Monthly Archives: October 2007

Change blindness and Enactive Perception

Change blindness is an interesting phenomenon that has many implications for theories of vision. Click on the following link of an animated gif that shifts between two slightly different images and see how long it takes you to notice what is different between the two. Likely, it will take you longer than you expect.

Click here for the change blindness example

Why does this phenomenon exist? Change blindness challenges traditional “stream of vision” theories that describe vision as a process of building up detailed representations of the world around us. The phenomenon of change blindness shows us that this idea of “visual richness” is deeply flawed. Clearly, we need a new conceptual framework for perception that allows for the continuity of vision while accounting for the illusion of rich perceptual representations. As a matter of fact, I discussed such a theory in the previous post: that of James Gibson’s ecological perception.

Another name for this approach to vision is enactive perception. As the name applies, this theory views perception as a form of action. Looking is an evolved skill, like everything else, and it is an exploratory process. Under this conceptual framework, change blindness can be explained in the following way: When we look at a picture, what we see is not a representation of reality. While this seems counterintuitive, it is quite obvious when you think about the fact that one never confuses a two-dimensional picture with the real world. Whenever you look at a picture, you are always aware that you are looking at a flat surface with information on it within the real three-dimensional world. What we see is rather a collection of information, not a representation or symbol of anything.

It is this embedded information that we actively seek out whenever we look at a picture. So in the change blindness example, it takes time to fully explore and extract all information in the picture. I want to really emphasize the process of exploration, because it is this active process that gives enactive perception its rich explanatory power. In addition to change blindness, enactive perception can potentially explain the phenomena of inattentional blindness, where you fail to perceive what is available for you to see. These effects are usually explained in terms of a limited cognitive capacity for attention, but I believe these attentional theories need to be placed in a broader conceptual framework for perception, and this is where enactive perception comes in. Under this framework, attention, looking, and perception are all brought under the same conceptual umbrella: that of seeking out information offered to us by the environment.


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James Gibson, Perception, and Dualism

Ever since Descartes, there has been a looming elephant in the room for psychologists and philosophers: how do you resolve the difference between mental acts and physical ones? Is there a ontological difference between the physical and mental? Is the problem even worth discussing? In this post I want to discuss the revolutionary ideas of James Gibson on perception and their implication for Cartesian dualism.

For early to mid 20th century psychologists, this dichotomy of the mental versus physical surfaced in the opposition of the behaviorist and Gestalt schools of thought. On one hand, the behaviorists wanted to completely ignore the “black box” of the mental domain in order to explain experience, while on the other hand, the Gestaltists wanted to use “top-down” mental rules to explain the same phenomena. James Gibson side-stepped this opposition completely and thought that the two schools were both being inhibited from progress by the same conceptual stumbling block: namely, implicit Cartesian dualism.

The behaviorists wanted to impose a distinction between physicalist forces and mental ones i.e. the physical world stimulates the mental realm and the mental realm then causes the body the react. The Gestalt psychologists also wanted to keep these two realms separate, but differed from the behaviorists by trying to phenomenologically investigate the mind through “cognitive” rules of Gestalt organization. In coming up with a theory of perception, Gibson was able to avoid this dualistic trap by focusing on an integration of the mental and the physical into a single domain: the realm of organisms meaningfully acting in an environment, as opposed to being a passive receivers of stimuli.

Gibson opposed this “response psychology” firstly by coming up with research methodologies that bypassed dualism, and secondly, by postulating theoretical frameworks for his research results to rest in. In this post I’d like to discuss the theoretical underpinnings of his pioneering research in the field of visual perception.

Possibly the most important concept in Gibsonian psychology is concept of “exploration” as being a critical aspect of perception. Going against hundreds of years of philosophical dogma, Gibson likened perception to an activity, or active skill that is employed to obtain information about the environment. As Gibson put it, “perception is active, not passive. It is exploratory, not merely receptive…Exploratory movements of the eyes, the head, and even locomotor exploration in the surrounding may all the thought of as a search for more information.”

Traditionally, perception has been analyzed in terms of distal versus proximal stimuli. That is, distal photons stimulate the proximal retinal photoreceptors, the mind interprets this information, and that is all there is to it. Gibson radically challenged this view. The alternative framework Gibson proposed seeks to analyze stimuli which excite the organism, not the retina. Thus, Gibsonian psychology seeks to explain perception in terms of an active organism exploring the environment and getting information about said environment for evolutionary purposes, as opposed to being a mere passive responder to physical stimuli hitting the retina.

