pain or loss. These research results however had been skewed to only highlight the positive influence.

There may, even probably, be a reward (and/or a punishment) system in the brain, but if there is, it is not identical to the supply of dopamine.

Four:

ScienceDaily had an item (here) by A Schindler and A Bartels; Parietal Cortx Codes for Egocentric Space beyond the Field of View; Current Biology 2012. Vision and the concept of the world from our personal viewpoint seem to be the same, but physically they are different projections. Here is the abstract:

Our subjective experience links covert visual and egocentric spatial attention seamlessly. However, the latter can extend beyond the visual field, covering all directions relative to our body. In contrast to visual representations, little is known about unseen egocentric representations in the healthy brain. Parietal cortex appears to be involved in both, because lesions in it can lead to deficits in visual attention, but also to a disorder of egocentric spatial awareness, known as hemispatial neglect. Here, we used a novel virtual reality paradigm to probe our participants’ egocentric surrounding during fMRI recordings. We found that egocentric unseen space was represented by patterns of voxel activity in parietal cortex, independent of visual information. Intriguingly, the best decoding performances corresponded to brain areas associated with visual covert attention and reaching, as well as to lesion sites associated with spatial neglect.

Five:

Deric Bownds had a post on intuition (here) from a paper: Wan and others; Developing Intuition: Neural Correlates of Cognitive-Skill Learning in Caudate Nucleus; The Journal of Neuroscience 2012 . It showed that the ability of experts to make decisions very quickly and accurately without consciously considering the situation is due to training the involves part of the basal ganglia, the caudate head. This part of unconscious cognition does not even appear to be primarily in the neocortex. Here is the abstract:

The superior capability of cognitive experts largely depends on automatic, quick information processing, which is often referred to as intuition. Intuition develops following extensive long-term training. There are many cognitive models on intuition development, but its neural basis is not known. Here we trained novices for 15 weeks to learn a simple board game and measured their brain activities in early and end phases of the training while they quickly generated the best next-move to a given board pattern. We found that the activation in the head of caudate nucleus developed over the course of training, in parallel to the development of the capability to quickly generate the best next-move, and the magnitude of the caudate activity was correlated with the subject’s performance. In contrast, cortical activations, which already appeared in the early phase of training, did not further change. Thus, neural activation in the caudate head, but not those in cortical areas, tracked the development of capability to quickly generate the best next-move, indicating that circuitries including the caudate head may automate cognitive computations.