User:Dibrisim/Perception draft

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Perception
Historically, perception was considered in terms established by René Descartes: perception → thoughts that lead to a decision (brain) and → action. Closely following Descartes’ thought was a period of automatons, like The Digesting Duck, but more importantly the period of punched cards used to automate textile looms, for example. And from there, there was just a small step left towards our modern computers based upon the principle: input → processing → output.

This picture has been entirely eroded in the second half of the last century that was crowned by seminal works of Richard L. Gregory, Benjamin Libet, Nancy Kanwisher and numerous other cognitive psychologists and neurologists. Aided by mostly non-intrusive equipment (eeg, fMRI etc.) they started to map subjective experiences with records of their physical manifestations. The accumulated finds lead to an inevitable conclusion that we are much more active or subjective in what and how we see than we suspected. Consequently, the previous theory of perception received “passive” as an attribute (PP) and the new, now widely accepted theory received “active” as an attribute. The rest of this article is, therefore, based upon Active Perception Theory (PA).

Support from Other Disciplines
Finds by cognitive psychologists received complementary support from genetics, sociology, anthropology and other disciplines. But the most surprising support came from the theory about Complex Adaptive Systems (an Artificial Intelligence discipline); especially neural networks. The processes in both bear remarkable similarity and many are now peeking through a keyhole on the door between them. This might well be the time to open this door...

The Basic Tenets
The Active Perception (PA) changes Descartes’ picture into dynamic interplay between environment ↔ our perception of it (default percepts) ↔ and our description of it (brain). The PA theory was originally conceived by philosopher David Hume. In his exchanges with a friend he explored how would a previously blind person interpret visual sensations? Unfortunately, a blind suddenly made see would be a miracle in his time... However, such cases were made possible in the last century. And Richard L. Gregory was lucky enough to follow up on the real life example of an older blind person suddenly made to see. Remarkably, his observations echoed very much Hume’s conclusions...

In short, PA theory states that we establish a set of more or less vague expectations (default percepts) for what we are about to experience in the next few moments of our lives. These sets are based upon a mental map or description of the place (situation) we are about to be in... And then we start a kind of virtual reality simulation just before the real stuff... As expectations of one moment are confirmed or corrected, the expectations for the following moments grow more specific until their time comes to be quickly confirmed or corrected. And this is much faster than building up the whole picture from the scratch every moment of our life...

Such simulations could be performed well in advance. And with a small ingredient - freedom, they are a solid launching pad for flights of imagination, art, thinking, planning... But, although fast, such simulations are prone to failures. If we do not expect to see a friend, for example, in a busy shopping mall, we may fail to see her waiving and yelling our name. Only later we may learn about our perceptual failure...

Consciousness
To psychologists, consciousness is like a spotlight cast at very few symbols in the dark sea of non-consciousness. It is not only timed by Dr. Benjamin Libet. Its capacity is also well measured with only seven symbols in the spotlight at the time - the Magical Number Seven, Plus or Minus Two...

It should be noted, though, that philosophers use term consciousness for another phenomenon that is here referred to as phenomenal consciousness or the hard problem of consciousness. While the consciousness (psychologists talk about) can be derived from phenomenal consciousness, we cannot draw any conclusion about phenomenal consciousness from findings psychologists made...

Memory
In each moment, seven of the most important sensations to us, are translated into symbols within our consciousness with emotional charges behind. The most of other sensations could be recorded non-consciously, but rather as related impressions translated in the emotional charges.

These emotionally charged symbols are stored into the short-term memory, as fresh memories, with good chances to be retained in the long-term memory.

The highly emotionally charged memories are constantly revisited in the context of other (long-term) memories of ours. And we even dream about these fresh memories.

This, constant repetition and verification of our fresh memories, turns them into long-time memories with the weight of repetitions behind them - habit. This iterative process could be well illustrated by the constant, internal repetition of the phone number of a nice girl we just met...

It should be noted that all of our emotions are reflected in the metabolic changes of our metabolism. Such changes are quickly propagated to every single cell of ours and reflected in the epigenome within them. Ultimately, these changes in our epigenome are passed to our offspring to benefit from our experiences...

