Sleep paralysis

From Wikipedia, the free encyclopedia

Sleep paralysis
The Nightmare by Henry Fuseli (1781) is thought to be a depiction of sleep paralysis perceived as a demonic visitation.
Specialty	Sleep medicine
Symptoms	Awareness but inability to move during waking or falling asleep
Duration	Less than a couple of minutes
Risk factors	Narcolepsy, obstructive sleep apnea, alcohol use, sleep deprivation
Diagnostic method	Based on description
Differential diagnosis	Narcolepsy, atonic seizure, hypokalemic periodic paralysis, night terrors
Treatment	Reassurance, sleep hygiene, cognitive behavioral therapy, antidepressants
Frequency	8–50%

Sleep paralysis is when, during awakening or falling asleep, a person is aware but unable to move or speak. During an episode, one may hallucinate (hear, feel, or see things that are not there), which often result in fear. Episodes generally last less than a couple of minutes. It may occur as a single episode or be recurrent.

The condition may occur in those who are otherwise healthy, those with narcolepsy, or may run in families as a result of specific genetic changes. The condition can be triggered by sleep deprivation, psychological stress, or abnormal sleep cycles. The underlying mechanism is believed to involve a dysfunction in REM sleep. Diagnosis is based on a person's description. Other conditions that can present similarly include narcolepsy, atonic seizure, and hypokalemic periodic paralysis.

Treatment options for sleep paralysis have been poorly studied. It is recommended that people be reassured that the condition is common and generally not serious. Other efforts that may be tried include sleep hygiene, cognitive behavioral therapy, and antidepressants.

Between 8% and 50% of people experience sleep paralysis at some time. About 5% of people have regular episodes. Males and females are affected equally. Sleep paralysis has been described throughout history. It is believed to have played a role in the creation of stories about alien abduction and other paranormal events.

Signs and symptoms

The central symptom of sleep paralysis is being unable to move during awakening.

Imagined sounds such as humming, hissing, static, zapping and buzzing noises are reported during sleep paralysis. Other sounds such as voices, whispers and roars are also experienced. These symptoms are usually accompanied by intense emotions such as fear and panic. People also have sensations of being dragged out of bed or of flying, numbness, and feelings of electric tingles or vibrations running through their body.

Sleep paralysis may include hypnogogic hallucinations, such as a supernatural creature suffocating or terrifying the individual, accompanied by a feeling of pressure on one's chest and difficulty breathing. Another example of a hallucination involves a menacing shadowy figure entering one's room or lurking outside one's window, while the subject is paralyzed. The content and interpretation of these hallucinations are driven by fear, somatic sensations, REM-induced sexual arousal, and REM mentation which are embedded in the sleeper's cultural narrative.

REM sleep physiology and somatic symptoms coupled with the awareness that one is paralyzed can generate a variety of psychological symptoms during sleep paralysis, including fear and worry that are aggravated by catastrophic cognitions about the attack. This can activate a fight-flight reaction and panic-like arousal. Consequently, when the person attempts to escape the paralysis, somatic symptoms and arousal are exacerbated, as an execution of motor programs in the absence of dampening proprioceptive feedback can lead to heightened sensations of bodily tightness and pressure, and even pain and spasms in limbs.

Pathophysiology

The pathophysiology of sleep paralysis has not been concretely identified, although there are several theories about its cause. The first of these stems from the understanding that sleep paralysis is a parasomnia resulting from dysfunctional overlap of the REM and waking stages of sleep. Polysomnographic studies found that individuals who experience sleep paralysis have shorter REM sleep latencies than normal along with shortened NREM and REM sleep cycles, and fragmentation of REM sleep. This study supports the observation that disturbance of regular sleeping patterns can instigate an episode of sleep paralysis, because fragmentation of REM sleep commonly occurs when sleep patterns are disrupted and has now been seen in combination with sleep paralysis.

Another major theory is that the neural functions that regulate sleep are out of balance in such a way that causes different sleep states to overlap. In this case, cholinergic sleep on neural populations are hyperactivated and the serotonergic sleep off neural populations are under-activated. As a result, the cells capable of sending the signals that would allow for complete arousal from the sleep state, the serotonergic neural populations, have difficulty in overcoming the signals sent by the cells that keep the brain in the sleep state. During normal REM sleep, the threshold for a stimulus to cause arousal is greatly elevated. Under normal conditions, medial and vestibular nuclei, cortical, thalamic, and cerebellar centers coordinate things such as head and eye movement, and orientation in space.

However, in individuals with SP, there is almost no blocking of exogenous stimuli, which means it is much easier for a stimulus to arouse the individual. There may also be a problem with the regulation of melatonin, which under normal circumstances regulates the serotonergic neural populations. Melatonin is typically at its lowest point during REM sleep. Inhibition of melatonin at an inappropriate time would make it impossible for the sleep off neural populations to depolarize when presented with a stimulus that would normally lead to complete arousal. The vestibular nuclei in particular has been identified as being closely related to dreaming during the REM stage of sleep. According to this hypothesis, vestibular-motor disorientation, unlike hallucinations, arise from completely endogenous sources of stimuli.

This could explain why the REM and waking stages of sleep overlap during sleep paralysis, and definitely explains the muscle paralysis experienced on awakening. If the effects of sleep on neural populations cannot be counteracted, characteristics of REM sleep are retained upon awakening. Common consequences of sleep paralysis include headaches, muscle pains or weakness and/or paranoia. As the correlation with REM sleep suggests, the paralysis is not complete: use of EOG traces shows that eye movement is still possible during such episodes; however, the individual experiencing sleep paralysis is unable to speak.

Research has found a genetic component in sleep paralysis. The characteristic fragmentation of REM sleep, hypnopompic, and hypnagogic hallucinations have a heritable component in other parasomnias, which lends credence to the idea that sleep paralysis is also genetic. Twin studies have shown that if one twin of a monozygotic pair (identical twins) experiences sleep paralysis that other twin is very likely to experience it as well. The identification of a genetic component means that there is some sort of disruption of a function at the physiological level. Further studies must be conducted to determine whether there is a mistake in the signaling pathway for arousal as suggested by the first theory presented, or whether the regulation of melatonin or the neural populations themselves have been disrupted.