The environment [consists of] a sort of reservoir of possible stimuli for both perception and action, light, heat, sound, odor, gravity, and potential contacts with objects that surround the individual…the sea of energy has variables of pattern and sequence which can be registered by sense organs.(Gibson, 1960c)

Gibson proposed that the fundamental distinction was not between different levels or forms of stimuli in perception, but rather, between “modes of activity”: voluntary behavior/perception versus “imposed” stimulation. The difference being in the former stimuli are obtained by active organisms on a functional level, and in the second it is merely “imposed” on any level.

Gibson’s belief that perception is the means whereby observers keep in touch with the valuable things around them thus led to his rejection, not only of behaviorism, but of the causal theory of perception as well. He came to consider perception an activity of motivated individuals, not the result of physical causes impinging on bodies inside of which minds are trapped.
-Reed, James Gibson and the Psychology of Perception

So, James Gibson was such a revolutionary figure in psychology, not only because he was a brilliant experimentalist, but also, because his theoretical frameworks were radical departures from the implicit Cartesian dualism that had plagued psychology and philosophy for hundreds of years. He was able to move beyond the stimulus-response framework and into a conceptual schema that took organisms in environments seriously.

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Thoughts on the Soul

soul-pineal gland

The soul-theory of the mind as long been popular amongst both armchair philosophers and also serious scholars. In this post I would like to explore some implications and problems of this theory and also, how it effects our day-to-day lives, even for those of us who don’t take it seriously.

First of all, why would men have come up with a soul-theory to explain the inner workings of our minds? I think one the most obvious answers is of course, immortality. As William James said,

Unquestionably, this is the stronghold of the spiritualistic belief,-as indeed the popular touchstone for all philosophies is the question, “What is their bearing on a future life?”
-James, Principles of Psychology, 1890

However, James is apt to point out some immediate problems with the soul-theory. One of which is that the kind of immortality offered up with the theory is not the sort that we care for. What he means by this is that the only things we are cognizant of, and thus care about, are the things in our stream-of-thought. But surely, our stream-of-thought ends when we die, so why would we, meaning our personal selves, care one way or another about something beyond what we are conscious of? In other words, the conscious stream-of-thought that soul-theorists use to substantiate their ideas on immortality gives no such guarantee merely because it is there on a phenomenal level.

James gives several more answers to the question of why scholars have utilized the soul-theory for ages. One reason is that it might give an account for the “closed individuality of each personal consciousness”, that is, the fact that our Thoughts are “insulated” from the thoughts of others. There are immediate problems with such accounts, as James notes, in pathological cases such. As he says, “the definitively closed nature of our personal consciousness is probably an average statistical resultant of many conditions, but not an elementary force or fact.”

Furthermore, the soul-theory does not have any explanatory power above and beyond non-soul theories. One can give a full phenomenological account of the subjective facts of consciousness without ever referring to a soul, and furthermore:

[If we] take the two formulations, first of a brain to whose processes pulse of thought simply correspond, and second, of one two whose processes pulse of thought in a Soul correspond, and compare them together, we see that at bottom the second formulation is only a more roundabout way than the first, of expressing the same bald fact. That bald fact is that when the brain acts, a thought occurs.
-James, ibid

James final conclusion is that “the substantial Soul…explains nothing and guarantees nothing.”

So, if the soul-theory does not give us an edge in our subjective descriptions of the mind nor in our scientific ones, why is it so pervasive? Perhaps, as Douglas Hofstadter said, we see the “‘soul’ emerge as a function not of any clearly defined inner state, but as a function of our own ability to project.” By this he is referring to the fact that as humans, we have a tendency to project “souls” into inanimate objects such as cars and toys. We “animate” our pets and teddy bears with mini-souls. However, as he notes, we also have the ability to be highly selective in our “attribution of soul”. For example, one might not be capable of killing an animal in cold blood, but still eat meat on a daily basis. An extreme example is the Nazis being capable of viewing Jews as mere animals. Some emotions then, such as patriotism, can act as a “valve, controlling the emotions that allow us to identify, to project,-to see our victim as as (a reflection of) ourselves.”

We all have a storehouse of empathy that is variously hard or easy to tap into, depending on our moods and on the stimulus. Sometimes, mere words or fleeting expressions hit the bull’s-eye and we soften. Other times we remain callous and ice, unmovable

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Buddhism and Experimental Psychology


Today, I’d like to discuss an article by the American Psychological Association on Buddhism and experimental psychology.