Default Percept
Defaults percepts are rather tagged outlines of what we are about to see. And in a shopping mall we do expect to see people, men, women, kids - a vague tagged outlines that can be quickly confirmed by our sensations. Few details for this outlines we can add are features of a good looking woman who attracts our attention. Otherwise, we simply do not bother and swiftly navigate through the crowd letting our attention to be grabbed every now and then by a window, another good looking woman...

However, our default percepts do not only help us to navigate through our physical environment. As a launching pad they are a base for our default (habitual) thoughts and actions. In essence, our navigation through the crowd would be severely impaired if there were not default actions there to avoid collisions. Few drinks, for example, could cause here quite a few collisions... And our feeling of annoyance when inconsiderate people block our way is also reflected in our default thoughts about that situation.

Practically all of our skills are based upon refined default percepts in the profession of our choosing. In a multiparty system voters always assign a more or less stable set of default percepts about each of major parties and candidates. They also weight their sincerity on the basis of their default actions. And politicians know this and they are hard at work to enhance the default imagery about themselves with their words about future actions... And at the same time they are hard at work to erode the default imagery about their opponents...

right|thumb|140px|WY license plate As long our defaults are confirmed all is fine. Our default image of a cube seen from above will be quickly (.2 sec ) confirmed in an image on the left. Our default image of a cowboy on a bucking bronco on a Wyoming licence plate on the rights will also be quickly confirmed. Our default expectation that there is signpost over there is also quickly confirmed. Our default expectation that a door is unlocked is also quickly confirmed most of the time.

But what if our defaults are rather vaguely confirmed or even rejected by our senses. A vague confirmation may make us to look and see a cube from below. A default image of a cowboy on a bucking bronco on the licence plate could also be something else. A default expectation of a signpost in the mist could also turn to be something else sometimes sending creeps through our spine. And the expectation of the unlocked door could fail sometimes resulting in a panic attack that prevents us to see that there is “PUSH”, not “PULL” on the door...

But that is rather another, mysterious world of non-defaults…

Recognition vs. Cognition
Ever got lost awakening in a hotel room? Where am I? This is not my room...

All is fine, when we move around a familiar territory or write on a familiar topic. We do not even think about steps we do when dancing a familiar dance... But what happens when we find ourselves on an unfamiliar hotel room, unfamiliar ground or dancing unfamiliar dance; when we, for example, move into a suburb of another city?

We hesitate. Our eyes try to identify landmarks we were told about. And although hesitant, we are doing here marvellous job. Highly abstract basic perceptual elements quickly establish a sketch we can start to fill in with details. Metaphorically we could list these elementary default percepts as: texture, object, distant, near, vertical, hot, wet, dry, red, round etc. etc. The details that that are filling in the basic sketch also follow a similar path...

How we behave on an unfamiliar territory could be illustrated with a crime scene investigation - a popular topic for movies and TV series. As investigators, we carefully search for "clues" - evidence. As investigators, we "tag" them. Unfortunately we do not have cameras in our brain to take pictures of these clues. As a punishment for this we have to return to the "crime scene" much more often than our TV investigators do; even in our dreams. In our dreams we visit the “crime scene” again and again. We do the same mentally while awake, but mostly nonconsciously and without the advantages of inhibited physical actions we have in our dreams. We also talk to other people about the “crime scene”; just as TV investigators do. Until, finally, the whole “crime scene” starts to make sense and we solve the “case” - the unfamiliar territory becomes familiar and we stop hesitating when moving around.

This hesitance, or start-stop type of perceptual actions, has been indirectly researched by Dr. Benjamin Libet. He was intrigued by the finding of Lüder Deecke and Hans Helmut Kornhuber that a volitional action is preceded by .8 seconds of brain activity. Hey, how come? We act immediately after we consciously decide, don’t we?



Intrigued, Dr. Benjamin Libet performed a series of experiments to find precisely when we decide. The surprising conclusion was that we only become conscious of a non-consciously generated urge (default action), to lift a hand for example, .5 seconds after our nonconsciousness “decides” to lift a hand. And at that point we can only veto and abort the initiated, default action.

And this gave us a clue to how a default percept could morph into a surprising non-default percept, but always with a noticeable delay - at least .5 seconds. This is how cube, on the left, viewed from above morphs into cube viewed from below. This is how Wyoming licence plate image on the right morphs from a cowboy on a bucking bronco into the wolf’s head on the Wisconsin licence plate on the right. This is how our default action from “PULL” into non-default “PUSH” brings a sigh of relief. And this is how a sudden change of signpost into a large wings flapping bird can bring a cry to our lips...