Hallucinations

A picture of a succubus-like vision, in contrast to the incubus. My Dream, My Bad Dream, 1915, by Fritz Schwimbeck

Several types of hallucinations have been linked to sleep paralysis: the belief that there is an intruder in the room, the presence of an incubus, and the sensation of floating. A neurological hypothesis is that in sleep paralysis the mechanisms which usually coordinate body movement and provide information on body position become activated and, because there is no actual movement, induce a floating sensation.

The intruder and Incubus hallucinations highly correlate with one another, and moderately correlated with the third hallucination, vestibular-motor disorientation, also known as out-of-body experiences, which differ from the other two in not involving the threat-activated vigilance system.

Several theories have been proposed to explain the hallucinations that may accompany sleep paralysis, but there is currently no research that supports a neurological model.

Threat hyper-vigilance

A hyper-vigilant state created in the midbrain may further contribute to hallucinations. More specifically, the emergency response is activated in the brain when individuals wake up paralyzed and feel vulnerable to attack. This helplessness can intensify the effects of the threat response well above the level typical of normal dreams, which could explain why such visions during sleep paralysis are so vivid. The threat-activated vigilance system is a protective mechanism that differentiates between dangerous situations and determines whether the fear response is appropriate.

The hyper-vigilance response can lead to the creation of endogenous stimuli that contribute to the perceived threat. A similar process may explain hallucinations, with slight variations, in which an evil presence is perceived by the subject to be attempting to suffocate them, either by pressing heavily on the chest or by strangulation. A neurological explanation holds that this results from a combination of the threat vigilance activation system and the muscle paralysis associated with sleep paralysis that removes voluntary control of breathing. Several features of REM breathing patterns exacerbate the feeling of suffocation. These include shallow rapid breathing, hypercapnia, and slight blockage of the airway, which is a symptom prevalent in sleep apnea patients.

According to this account, the subjects attempt to breathe deeply and find themselves unable to do so, creating a sensation of resistance, which the threat-activated vigilance system interprets as an unearthly being sitting on their chest, threatening suffocation. The sensation of entrapment causes a feedback loop when the fear of suffocation increases as a result of continued helplessness, causing the subjects to struggle to end the SP episode.

Diagnosis

Sleep paralysis is mainly diagnosed via clinical interview and ruling out other potential sleep disorders that could account for the feelings of paralysis. The main disorder that is checked for is narcolepsy due to the high prevalence of narcolepsy in conjunction with sleep paralysis. The availability of a genetic test for narcolepsy makes this an easy disorder to rule out. Several measures are available to reliably diagnose (e.g., the fearful isolated sleep paralysis interview) or screen (Munich Parasomnia Screening) for recurrent isolated sleep paralysis.

Classification

Episodes of sleep paralysis can occur in the context of several medical conditions (e.g., narcolepsy, hypokalemia). When episodes occur independent of these conditions or substance use, it is termed "isolated sleep paralysis" (ISP). When ISP episodes are more frequent and cause clinically significant distress and/or interference, it is classified as "recurrent isolated sleep paralysis"(RISP). Episodes of sleep paralysis, regardless of classification, are generally short (1–6 minutes), but longer episodes have been documented. With RISP the individual can also suffer back-to-back episodes of sleep paralysis in the same night, which is unlikely in individuals who suffer from ISP.

It can be difficult to differentiate between cataplexy brought on by narcolepsy and true sleep paralysis, because the two phenomena are physically indistinguishable. The best way to differentiate between the two is to note when the attacks occur most often. Narcolepsy attacks are more common when the individual is falling asleep; ISP and RISP attacks are more common upon awakening.

Prevention

Several circumstances have been identified that are associated with an increased risk of sleep paralysis. These include insomnia, sleep deprivation, an erratic sleep schedule, stress, and physical fatigue. It is also believed that there may be a genetic component in the development of RISP, because there is a high concurrent incidence of sleep paralysis in monozygotic twins. Sleeping in the supine position has been found an especially prominent instigator of sleep paralysis.

Sleeping in the supine position is believed to make the sleeper more vulnerable to episodes of sleep paralysis because in this sleeping position it is possible for the soft palate to collapse and obstruct the airway. This is a possibility regardless of whether the individual has been diagnosed with sleep apnea or not. There may also be a greater rate of microarousals while sleeping in the supine position because there is a greater amount of pressure being exerted on the lungs by gravity.

While many factors can increase the risk for ISP or RISP, they can be avoided with minor lifestyle changes. By maintaining a regular sleep schedule and observing good sleep hygiene, one can reduce chances of sleep paralysis. It helps subjects to reduce the intake of stimulants and stress in daily life by taking up a hobby or seeing a trained psychologist who can suggest coping mechanisms for stress. However, some cases of ISP and RISP involve a genetic factor—which means some people may find sleep paralysis unavoidable. Practicing meditation regularly might also be helpful in preventing fragmented sleep, and thus the occurrence of sleep paralysis. Research has shown that long-term meditation practitioners spend more time in slow wave sleep, and as such regular meditation practice could reduce nocturnal arousal and thus possibly sleep paralysis.

Treatment

Medical treatment starts with education about sleep stages and the inability to move muscles during REM sleep. People should be evaluated for narcolepsy if symptoms persist. The safest treatment for sleep paralysis is for people to adopt healthier sleeping habits. However, in more serious cases tricyclic antidepressants or selective serotonin reuptake inhibitors (SSRIs) may be used. Despite the fact that these treatments are prescribed there is currently no drug that has been found to completely interrupt episodes of sleep paralysis a majority of the time.

Medications

Though no large trials have taken place which focus on the treatment of sleep paralysis, several drugs have promise in case studies. Two trials of GHB for people with narcolepsy demonstrated reductions in sleep paralysis episodes.