Interdisciplinary work is currently at the forefront of research into the mind and brain, and what could be more interdisciplinary than working with Buddhist monks? After all, Buddhists have a 2,000 year headstart on empirical investigations into how the mind influences the body and vice versa. The essence of Buddhism’s approach to psychology has never been dogmatic, but rather, has always been empirical in nature. It has always been emphasized that lifelong experience through meditation is the gateway into understanding your own mind, which of course, leads to the understanding of others. It is this emphasis on experience that makes Buddhism so compatible with empirical psychology.

One line of research currently being explored by Paul Ekman is concerned with meditation and emotional control. At its heart, meditation emphasizes mindfulness of mental states. This form of mindfulness is an attentive awareness of your thoughts and emotions. In the same way that one might sit in a cafe and passively attend to the various people walking by, in meditation, one strives to not react to emotions, but rather, only be passively “mindful” of them.

In a series of yet unpublished experiments, Ekman exposed one Tibetan Buddhist monk to a sudden sound as loud as a firecracker and monitored the participant’s blood pressure, muscle movements, heart rate and skin temperature for signs of startle. The Buddhist monk, possibly due to hours of practice regulating his emotions through meditation, registered little sign of disturbance.

This ability to passively sail through mental storms almost certainly has to do with inhibitory control, which is mediated by the frontal lobes of the brain, responsible for the “executive functions” of attention, planning, socialization, and impulse control. So, it seems evident that through mindfulness training, one can increase your ability to attend to thoughts and emotions, and as they say, knowledge is power. Enhance your awareness and mental control will follow. However, because of our evolutionary history, automatic reactions to emotions is deeply ingrained in our forebrains, and consequently the meditative path is long and arduous. But take the following words to heart and be at peace,

I am about to tread the very path that has been walked by the Buddha and by his great and holy disciples. An indolent person cannot follow that path. May my energy prevail. May I succeed.


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The Reticular Nucleus and Meditation

In this post, I want to discuss the reticular nucleus of the brain and its potential relation to meditation as outlined in James Austin’s seminal work Zen and the Brain.

The reticular nucleus is best described as a thin sheet of nerve cells that surrounds the thalamus, like a cap. Here is a picture of the thalamus to give you an idea of what exactly it is encapsulating:

The reticular nucleus contains large GABA nerve cells, which are inhibitory in nature. It receives input from a network of long dendrites that cover the outer surface of the thalamus. This network monitors the information flow between the thalamus and the cortex. When too many impulses flow in, the GABA cells fire and serve to inhibit the information that would otherwise be going to the “sensory relay cells” of the thalamus.

Most of the nerve cells of the reticular nucleus fire have average firing rate between 5 and thirty-four times a second. Some, however, can fire much faster, at rates between 200 to 350 times a second. After these bursts of inhibition shut down the thamalic gate, they can pause for as long as three to four seconds, which is a relatively long time in brain terms. Because of variability of firing rates and the potential for long pauses , it is believed that these GABA cells of the reticular nucleus don’t just “shut down” the thalamus in a crude fashion, but rather, regulate its activity through complex, rhythmic oscillations of hyperpolarization and depolarization.

Okay, so what is the relation between all of this and meditation? Well, in his book, James Austin proposes two hypothesis:

The first is that the brain could shut off sensation during internal absorption by prolonging its burst phase of GABA activity. The second is that prolonging the pause phase could help sponsor the entry into consciousness of several kinds of “quickenings.” At the sensate end of the spectrum, these events could include visual episodes perceived as blinding white lights. At the mental end, brief “illuminations” of other kinds might be added to.


Cortical excitatory states descend to excite the reticular nucleus and block sensation and brainstem excitatory states ascend to inhibit the nucleus and allow more sensate messages to flow through the thalamus into consciousness

…what this thin GABA nucleus seems to be preparing us for is a relatively novel concept: a high-level blockade caused by strong afferent inhibition. Even so, outside of it,-creating it in fact- are other lays of extra excitation, still going on elsewhere.

What this means for meditation is that the prefrontal areas associated with executive control and attention can send down messages to activate the reticular nucleus, blocking sensation. This would result in various sensory modalities dropping out to the point of losing your “reference frame” and subsequently your sense of self would dissipate. Furthermore, when the lower-centres of the brain send up information to the block the inhibitory functions of the nucleus, this would result in an increase of “novel, secondary fluctuations of cognitive functions as high up as in the frontal lobe.”