Free Will
The finds of Dr. Benjamin Libet were often grossly misinterpreted, mildly said. Many jumped to a conclusion that we (our consciousness) do not have a free will. Well, things are not as simple as others are putting them... For the beginning we can veto consciously undesireable default percepts...

But most importantly, we do decide consciously to buy a loaf of bread, for example. On familiar territory this decision will prepare sets of default actions and percepts that will take us effortlessly to a bakery. On unfamiliar territory we will have start-stop kind of perceptual actions, but - that's about all. We will still act and see how to find the bakery - acting on our original free will choice about what to do next...

Our free will could be impeded though, by malfunctions in the critical areas of our brain, like hippocampus for example...

The Scales of the Lady Justice
As in situated cognition we could find ourselves in a wilderness. Our previous experiences (memory) of a farm may start to intermingle with the wilderness and we imagine a farm within it. As our thoughts bounce of the details of the environment, the imagined farm grows more real. As we think when, how, what and where, our imagined farm grows even more real with a doable plan of habitual actions behind. If we do not have all habitual actions needed, we imagine which people might have them - experts like carpenters, plumbers... And then we discuss the when, how, what and where - giving final touches to the plan. And then all we need is “let’s do it” to put all these habitual actions in their motions.

But neither the overall plan nor individual plans within it are perfect. Experienced professionals know this and add additional checks, allow for rescheduling etc. etc. But, not too much - that would be a red tape...

Such balancing act of recognition (defaults) on one side and cognition (non-defaults) on the other - we need in every walk of life. An inexperienced programmer will leap into the coding and get tangled in the web of contradictions between defaults. An inexperienced manager will get tangled in the web of contradictions between defaults or stereotypes and prejudices of the people he manages - and red tapes pop up everywhere; mechanically accounting for as many non-defaults as possible. And the bureaucracy grows to satisfy the needs of the growing bureaucracy...

Our political correctness is also such red tape. It prevents us from expressing some of our prejudices in public - only. Our political correctness lists default vetoes to our consciousness for some of our prejudging defaults. But it does not encourage us to test non-defaults against the real person before us, privately and publicly. And every human being has a potential for countless pleasant surprises... And glass ceilings will start to shatter all around us...

Non-monotonic logic
We do not only see according to the given outlines, we also think and act according to similar outlines. And these outlines are partly formalised in the non-monotonic logic.

Default Reasoning and Abductive Reasoning could easily be related to default percepts, habitual thoughts and acts. Reasoning about Knowledge indicates how we become aware that sensations do not match our defaults while Belief Revision indicates the process of how we perceive non-default percepts or think or act in a non-habitual way.

The only difference seems to be that we are much more emotional and habitual then we usually suspect. And this is often experienced as a kind of uneasiness in the face of non-default percepts or non-habitual thoughts or actions. Further improvements in non-monotonic logic may well be in adding emotional and habitual weights. Emotional weight could be a ranked importance to an agent, while habitual weight could be expressed as the number of successful direct and indirect, but successful, resolutions of default/habitual percepts, thoughts and actions. Unsuccessful resolutions should result in decreases of both, emotional and habitual weights. Successful resolutions should increase a weight of either. With this we could have an iterative process for Belief Revision relations.

Kolmogorov Complexity
Russian mathematician Andrey Kolmogorov tried and mostly succeeded to mathematically express how to shorten a description without losing any of its meanings. We also are doing something similar under the weights of exceptions to our default percepts. Regardless of how many billions of neurons we hide in our skull, our memory is still limited. And if we fill all of the “free space on the disk” with exceptions, that’s it. We can forget our “PC”.

Despite all of the free time our consciousness might have; our non-consciousness is all the time busy trying to find a solution for accumulated exceptions and contradictions. And it works pretty much according to the rules Kolmogorov established; ensuring that there will be enough “free disk space” for our future fresh memories...

And each compression of our knowledge always results in a higher level of abstraction...

The Triangle of Perception
The developmental psychology established the three corners of perception: genome with its expressions, culture and personal experiences. Through genome we inherit parts of vague expectations. These parts are later complemented with their cultural complementary pairs forming a base for our own experiences. And as we grow into our culture we draw the lines to the third angle, filling up the triangle with our own experiences...