Cognitive-behavior therapy

Some of the earliest work in treating sleep paralysis was done using a culturally sensitive cognitive-behavior therapy called CA-CBT. The work focuses on psycho-education and modifying catastrophic cognitions about the sleep paralysis attack. This approach has previously been used to treat sleep paralysis in Egypt, although clinical trials are lacking.

The first published psychosocial treatment for recurrent isolated sleep paralysis was cognitive-behavior therapy for isolated sleep paralysis (CBT-ISP). CBT-ISP is manualized, has an adherence manual for research purposes, and is intended to both prevent and disrupt ISP episodes. It begins with self-monitoring of symptoms, cognitive restructuring of maladaptive thoughts relevant to ISP (e.g., "the paralysis will be permanent"), and psychoeducation about the nature of sleep paralysis. Prevention techniques include ISP-specific sleep hygiene and the preparatory use of various relaxation techniques (e.g. diaphragmatic breathing, mindfulness, progressive muscle relaxation, meditation). Episode disruption techniques are first practiced in session and then applied during actual attacks. No controlled trial of CBT-ISP has yet been conducted to prove its effectiveness.

Meditation-relaxation therapy

Meditation-relaxation (MR) therapy is a published direct treatment for sleep paralysis. The treatment was partly derived from the neuroscientific hypothesis suggesting that attempting movement during sleep paralysis (e.g., due to panic-like reactions) can lead to neurological distortions of one's "body image", possibly triggering hallucinations of shadowy human-like figures. The therapy is based on four steps applied during sleep paralysis: (1) reappraisal of the meaning of the attack (cognitive reappraisal); which entails closing one's eyes, avoid panicking and re-appraising the meaning of the attack as benign. (2) psychological and emotional distancing (emotion regulation); the sleeper reminds him- or herself that catastrophizing the event (i.e., fear and worry) will worsen and possibly prolong it; (3) inward focused-attention meditation; focusing attention inward on an emotionally salient positive object; 4) muscle relaxation; relaxing one's muscles, avoid controlling breathing and avoid attempting to move.There are preliminary case reports supporting this treatment, although no randomized clinical trials yet to show its effectiveness.

Epidemiology

Sleep paralysis is equally experienced in both males and females. Lifetime prevalence rates derived from 35 aggregated studies indicate that approximately 8% of the general population, 28% of students, and 32% of psychiatric patients experience at least one episode of sleep paralysis at some point in their lives. Rates of recurrent sleep paralysis are not as well known, but 15%-45% of those with a lifetime history of sleep paralysis may meet diagnostic criteria for Recurrent Isolated Sleep Paralysis. In surveys from Canada, China, England, Japan and Nigeria, 20% to 60% of individuals reported having experienced sleep paralysis at least once in their lifetime. In general, non-whites appear to experience sleep paralysis at higher rates than whites, but the magnitude of the difference is rather small. Approximately 36% of the general population that experiences isolated sleep paralysis is likely to develop it between 25 and 44 years of age.

Isolated sleep paralysis is commonly seen in patients that have been diagnosed with narcolepsy. Approximately 30–50% of people that have been diagnosed with narcolepsy have experienced sleep paralysis as an auxiliary symptom. A majority of the individuals who have experienced sleep paralysis have sporadic episodes that occur once a month to once a year. Only 3% of individuals experiencing sleep paralysis that is not associated with a neuromuscular disorder have nightly episodes.

Sleep paralysis could lead the individual to acquire conditioned fear of the experience ("worry attacks"), resulting in more nighttime awakening and fragmented sleep (because of nocturnal arousal and hyper-alertness to symptoms of paralysis), making the person more likely to have sleep paralysis in the future.

Society and culture

Etymology

A 19th century version of Füssli's The Nightmare (1781)

The original definition of sleep paralysis was codified by Samuel Johnson in his A Dictionary of the English Language as nightmare, a term that evolved into our modern definition. The term was first used and dubbed by British neurologist, S.A.K. Wilson in his 1928 dissertation, The Narcolepsies. Such sleep paralysis was widely considered the work of demons, and more specifically incubi, which were thought to sit on the chests of sleepers. In Old English the name for these beings was mare or mære (from a proto-Germanic *marōn, cf. Old Norse mara), hence comes the mare in the word nightmare. The word might be cognate to Greek Marōn (in the Odyssey) and Sanskrit Māra.

Cultural significance and priming

Le Cauchemar (The Nightmare), by Eugène Thivier (1894)

 

Although the core features of sleep paralysis (e.g., atonia, a clear sensorium, and frequent hallucinations) appear to be universal, the ways in which they are experienced vary according to time, place, and culture. Over 100 terms have been identified for these experiences. Some scientists have proposed sleep paralysis as an explanation for reports of paranormal phenomena such as ghosts, parasites, alien visits, demons or demonic possession, alien abduction experiences, the Night Hag and shadow people haunting.

The night hag is a generic name for a fantastical creature from the folklore of various peoples which is used to explain the phenomenon of sleep paralysis. A common description is that a person feels a presence of a supernatural malevolent being which immobilizes the person as if sitting on his/her chest. Various cultures have various names for this phenomenon and/or supernatural character. For example, sleep paralysis is referred to as a Pandafeche attack in Italy.

Among Italians the Pandafeche may refer to an evil witch, sometimes a ghost-like-spirit or a terrifying cat-like creature. Sleep paralysis among Cambodians is known as, “the ghost pushes you down,” and entails the belief in dangerous visitations from deceased relatives. In Egypt, sleep paralysis is conceptualized as a terrifying Jinn attack. The Jinn (i.e., evil genies) may terrorize and even kill its victims. Sleep paralysis is sometimes interpreted as space alien abduction in the United States.

According to some scientists culture may be a major factor in shaping sleep paralysis. When sleep paralysis is interpreted through a particular cultural filter, it may take on greater salience. For example, if sleep paralysis is feared in a certain culture, this fear could lead to conditioned fear, and thus worsen the experience, in turn leading to higher rates. Consistent with this idea, high rates and long durations of immobility during sleep paralysis have been found in Egypt, where there are elaborate beliefs about sleep paralysis, involving malevolent spirit-like creatures, the Jinn. 