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The Turing Test

I, Robot

In 1950, Alan Turing published a landmark paper in the journal Mind entitled “Computing Machinery and Intelligence”. In this paper he asked the question “Can machines think?” and proposed a method for determining whether a machine thought intelligently or not. This method became known as the Turing Test.

The test runs as follows, from wikipedia:

a human judge engages in a natural language conversation with one human and one machine, each of which try to appear human; if the judge cannot reliably tell which is which, then the machine is said to pass the test. In order to keep the test setting simple and universal (to explicitly test the linguistic capability of the machine instead of its ability to render words into audio), the conversation is usually limited to a text-only channel.

It is interesting to note that Turing himself thought that question itself(can machines think?) was “too meaningless to deserve discussion”. By this he meant that the most common objections to the question were usually drenched in emotional overtones to such a degree as to make them irrelevant. Nevertheless, Turing went on to discuss several objections to the idea can machines could ever properly be said to “think”.

Some of the objections he dismissed outright as ridiculous(such as the “head in the sand” objection that it would simply be too dreadful if machines thought), but others he gave more careful consideration of. The objection that I would like to discuss in this post is the “Argument from Consciousness” which denies the validity of the Turing Test because “No mechanism could feel(and not merely artificially signal, an easy contrivance) pleasure at its successes, grief when its valves fuse, be warmed by flattery, be made miserable by its mistakes, be charmed by sex, be angry or depressed when it cannot get what it wants.”

Turing counters this objection in the following way:

According to the most extreme form of this view the only way by which one could be sure that a machine thinks is to be the machine and to feel oneself thinking. One could then describe these feelings to the world, but of course no one would be justified in taking any notice. Likewise according to this view the only way to know that a man thinks is to be that particular man. It is in fact the solipsist point of view.

Turing goes on to note that if you do not accept this extreme viewpoint, then necessarily one must accept the validity of the terms of his test, namely that if a machine could fool you through textual-typing that it was human, then by all intents and purposes, that machine could be said to have thoughts. This might be seem silly at first because you might object and say “well, I can imagine a machine who thinks, but doesn’t have any emotions. It doesn’t care about anything”. The Turing test gets around this obvious objection because it postulates that what matters about minds is whether or not they can act in an intelligent way. He further argues that if any machine could act(type) in such a way as to convince any human observer that it was intelligent, then surely, it simply is intelligent. Furthermore, an example of intelligent thinking that would necessarily include an understanding of emotional overtones would be the reading of a good novel. This example illustrates the fact that emotion and intelligence are interlinked in such a way as to make it impossible to extract the two.

One might still object by saying that a machine could only possibly “represent” intelligent thoughts, but representations are not the same thing as real thoughts. My favorite philosopher Daniel Dennett has a fascinating reply to this objection. He asks us to imagine a computer simulation of a mathematician. Would it not be silly to complain that this simulated mathematician only gave mere representations of mathematical proofs, but not real proofs? Dennett, of course, says that representations of proofs are proofs because if this simulation of a mathematician produced proofs, would it not be valuable as a “colleague” to any proof-producing math department?

The moral of this simulated mathematician is that the criteria for what we consider thoughts depends not on whether it is represented or not represented, but rather, on the organizational pattern. In the same way that we would not care if a mathematical proof is “merely represented” if it is in fact a real proof, the question of whether “represented thoughts” were really thoughts becomes moot. We must take the Zen approach, and “unask” the question because it only obfuscates the important qualities of thought, namely its real-world effects.


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Thoughts on the self


The universal conscious fact is not “feelings and thoughts exist,” but “I think” and “I feel”. No psychology, at any rate, can question the existence of personal selves. The worst a psychology can do is so to interpret the nature of these selves as to rob them of their worth.
-William James, Principles of Psychology, 1890

So, if we are to take William James seriously, the question seems to be: should we we rob our “personal selves” of their worth by postulating neuroscientific models that attempt to “reduce” the self to nothing but the firing of neurons? Or perhaps we should fully accept the perceived phenomenological “existence” of the self as something that simply cannot be explained in brain terms? Regardless, I think there is something of a false dichotomy going on here. We need not rely solely on intra-cranial brain theories of the self, but rather, we should inject them with a radical conceptual framework that moves away from traditional Western conception of the self as a unitary, executive controller.