This view is now receiving increased support from genetics, sociology and anthropology. Several studies showed differences in genome and epigenome on the basis of accumulated experiences of our parents and our own accumulated experiences as we live within our culture. Anthropological studies in perceptual differences between cultures, now facilitated by brain scanning equipment, have also provided strong support.

Comments
I am reminded of Abraham Lincoln's message to Congress, December 1, 1862: "What makes a country? First, the land, then the people, then the laws". This paraphrase is by Bernard De Voto. --Ancheta Wis (talk) 12:13, 18 February 2010 (UTC)

But perhaps the word "Perception" in the middle of the triangle? --Ancheta Wis (talk) 12:24, 18 February 2010 (UTC)

When I realized I didn't have the words right, I also realized something about a perception: we only perceive as much as we need to. When I was confronted with the realization that I had read straight past the words to the concepts behind them, I was forced to pay attention to the phrases which made up the concept, and reflect those more faithfully in the labels on the diagram. In other words, when my hierarchy of values kicks in, then my perception of the object becomes more nuanced and accurate. Some people (i.e., me in reading the words) are just not going to care about something until it matters to them. So for example, if a deer is attempting to cross a road, there is the well-known phenomenon of the deer 'freezing' in the headlights of a car, and the deer is likely to stand there until its internal alarms set it free to run away. Another example would then be the spectrum of the diffusion of innovations: first, innovators, then the early adopters, then the early majority, and finally, the late majority, with some laggards never adopting the innovation, because it simply doesn't matter to them. In other words, the triangle explains why some people will never perceive a phenomenon which is visible to others, because one or more of the corners of the triangle is keeping them from perceiving; it is not until they are 'ready' will they perceive. For example, the author of Amazing Grace was blind to the evils of slavery until he saw what he himself had done. In another example, an emperor of China did not perceive that Chinese was a tonal language until a philosopher uttered a sentence composed of the same sound in 4 different tones. --Ancheta Wis (talk) 17:32, 18 February 2010 (UTC)

Reply
Dear Ancheta,

Job well done with the triangle!

Now about your insight while doing the graphics: spot on. This is why I was thinking about animated graphics. But let me explain this in words with this article as an example. —Preceding unsigned comment added by Dibrisim (talk • contribs) 21:30, 18 February 2010 (UTC)

All of us started from our own subculture; statistics, logic, genetics etc. Consequently, readers who feel comfortable within their own subculture - will have difficulties to read/comprehend parts of this article fully. To overcome this, I have tied my vocabulary to express relations to other subcultures; enabling the reader to venture into other subcultures. Graphically, this would mean moderate extension of the base line towards cultural angle. Tiny extension of the base line towards genome corner takes ages and I would not worry about that. However, the top of the triangle should move upwards the fastest, because the reader’s own experiences will give plenty of unarticulated meanings to words he/she is reading; facilitating further growth of the triangle.

The best tags to corners would be “genome”, “culture” and “our own” with “experiences” in the middle.

I have been thinking about diffusion of innovations also, but this would double the scope of the article. I must correct your realisation about perception a bit, though. You are only partly right that we perceive only as much as we “need to”. The most of people are reluctant to step out of their comfort zone, but there are those who muster the courage for the leap of faith. And hopefully, this article will encourage few to do so. And when others see it possible, they will follow. And this is the psychology behind the diffusion of innovations...

Thank you very much, Damir Ibrisimovic (talk) 21:22, 18 February 2010 (UTC)

I would also move the picture next to Our Own Corner section, for there will all three components fall in their places to the reader. I'll try to find some wording that will prompt the reader to contemplate the picture. Thanks, Damir Ibrisimovic (talk) 23:34, 18 February 2010 (UTC)

The Genetics' Corner
In the last two decades our attention has been drown towards expressions of our genome; i.e. epigenome. It has been shown that expressions of the same genetic sequence change as we accumulate our experiences. This has shaken the whole world of geneticists who believed that our genes are set in the stone. And who believed that only “errors in copying” (mutations) were a driving force behind evolution.

Further research has indicated the impact of our physical and not so physical environment - i.e. culture. Two independent researches looked at a single-letter within a gene sequence and practically replicated Europe's map. Research in hyperlexia has also indicated that more detailed parts of our literacy can impact our genetic makeup.