Research has found that sleep paralysis is associated with great fear and fear of impending death in 50% of sufferers in Egypt. A study comparing rates and characteristics of sleep paralysis in Egypt and Denmark found that the phenomenon is three times more common in Egypt versus Denmark. In Denmark, unlike Egypt, there are no elaborate supernatural beliefs about sleep paralysis, and the experience is often interpreted as an odd physiological event, with overall shorter sleep paralysis episodes and fewer people (17%) fearing that they could die from it.

Literature

Various forms of magic and spiritual possession were also advanced as causes in literature. In nineteenth century Europe, the vagaries of diet were thought to be responsible. For example, in Charles Dickens's A Christmas Carol, Ebenezer Scrooge attributes the ghost he sees to "... an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato..." In a similar vein, the Household Cyclopedia (1881) offers the following advice about nightmares:

Great attention is to be paid to regularity and choice of diet. Intemperance of every kind is hurtful, but nothing is more productive of this disease than drinking bad wine. Of eatables those which are most prejudicial are all fat and greasy meats and pastry... Moderate exercise contributes in a superior degree to promote the digestion of food and prevent flatulence; those, however, who are necessarily confined to a sedentary occupation, should particularly avoid applying themselves to study or bodily labor immediately after eating... Going to bed before the usual hour is a frequent cause of night-mare, as it either occasions the patient to sleep too long or to lie long awake in the night. Passing a whole night or part of a night without rest likewise gives birth to the disease, as it occasions the patient, on the succeeding night, to sleep too soundly. Indulging in sleep too late in the morning, is an almost certain method to bring on the paroxysm, and the more frequently it returns, the greater strength it acquires; the propensity to sleep at this time is almost irresistible.

J. M. Barrie, the author of the Peter Pan stories, may have had sleep paralysis. He said of himself ‘In my early boyhood it was a sheet that tried to choke me in the night.’ He also described several incidents in the Peter Pan stories that indicate that he was familiar with an awareness of a loss of muscle tone whilst in a dream-like state. For example, Maimie is asleep but calls out ‘What was that....It is coming nearer! It is feeling your bed with its horns-it is boring for [into] you’. and when the Darling children were dreaming of flying, Barrie says ‘Nothing horrid was visible in the air, yet their progress had become slow and labored, exactly as if they were pushing their way through hostile forces. Sometimes they hung in the air until Peter had beaten on it with his fists.’ Barrie describes many parasomnias and neurological symptoms in his books and uses them to explore the nature of consciousness from an experiential point of view.

Documentary films

The Nightmare is a 2015 documentary that discusses the causes of sleep paralysis as seen through extensive interviews with participants, and the experiences are re-enacted by professional actors. In synopsis, it proposes that such cultural memes as alien abduction, the near death experience and shadow people can, in many cases, be attributed to sleep paralysis. The "real-life" horror film debuted at the Sundance Film Festival on January 26, 2015 and premiered in theaters on June 5, 2015.

Direct and indirect realism

From Wikipedia, the free encyclopedia

Direct realism argues we perceive the world directly

 

The question of direct or naïve realism, as opposed to indirect or representational realism, arises in the philosophy of perception and of mind out of the debate over the nature of conscious experience; the epistemological question of whether the world we see around us is the real world itself or merely an internal perceptual copy of that world generated by neural processes in our brain. Naïve realism is known as direct realism when developed to counter indirect or representative realism, also known as epistemological dualism, the philosophical position that our conscious experience is not of the real world itself but of an internal representation, a miniature virtual-reality replica of the world.

Indirect realism is broadly equivalent to the accepted view of perception in natural science that states that we do not and cannot perceive the external world as it really is but know only our ideas and interpretations of the way the world is. Representationalism is one of the key assumptions of cognitivism in psychology. The representational realist would deny that "first-hand knowledge" is a coherent concept, since knowledge is always via some means. Our ideas of the world are interpretations of sensory input derived from an external world that is real (unlike the standpoint of idealism, which holds that only ideas are real, but mind-independent things are not).

The main alternative to representationalism is anti-representationalism, the view according to which perception is not a process of constructing internal representations.

History

Aristotle was the first to provide a description of direct realism. In On the Soul he describes how a see-er is informed of the object itself by way of the hylomorphic form carried over the intervening material continuum with which the eye is impressed.

In medieval philosophy, direct realism was defended by Thomas Aquinas.

Indirect realism was popular with several early modern philosophers, including René Descartes, John Locke, G. W. Leibniz, George Berkeley, and David Hume.

Locke categorized qualities as follows:

• Primary qualities are qualities which are "explanatorily basic" – which is to say, they can be referred to as the explanation for other qualities or phenomena without requiring explanation themselves – and they are distinct in that our sensory experience of them resembles them in reality. (For example, one perceives an object as spherical precisely because of the way the atoms of the sphere are arranged.) Primary qualities cannot be removed by either thought or physical action, and include mass, movement, and, controversially, solidity (although later proponents of the distinction between primary and secondary qualities usually discount solidity).
• Secondary qualities are qualities which one's experience does not directly resemble; for example, when one sees an object as red, the sensation of seeing redness is not produced by some quality of redness in the object, but by the arrangement of atoms on the surface of the object which reflects and absorbs light in a particular way. Secondary qualities include colour, smell, and taste.

Thomas Reid, a notable member of the Scottish common sense realism was proponent of direct realism. Direct realist views have been attributed to Baruch Spinoza. Immanuel Kant's empirical realism has also been interpreted as a form of direct realism.

Late modern philosophers, J. G. Fichte and G. W. F. Hegel followed Kant in adopting empirical realism. Gottlob Frege (in his paper "Über Sinn und Bedeutung") also subscribed to indirect realism.

In contemporary philosophy, indirect realism has been defended by Edmund Husserl and Bertrand Russell Direct realism has been defended by Hilary Putnam, John McDowell, Galen Strawson, and John R. Searle.