Buddhists have long conceptualized the self as something impermanent and illusionary, not capable of being meaningfully described or categorized. This concept of “no-self” has important philosophical implications that I think are relevant to the Western brain-based scavenger hunt for the soul. Perhaps a simple thought experiment can illustrate why a illusionary conception of the self can have useful explanatory power:

Imagine you are stranded in some remote location and you have a teletransporter that will dismantle the atoms that constitute your body and zap them through space to another location to be re-assembled. Would you use it? Well, of course! But stop and think for a moment about who steps out of the transporter. Would it be you stepping out, or a mere replica. Afterall, your original body and brain was vanquished and if your self-hood isn’t stored in your body and brain, where is it stored? Does it even make sense to conceive of the self as something capable of being destroyed? Does it even make sense to conceive the self at all? Surely, you might think, there has to be something there to be explained, but what could it be?

I am found of how philosopher Daniel Dennett deals with this problem of perceived, but illusionary selves. He attributes these philosophical problems to the fact that often we think in all-or-nothing terms. Either the self exists or it doesn’t. Dennett thinks that this line of thought leads to the conceptual pitfalls and muddles that arise in thought experiments such as the teleportation case. Under Dennetts view, the self is best viewed as a “center of narrative gravity”. Essentially, we build up a series of micro-stories about ourselves and our place in the world and this autobiographical conglomeration gives rise to the illusion of a central self simply due to the fact that all these stories happen to a single body. However, this “bundle view” of the self has important philosophical ramifications simply because it calls into question ideas concerning responsibility and agency. As Dennett phrases it, “Our tales are spun, but for the most part we don’t spin them;they spin us. Our human consciousness, and our narrative selfhood, is their product, not their source”.

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The Binding Problem-continued


Well, in my last post I kind of left my audience hanging in the fact that I discussed the binding problem, but didn’t give any proposed solution. In this post I want to discuss and speculate on a possible answer to the question of how vision is bound together.

Possibly, the most well-known solution is Treisman’s feature-integration theory, which is an attention-based theory of the human visual system. In a nutshell, Treisman has proposed that when you attend to an object, the fact that you are attending to it necessarily integrates all the salient features of the object together aka the features are bound together. Furthermore, he postulates “feature maps” in the parietal lobes of the brain are used to select the features being bound together for any particular object.

His theory can be tested and has been tested in the following way:

Two white digits are briefly presented in the center of a computer screen, one of which is physically large than the other; the subjects’ task is to report the larger of the two digits, a task that requires attention to be directed at the center of the screen. Simultaneously with the digits, two colored letters are briefly presented in the periphery, one of which is always an F or X accompanied by a distractor letter(such as an O). Thus, after reporting on the digits, subjects are asked which of the two target letters occurred(F or X) and, most importantly the color in which that target was presented. If attention is required for binding, one might expect to observe “illusory conjunctions” in this paradigm such that subjects miscombine the features making up the two peripheral letters. And, in fact, that is just what is observed-when a red O and a yellow X are presented, for example, subjects often report seeing a red X more often than would be predicted by chance.(Hunt & Ellis, 2004)

Furthermore, empirical support for Treisman’s theory has been found in patients who have sustained damage to the parietal lobes.

When presented with multiple objects, a patient [who had sustained bi-lateral damage to the parietal lobes] could only report the individual features making up various objects; he was unable to correctly report which features belonged to the same object!(Hunt & Ellis, 2004)

However, I’d like to point out that while there is a lot of empirical support for Treisman’s theory and various other cognitive/neural explanations, there is still an explanatory gap in the following way: as far as I am aware, no theory of vision that attempts to account for the binding problem adequately gives an evolutionary explanation. This is a problem because it seems logical for reasons of parsimony to assume that at some point in our evolutionary history salient features weren’t bound together, so in order to give a satisfactory answer to the binding problem, one must propose some sort of evolutionary pressure explaining how and why they got bound together. I will not go into details in this post, but I speculate that one can get around this “why” problem if one has a wider conceptual framework to substantiate “brain bound” theories such as Treisman’s. Without better conceptual frameworks, these neural theories of perception will necessarily have limited explanatory power, despite being supported by empirical evidence.

Hunt, R., & Ellis, H. (2004). Fundamentals of Cognitive Psychology (Seven ed.): McGraw-Hill Higher Education.