The current efforts are focused at the role of the combined parents’ epigenome might play during fertilisation. And the tentative expectation is that it does play a big part as parents’ genome halves are combined into the child’s. Another branch of research is focused at probable impact mother’s epigenome (experiences) on foetus.

A bigger picture is also being now contemplated. The evolution is now being looked at through the prism of organism's experiences within an environment. And although there are no instantly obvious changes, the positive and negative experiences of an organism within an environment might be behind changes in physical appearances...

The Culture's Corner
The assessment that there was a shift away from the transfer of the accumulated knowledge by genes only is not new. It was rather obvious. While calves start running around few hours or days after birth, our kids need almost a year to start to crawl around...

But this was just an educated guess. We did not have enough evidence to justify and explain fully the transfer of knowledge by cultural means. For a while such transfer of knowledge was only indicated by developmental psychologists. But the last two decades with the aid of brain scanning and other newly developed equipment changed all this. A strong link with genetics is now established and virtually, all of our culture started to reveal itself in activities of our brains.

Ever thought how South-East Asians seem to be enigmatic? Ever thought that we might be puzzling to them also? And both impressions seem valid on the surface, for each of the two cultures learned to search for emotional clues at different places. While Caucasians manifest emotions mostly around eyes and mouth and look for them there, South-East Asians manifest and look for the same clues in the body language.

South East Asians will also see in a picture a jungle (texture) that happens to have an elephant in it. Caucasians tend to see in the same picture the elephant (object) first and the jungle as a background. This might seem as a minor difference, but the cognitive processes in the background are entirely reversed.

And Our Own Corner
From the moment of our conception, we start to accumulate our own perceptual experiences that will form a launching pad for our physical activities after our birth. Unfortunately, and probably because of dramatic changes we undergo during our early childhood, our memories from this period are rare and sporadic. So what we did and experienced then is better left to others to tell us, especially experts...

As we grow older, more and more experiences grow harder and harder to recall. But they do stay in the background we call non-consciousness, unless something dramatic happens to us that forces us to re-evaluate them. And even this re-evaluation could be performed non-consciously and in our dreams without us even noticing that. And every now and then we end up with our experiences (knowledge) consolidated and more stable to face a new day...

Although we often do not notice this consciously, gradual but dramatic changes in our perceptual base do happen occasionally. This could be seen in two individuals falling in love. The manifestation of this process could be seen in the frequent replacements of “I” (me, individual) with “us” (a greater whole). The first time parents also manifest similar dramatic changes of perspective - suddenly their whole world is centred on their child.

We may have been born into one of many farmers’ subcultures. And a teacher may have helped us to discover a whole new world in biology, for example - another subculture. And our perceptual baseline, as in graphics on the right, inched a bit towards cultural angle as we study biology; while our own angle shoots upward under pressure of newly accumulated experiences...

And we may be here in a similar position reading this article and discovering new subcultures - math, logic, psychology are just a few. And with each visit to another subculture we add to our own experiences pushing the angles of the triangle further away... And our own angle might be shooting upwards, right now - to the moon...

Note also that our perceptual baseline is also inching towards genome angle as we shoot upwards towards the moon. This is, however, reflected mostly in our epigenome - and such changes are very slow in time...

Comments
Could we have a simpple animated gif or png of a triangle of our experiences growing from the genetic and cultural base? I have a clear picture of it, but do not know how to do that in Wikipedia. Thanks for all help. Kind regards, Damir Ibrisimovic (talk) 04:05, 16 February 2010 (UTC)

Relations to the conscious and to the subconscious added in the reference. Damir Ibrisimovic (talk) 06:30, 18 February 2010 (UTC)
 * Well, here is a disconnect. When I read the description before I was seeing it statically. But if your concept is a flow from two corners to the top, then this picture does not follow from the words which I read. Logically, there could also be a flow from genetic & personal to cultural. I would visualize this as a flow in another direction than from bottom to top. --Ancheta Wis (talk) 18:55, 18 February 2010 (UTC)


 * Ancheta, I hope that my reply above and the description in the above section answered your "disconnect". Of course, you are right - all angles are impacted. Thanks, Damir Ibrisimovic (talk) 00:32, 19 February 2010 (UTC)

Early Development
Foetal stage of our perceptual development is the least known stage. However, for ethical reasons, rare experiments have shown that a foetus reacts to a disturbing sound, for example. With the repeated exposure, the foetus adapts, i.e. is not disturbed by the sound. And the foetus remembers, for few weeks later the foetus will not be disturbed either. It is therefore assumed that in our foetal stage we start to accumulate non-visual sensations and start growing our perceptual base, i.e. experiences.