However, epistemological dualism has come under sustained attack by other contemporary philosophers, such as Ludwig Wittgenstein (the private language argument) and Wilfrid Sellars in his seminal essay "Empiricism and the Philosophy of Mind". Indirect realism is argued to be problematical because of Ryle's regress and the homunculus argument. However, recently reliance on the private language argument and the Homunculus Objection has itself come under attack. It can be argued that those who argue for "inner presence", to use Antti Revonsuo's term, are not proposing a private "referent", with the application of language to it being "private" and thus unshareable, but a private use of public language. There is no doubt that each of us has a private understanding of public language, a notion that has been experimentally supported; George Steiner refers to our personal use of language as an "idiolect", one particular to ourselves in its detail. The question has to be put how a collective use of language can go on when, not only do we have differing understandings of the words we use, but our sensory registrations differ.

Arguments against direct realism

The argument from illusion

Illusion creates a problem for naïve realists as it suggests our senses are fallible, perceiving things that aren't there. In this illusion, the lines are horizontal, despite how they appear.

 

This argument was "first offered in a more or less fully explicit form in Berkeley (1713)." It is also referred to as the problem of conflicting appearances (e.g. Myles Burnyeat's article Conflicting Appearances). It has been argued that "informed commonsense" indicates that perceptions often depend on organs of perception. For example, humans would receive visual information very differently if they, like flies, had compound eyes, and may not even be able to imagine how things would appear with entirely different sense organs such as infra-red detectors or echo-location devices. Furthermore, perception systems can misrepresent objects even when in full working order, as shown, for example, by optical illusions like the Müller-Lyer illusion. More dramatically, sometimes people perceive things which are not there at all, which can be termed instances of "hallucination" or "perceptual delusion".

Illusions are present in nature. Rainbows are an example of a perceptual delusion. "For, unlike an architectural arch, a rainbow recedes as we approach it, never to be reached."

 

The argument from illusion allegedly shows the need to posit sense-data as the immediate objects of perception. In cases of illusion or hallucination, the object has qualities that no public physical object in that situation has and so must be distinct from any such object. Naïve realism may accommodate these facts as they stand by virtue of its very vagueness (or "open-texture"): it is not specific or detailed enough to be refuted by such cases. A more developed direct realist might respond by showing that various cases of misperception, failed perception, and perceptual relativity do not make it necessary to suppose that sense-data exist. When a stick submerged in water looks bent a direct realist is not compelled to say the stick actually is bent but can say that the stick can have more than one appearance: a straight stick can look bent when light reflected from the stick arrives at one's eye in a crooked pattern, but this appearance is not necessarily a sense-datum in the mind. Similar things can be said about the coin which appears circular from one vantage point and oval-shaped from another. Pressing on your eyeball with a finger creates double vision but assuming the existence of two sense-data is unnecessary: the direct realist can say that they have two eyes, each giving them a different view of the world. Usually the eyes are focused in the same direction; but sometimes they are not. 

However, this response is presumably based on previously observed data. If one were to be able to observe nothing other than the stick in the water, with no previous information, it would appear that the stick was bent. Visual depth in particular is a set of inferences, not an actual experience of the space between things in a radial direction outward from the observation point. If all empirical evidence is based upon observation then the entire developed memory and knowledge of every perception and of each sense may be as skewed as the bent stick. Since objects with different qualities are experienced from each of the different perspectives there is no apparent experiential basis for regarding one out of any such set of related perceptual experiences as the one in which the relevant physical object is itself immediately experienced. The most reasonable conclusion is that the experienced object is always distinct from the physical object or at least that there is no way to identify which, if any, of the immediately experienced objects is the physical object itself. Epistemologically it is as though physical objects were never given, whether or not that is in fact the case.

Another potential counter-example involves vivid hallucinations: phantom elephants, for instance, might be interpreted as sense-data. A direct realist response would differentiate hallucination from genuine perception: no perception of elephants is going on, only the different and related mental process of hallucination. However, if there are visual images when we hallucinate it seems reasonable that there are visual images when we see. Similarly if dreaming involves visual and auditory images in our minds it seems reasonable to think there are visual and auditory images, or sense-data, when we are awake and perceiving things. This argument has been challenged in a number of different ways. First it has been questioned whether there must be some object present that actually has the experienced qualities, which would then seemingly have to be something like a sense-datum. Why couldn't it be that the perceiver is simply in a state of seeming to experience such an object without any object actually being present? Second, in cases of illusion and perceptual relativity there is an object present which is simply misperceived, usually in readily explainable ways, and no need to suppose that an additional object is also involved. Third, the last part of the perceptual relativity version of the argument has been challenged by questioning whether there is really no experiential difference between veridical and non-veridical perception; and by arguing that even if sense-data are experienced in non-veridical cases and even if the difference between veridical and non-veridical cases is, as claimed, experientially indiscernible, there is still no reason to think that sense-data are the immediate objects of experience in veridical cases. Fourth, do sense-data exist through time or are they momentary? Can they exist when not being perceived? Are they public or private? Can they be themselves misperceived? Do they exist in minds or are they extra-mental, even if not physical? On the basis of the intractability of these questions, it has been argued that the conclusion of the argument from illusion is unacceptable or even unintelligible, even in the absence of a clear diagnosis of exactly where and how it goes wrong.

Direct realists can potentially deny the existence of any such thing as a mental image but this is difficult to maintain, since we seem able to visually imagine all sorts of things with ease. Even if perception does not involve images other mental processes like imagination certainly seem to. One view, similar to Reid's, is that we do have images of various sorts in our minds when we perceive, dream, hallucinate and imagine but when we actually perceive things, our sensations cannot be considered objects of perception or attention. The only objects of perception are external objects. Even if perception is accompanied by images, or sensations, it is wrong to say we perceive sensations. Direct realism defines perception as perception of external objects where an "external object" is allowed to be a photon in the eye but not an impulse in a nerve leading from the eye. Recent work in neuroscience suggests a shared ontology for perception, imagination and dreaming, with similar areas of brain being used for all of these.