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The Binding Problem of Perception

Imagine looking at an apple. If asked to describe it you would probably begin describing it’s various features: its color, brightness, hue, shape, size, texture, spatial location, etc. Now suppose if someone asked you where are all these features located, you would probably give them a funny look. Isn’t it obvious? Out there! In the apple! They are the apple! That is what an apple is: a conglomeration of various features integrated into one continuous percept. This seems like a perfectly sensible explanation, but if this obstinate person continues his line of questioning and asks you where this apple-percept is located in the brain, you might have to crack open a textbook and get back to him, because now the answer is not so simple.

After studiously pouring over the latest research on visual processing, you are finally ready to give the questioner an answer: no where. Simply put, the various features that make up an “apple” are represented in a highly complicated manner across a dazzlingly diverse array of brain tissue. For the sake of simplicity, brain researcher’s often distinguish between two primary information pathways that sensory data takes: the what and where streams. These two streams form the basis of an exemplary conceptual framework for how the brain processes various features of the objects around us to form a more-or-less continuous percept of objects such as apples. It is these continuous percepts that allows us to manipulate and verbally describe them accurately.

So, if all these apple-features are neurally processed in a separated fashion, why do they appear to be bound together? One obvious answer is they are bound together for the sake of convenience for the perceiver, otherwise how could a person act in a meaningful way? In order to pick up an apple to eat it, a subject must have a more-or-less continuous perception of all the various apple-features integrated into a single spatio-temporal location. This seems to make sense, but who or what are these high-order representations being convenient for? Why would neurons care if things are bound together or not? Surely, from an evolutionary perspective, where efficiency and survival are involved, doing all this extra processing to bind all these neurally separate features together for some subject seems bizarre. Who is this perceiving subject and why would the brain decide to stop the important business of surviving and begin integrating all these salient features into a continuous perception? There needs to be a perceiver for there to be a perception and in order for there to be a perceiver, there needs to be some sort of integrated perception. It seems as if we are at a chicken-and-egg impasse, thus making the phenomenon of unity perception a problem for students of the mind.

There have been many proposed solutions to the binding problem over the years, but they are beyond the scope of this post, in which I only wanted to outline the problem. Whether or not I will attempt to discuss any of these solutions is yet to be determined as of now, but I wouldn’t be surprised if it was the focus of a forthcoming post. Sorry to leave you hanging!

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Consciousness and sleep


In this post, I want to discuss a paper entitled the “Breakdown of Cortical Effective Connectivity During Sleep”. In plain English, this paper discusses the theoretical possibility that consciousness fades during the night because the cortex essentially doesn’t talk to itself as much. More specifically, this study focused on NREM sleep, which accounts for roughly 75-80% of our total sleep time. During NREM sleep, people often report no dream experiences, and it is this lack conscious activity that the researchers wanted to investigate. What goes on in our brains during this period of non-consciousness?

In order to answer this question, the researchers used a combination of transcranial magnetic stimulation(TMS) and electroencephalography(EEG). Using TMS was advantageous for the researchers because they could stimulate the cortex directly without activating the subcortical reticular formation and the thalamo gating relay.

The researchers used TMS to stimulate the rostral portion of the right premotor cortex, which has dense connections with the rest of the cortex area, which is heavily correlated to typical wakeful consciousness. Now for the results:

During wakefulness, TMS induced a sustained response made of recurrent waves of activity…With the onset of NREM sleep, the brain response to TMS changed markedly. After [the initial] large wave, no further TMS-locked activity could be detected.

Thus, during wakefulness, the perturbation of the rostral premotor cortex was followed by spatially and temporally differentiated patterns of activation that appeared to propagate along its anatomical connections. In striking contrast, during NREM sleep the location of maximum current density remained confined to the stimulated area.

During wakefulness, the site of maximum activation moved back and forth among premotor and prefrontal areas in both hemispheres and, in some subjects, it also involved the motor and posterior parietal cortex. During NREM sleep, by contrast, the activity evoked by TMS did not propagate in space and time in any of the subjects.

Thus, an impairment in the ability to integrate information among specialized thalamocortical modules—a proposed theoretical requirement for consciousness—may underlie the fading of consciousness in NREM sleep early in the night.

The researcher’s speculation on the potential neural mechanisms behind this decreased cortical activity during NREM sleep is a little beyond the scope of this blog, but it could have something to do with “down states” of depolarization being triggered more easily. Regardless,

Whatever the precise mechanisms, they are most likely engaged by the progressive reduction of the firing of diffuse neuromodulatory systems that occurs when we fall asleep.

For those interested, a good summary article of this research can be found here

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