Our early development, after birth, is rather observed and explained by others, especially by trained experts like developmental psychologists and pediatricians. As babies we are exposed to our physical environment under protection of our parents. And as babies do, we explore it with all of our senses - including putting toys into our mouth. And we indicate some basic communicational skills with our sounds full of emotions; sounds other babies and our moms intuitively interpret well.

As babies we later try to translate what we perceived into our own action to - sit for example. And it takes quite a while until such deliberate actions of ours become natural, i.e. habitual. Slowly we start to crawl also, and it is amazing how proficient we were in this skill. Another entirely separate world - speech is also being mastered. But we forgot that, for this stage was rather a stage of exploring and developing habitual actions, like waking, that will carry us around within our environment. At this stage we are also quite proficient in forming mental maps also, but planning further ahead is still out of our reach. To reach it we need to change radically and be born into the world of the social aspect of our consciousness. This is probably the reason why our memories before our second birth (period between age four and six) are so rare...

Social Aspects
The first signs of the social aspect of our consciousness are fostered by our parents; nudging us to share toys, for example. And reluctantly, we start to see ourselves as other’s would. In our mind’s eye we see ourselves as others would see us more frequently until such perspective becomes habitual. In this stage, other strange feelings start to germinate, like empathy for example. Our previous need for a close physical contact to interpret emotions of others is also gradually replaced with reading emotions from words and physical actions of others...

But these are only beginnings in the development of the social aspect of our consciousness. We are still far from really seeing ourselves as our parents would. To really see ourselves as our parents would we have to become parents ourselves; or at least fully committed to a child. And to really see ourselves as our parents would we need to discover that we behave towards our child almost exactly as our parents did towards us...

But the development of the social aspect of our consciousness is not complete yet. We need to see ourselves as people of another culture or religion would. And that seems to be a never ending story... But there is a hope. If we build into our cultures perspectives like these, our kids will have much easier job here.

What Could Go Wrong
Apart from handicaps like autism, many things can go wrong, especially during development of our social aspect of consciousness. If physical proximity of our parents is not frequent or if it does not radiate positive emotions, we could grow reluctant towards experiences human touch brings. And this could result in serious impediments in our future social life.

Our parents, maybe under the stress of hardship, could neglect the development of the social aspect of our consciousness. Consequently, empathy and sharing may be a strange feeling to us. And the most of such deviations usually ends up in prisons... Only harmless deviations that are tolerated by the society do not...

Our reading of emotions from words and actions of others, especially our parents, could also be misleading. A son of a bashed mother, for example, might get an impression that outbursts of violence are outbursts of love. A daughter of a bashed mother might also form a similar impression. And such impressions could stay with us for the rest of our lives. The son will bash his wife and the wife will be disappointed if she is not bashed. And the cycle of domestic violence repeats itself...

Reality?
How certain we are, after all this, that what we see is real?

Our reality is actually the hard problem of consciousness in philosophy. If we look at two apples close enough, we will quickly start to notice more and more differences. And the closer we look, the more differences we find; until we realise that there is infinity of differences; until we realise that each of the apples is actually unique. And how to compare unique phenomena?

Statistics also has a really hard problem here. A unique phenomenon is statistically impossible – 1/∞ = 0. To bypass this problem we have now Kolmogorov's zero-one law.

And the answer is quite simple. Both apples have common properties; properties we cannot find in their atoms - emergent properties; like saltines of salt that looks and feels nothing like properties of sodium or chlorine. And we can count the same emergent properties of otherwise unique phenomena... And shared properties of apples enable us to count them... And we grow into skilled mathematicians... Can we have a simpler number theory?...

But, are those emergent properties real?...

Theories of Perception

 * Interactive Activation and Competition
 * Irving Biederman's Recognition by Components Theory
 * Anne Treisman's Feature Integration Theory
 * Situated cognition

Removed Sections

 * 1) Perception and Reality
 * 2) Perceptual Threshold (to be addressed in the percept article)