Problems with the indirect theory

A problem with representationalism is that if simple data flow and information processing is assumed then something in the brain must be interpreting incoming data as a "percept". This something is often described as a homunculus, although the term homunculus is also used to imply an entity that creates a continual regress, and this need not be implied. This suggests that some phenomenon other than simple data flow and information processing is involved in perception. This is more of an issue now than it was for rationalist philosophers prior to Newton, such as Descartes, for whom physical processes were poorly defined. Descartes held that there is a "homunculus" in the form of the soul, belonging to a form of natural substance known as res cogitans that obeyed different laws from those obeyed by solid matter (res extensa). Although Descartes' duality of natural substances may have echoes in modern physics (Bose and Fermi statistics) no agreed account of 'interpretation' has been formulated. Thus representationalism remains an incomplete description of perception. Aristotle realized this and simply proposed that ideas themselves (representations) must be aware—in other words that there is no further transfer of sense impressions beyond ideas.

The representational theory of perception

A potential difficulty with representational realism is that, if we only have knowledge of representations of the world, how can we know that they resemble in any significant way the objects to which they are supposed to correspond? Any creature with a representation in its brain would need to interact with the objects that are represented to identify them with the representation. This difficulty would seem reasonably to be covered by the learning by exploration of the world that goes on throughout life. However, there may still be a concern that if the external world is only to be inferred, its 'true likeness' might be quite different from our idea of it. The representational realist would answer to this that "true likeness" is an intuitive concept that falls in the face of logic, since a likeness must always depend on the way in which something is considered.

A semantic difficulty may arise when considering reference in representationalism. If a person says "I see the Eiffel Tower" at a time when they are indeed looking at the Eiffel Tower, to what does the term "Eiffel Tower" refer? The direct realist might say that in the representational account people do not really see the tower but rather 'see' the representation. However, this is a distortion of the meaning of the word see which the representationalist does not imply. For the representationalist the statement refers to the Eiffel Tower, which implicitly is experienced in the form of a representation. The representationalist does not imply that when a person refers to the Eiffel Tower, they are referring to their sense experience, and when another person refers to the Tower, they are referring to their sense experience. 

Furthermore, representative realism claims that we perceive our perceptual intermediaries-we can attend to them-just as we observe our image in a mirror. However, as we can scientifically verify, this is clearly not true of the physiological components of the perceptual process. This also brings up the problem of dualism and its relation to representative realism, concerning the incongruous marriage of the metaphysical and the physical.

The new objection to the Homunculus Argument claims that it relies on a naive view of sensation. Because the eyes respond to light rays is no reason for supposing that the visual field requires eyes to see it. Visual sensation (the argument can be extrapolated to the other senses) bears no direct resemblance to the light rays at the retina, nor to the character of what they are reflected from or pass through or what was glowing at the origin of them. The reason given is that they only bear the similarities of co-variation with what arrives at the retinas. Just as the currents in a wire going to a loudspeaker vary proportionately with the sounds that emanate from it but have no other likeness, so too does sensation vary proportionately (and not necessarily directly) with what causes it but bears no other resemblance to the input. This implies that the color we experience is actually a cortical occurrence, and that light rays and external surfaces are not themselves colored. The proportional variations with which cortical color changes are there in the external world, but not color as we experience it. Contrary to what Gilbert Ryle believed, those who argue for sensations being brain processes do not have to hold that there is a "picture" in the brain since this is impossible according to this theory since actual pictures in the external world are not colored. It is plain that Ryle unthinkingly carried over what the eyes do to the nature of sensation; A. J. Ayer at the time described Ryle's position as "very weak". So there is no "screen" in front of cortical "eyes", no mental objects before one. As Thomas Hobbes put it: "How do we take notice of sense?—by sense itself". Moreland Perkins has characterized it thus: that sensing is not like kicking a ball, but rather "kicking a kick". Today there are still philosophers arguing for color being a property of external surfaces, light sources, etc.

A more fundamental criticism is implied in theories of this type. The differences at the sensory and perceptual levels between agents require that some means of ensuring at least a partial correlation can be achieved that allows the updatings involved in communication to take place. The process in an informative statement begins with the parties hypothetically assuming that they are referring to the "same" entity or "property", even though their selections from their sensory fields cannot match; we can call this mutually imagined projection the "logical subject" of the statement. The speaker then produces the logical predicate which effects the proposed updating of the "referent". If the statement goes through, the hearer will now have a different percept and concept of the "referent"—perhaps even seeing it now as two things and not one. The radical conclusion is that we are premature in conceiving of the external as already sorted into singular "objects" in the first place, since we only need to behave as if they are already logically singular. The diagram at the beginning of this entry would thus be thought of as a false picture of the actual case, since to draw "an" object as already selected from the real is only to treat the practically needful, but strictly false, hypothesis of objects-as-logically-singular as ontologically given. The proponents of this view thus argue that there is no need actually to believe in the singularity of an object since we can manage perfectly well by mutually imagining that 'it' is singular. A proponent of this theory can thus ask the direct realist feels why he or she thinks it is necessary to move to taking the imagining of singularity for real when there is no practical difference in the outcome in action. Therefore, although there are selections from our sensory fields which for the time being we treat as if they were objects, they are only provisional, open to corrections at any time, and, hence, far from being direct representations of pre-existing singularities, they retain an experimental character. Virtual constructs or no, they remain, however, selections that are causally linked to the real and can surprise us at any time—which removes any danger of solipsism in this theory. This approach dovetails with the philosophy known as social constructivism.

The character of experience of a physical object can be altered in major ways by changes in the conditions of perception or of the relevant sense-organs and the resulting neurophysiological processes, without change in the external physical object that initiates this process and that may seem to be depicted by the experience. Conversely any process that yields the same sensory/neural results will yield the same perceptual experience, no matter what the physical object that initiated the process may have been like. Furthermore, the causal process that intervenes between the external object and the perceptual experience takes time, so that the character of the experience reflects, at the most, an earlier stage of that object than the one existing at the moment of perception. As in observations of astronomical objects the external object may have ceased to exist long before the experience occurs. These facts are claimed to point to the conclusion that the direct object of experience is an entity produced at the end of this causal process, distinct from any physical object that initiates the process."

The adverbial theory

The above argument invites the conclusion of a perceptual dualism that raises the issue of how and whether the object can be known by experience. The adverbial theory proposes "that this dualism is a dualism of objects, with perceptual experience being a more direct experience of objects of a different sort, sense-data." Perceptual dualism implies:

both an act of awareness (or apprehension) and an object (the sense-datum) which that act apprehends or is an awareness of. The fundamental idea of the adverbial theory, in contrast, is that there is no need for such objects and the problems that they bring with them (such as whether they are physical or mental or somehow neither). Instead, it is suggested, merely the occurrence of a mental act or mental state with its own intrinsic character is enough to account for the character of immediate experience.

According to the adverbial theory, when, for example, I experience a silver elliptical shape (as when viewing a coin from an angle) I am in a certain specific state of sensing or sensory awareness or of being appeared to: I sense in a certain manner or am appeared to in a certain way, and that specific manner of sensing or of being appeared to accounts for the content of my experience: I am in a certain distinctive sort of experiential state. There need be no object or entity of any sort that is literally silver and elliptical in the material world or in the mind. I experience a silver and elliptical shape because an object or entity that literally has that color and shape is directly before my mind. But the nature of these entities and the way in which they are related to the mind are difficult to understand. The adverbial theory has the advantage of being metaphysically simpler, avoiding issues about the nature of sense-data, but we gain no real understanding of the nature of the states in question or of how exactly they account for the character of immediate experience."

Mental image

From Wikipedia, the free encyclopedia

A mental image or mental picture is the representation in a person's mind of the physical world outside that person. It is an experience that, on most occasions, significantly resembles the experience of perceiving some object, event, or scene, but occurs when the relevant object, event, or scene is not actually present to the senses. There are sometimes episodes, particularly on falling asleep (hypnagogic imagery) and waking up (hypnopompic), when the mental imagery, being of a rapid, phantasmagoric and involuntary character, defies perception, presenting a kaleidoscopic field, in which no distinct object can be discerned. Mental imagery can sometimes produce the same effects as would be produced by the behavior or experience imagined.

The nature of these experiences, what makes them possible, and their function (if any) have long been subjects of research and controversy in philosophy, psychology, cognitive science, and, more recently, neuroscience. As contemporary researchers use the expression, mental images or imagery can comprise information from any source of sensory input; one may experience auditory images, olfactory images, and so forth. However, the majority of philosophical and scientific investigations of the topic focus upon visual mental imagery. It has sometimes been assumed that, like humans, some types of animals are capable of experiencing mental images. Due to the fundamentally introspective nature of the phenomenon, there is little to no evidence either for or against this view.

Philosophers such as George Berkeley and David Hume, and early experimental psychologists such as Wilhelm Wundt and William James, understood ideas in general to be mental images. Today it is very widely believed that much imagery functions as mental representations (or mental models), playing an important role in memory and thinking. William Brant (2013, p. 12) traces the scientific use of the phrase "mental images" back to John Tyndall's 1870 speech called the "Scientific Use of the Imagination". Some have gone so far as to suggest that images are best understood to be, by definition, a form of inner, mental or neural representation; in the case of hypnagogic and hypnapompic imagery, it is not representational at all. Others reject the view that the image experience may be identical with (or directly caused by) any such representation in the mind or the brain, but do not take account of the non-representational forms of imagery.

In 2010, IBM applied for a patent on a method to extract mental images of human faces from the human brain. It uses a feedback loop based on brain measurements of the fusiform face area in the brain that activates proportionate with degree of facial recognition. It was issued in 2015.

The mind's eye

The notion of a "mind's eye" goes back at least to Cicero's reference to mentis oculi during his discussion of the orator's appropriate use of simile.

In this discussion, Cicero observed that allusions to "the Syrtis of his patrimony" and "the Charybdis of his possessions" involved similes that were "too far-fetched"; and he advised the orator to, instead, just speak of "the rock" and "the gulf" (respectively)—on the grounds that "the eyes of the mind are more easily directed to those objects which we have seen, than to those which we have only heard".

The concept of "the mind's eye" first appeared in English in Chaucer's (c. 1387) Man of Law's Tale in his Canterbury Tales, where he tells us that one of the three men dwelling in a castle was blind, and could only see with "the eyes of his mind"; namely, those eyes "with which all men see after they have become blind". The phrase remained rarely used and the OED incorrectly ascribes it to Shakespeare, as the first time the literally introspective phrase ‘the mind's eye’ is used in English was in Hamlet. As an example of introspection, it demonstrates that the internal life of the mind rarely came into focus in literature until the introspective realism movement in the 19th century.

Physical basis

The biological foundation of the mind's eye is not fully understood. Studies using fMRI have shown that the lateral geniculate nucleus and the V1 area of the visual cortex are activated during mental imagery tasks. Ratey writes:

The visual pathway is not a one-way street. Higher areas of the brain can also send visual input back to neurons in lower areas of the visual cortex. [...] As humans, we have the ability to see with the mind's eye – to have a perceptual experience in the absence of visual input. For example, PET scans have shown that when subjects, seated in a room, imagine they are at their front door starting to walk either to the left or right, activation begins in the visual association cortex, the parietal cortex, and the prefrontal cortex - all higher cognitive processing centers of the brain.

The rudiments of a biological basis for the mind's eye is found in the deeper portions of the brain below the neocortex, or where the center of perception exists. The thalamus has been found to be discrete to other components in that it processes all forms of perceptional data relayed from both lower and higher components of the brain. Damage to this component can produce permanent perceptual damage, however when damage is inflicted upon the cerebral cortex, the brain adapts to neuroplasticity to amend any occlusions for perception. It can be thought that the neocortex is a sophisticated memory storage warehouse in which data received as an input from sensory systems are compartmentalized via the cerebral cortex. This would essentially allow for shapes to be identified, although given the lack of filtering input produced internally, one may as a consequence, hallucinate - essentially seeing something that isn't received as an input externally but rather internal (i.e. an error in the filtering of segmented sensory data from the cerebral cortex may result in one seeing, feeling, hearing or experiencing something that is inconsistent with reality). 

Not all people have the same internal perceptual ability. For many, when the eyes are closed, the perception of darkness prevails. However, some people are able to perceive colorful, dynamic imagery. The use of hallucinogenic drugs increases the subject's ability to consciously access visual (and auditory, and other sense) percepts.

Furthermore, the pineal gland is a hypothetical candidate for producing a mind's eye; Rick Strassman and others have postulated that during near-death experiences (NDEs) and dreaming, the gland might secrete a hallucinogenic chemical N,N-Dimethyltryptamine (DMT) to produce internal visuals when external sensory data is occluded. However, this hypothesis has yet to be fully supported with neurochemical evidence and plausible mechanism for DMT production.

The hypothesized condition where a person lacks a mind's eye is called aphantasia. The term was first suggested in a 2015 study.

Common examples of mental images include daydreaming and the mental visualization that occurs while reading a book. Another is of the pictures summoned by athletes during training or before a competition, outlining each step they will take to accomplish their goal. When a musician hears a song, he or she can sometimes "see" the song notes in their head, as well as hear them with all their tonal qualities. This is considered different from an after-effect, such as an after-image. Calling up an image in our minds can be a voluntary act, so it can be characterized as being under various degrees of conscious control.

According to psychologist and cognitive scientist Steven Pinker, our experiences of the world are represented in our minds as mental images. These mental images can then be associated and compared with others, and can be used to synthesize completely new images. In this view, mental images allow us to form useful theories of how the world works by formulating likely sequences of mental images in our heads without having to directly experience that outcome. Whether other creatures have this capability is debatable.

There are several theories as to how mental images are formed in the mind. These include the dual-code theory, the propositional theory, and the functional-equivalency hypothesis. The dual-code theory, created by Allan Paivio in 1971, is the theory that we use two separate codes to represent information in our brains: image codes and verbal codes. Image codes are things like thinking of a picture of a dog when you are thinking of a dog, whereas a verbal code would be to think of the word "dog". Another example is the difference between thinking of abstract words such as justice or love and thinking of concrete words like elephant or chair. When abstract words are thought of, it is easier to think of them in terms of verbal codes—finding words that define them or describe them. With concrete words, it is often easier to use image codes and bring up a picture of a human or chair in your mind rather than words associated or descriptive of them. 

The propositional theory involves storing images in the form of a generic propositional code that stores the meaning of the concept not the image itself. The propositional codes can either be descriptive of the image or symbolic. They are then transferred back into verbal and visual code to form the mental image.

The functional-equivalency hypothesis is that mental images are "internal representations" that work in the same way as the actual perception of physical objects. In other words, the picture of a dog brought to mind when the word dog is read is interpreted in the same way as if the person looking at an actual dog before them. 

Research has occurred to designate a specific neural correlate of imagery; however, studies show a multitude of results. Most studies published before 2001 suggest neural correlates of visual imagery occur in brodmann area 17. Auditory performance imagery have been observed in the premotor areas, precunes, and medial brodmann area 40. Auditory imagery in general occurs across participants in the temporal voice area (TVA), which allows top-down imaging manipulations, processing, and storage of audition functions. Olfactory imagery research shows activation in the anterior piriform cortex and the posterior piriform cortex; experts in olfactory imagery have larger gray matter associated to olfactory areas. Tactile imagery is found to occur in the dorsolateral prefrontal area, inferior frontal gyrus, frontal gyrus, insula, precentral gyrus, and the medial frontal gyrus with basil ganglia activation in the ventral posteriomedial nucleus and putamen (hemisphere activation corresponds to the location of the imagined tactile stimulus). Research in gustatory imagery reveals activation in the anterior insular cortex, frontal operculum, and prefrontal cortex. Novices of a specific form of mental imagery show less gray matter than experts of mental imagery congruent to that form. A meta-analysis of neuroimagery studies revealed significant activation of the bilateral dorsal parietal, interior insula, and left inferior frontal regions of the brain.

Imagery has been thought to co-occur with perception; however, participants with damaged sense-modality receptors can sometimes perform imagery of said modality receptors. Neuroscience with imagery has been used to communicate with seemingly unconscious individuals through fMRI activation of different neural correlates of imagery, demanding further study into low quality consciousness. A study on one patient with one occipital lobe removed found the horizontal area of their visual mental image was reduced.

Neural substrates of visual imagery

Visual imagery is the ability to create mental representations of things, people, and places that are absent from an individual’s visual field. This ability is crucial to problem-solving tasks, memory, and spatial reasoning. Neuroscientists have found that imagery and perception share many of the same neural substrates, or areas of the brain that function similarly during both imagery and perception, such as the visual cortex and higher visual areas. Kosslyn and colleagues (1999) showed that the early visual cortex, Area 17 and Area 18/19, is activated during visual imagery. They found that inhibition of these areas through repetitive transcranial magnetic stimulation (rTMS) resulted in impaired visual perception and imagery. Furthermore, research conducted with lesioned patients has revealed that visual imagery and visual perception have the same representational organization. This has been concluded from patients in which impaired perception also experience visual imagery deficits at the same level of the mental representation.

Behrmann and colleagues (1992) describe a patient C.K., who provided evidence challenging the view that visual imagery and visual perception rely on the same representational system. C.K. was a 33-year old man with visual object agnosia acquired after a vehicular accident. This deficit prevented him from being able to recognize objects and copy objects fluidly. Surprisingly, his ability to draw accurate objects from memory indicated his visual imagery was intact and normal. Furthermore, C.K. successfully performed other tasks requiring visual imagery for judgment of size, shape, color, and composition. These findings conflict with previous research as they suggest there is a partial dissociation between visual imagery and visual perception. C.K. exhibited a perceptual deficit that was not associated with a corresponding deficit in visual imagery, indicating that these two processes have systems for mental representations that may not be mediated entirely by the same neural substrates. 

Schlegel and colleagues (2013) conducted a functional MRI analysis of regions activated during manipulation of visual imagery. They identified 11 bilateral cortical and subcortical regions that exhibited increased activation when manipulating a visual image compared to when the visual image was just maintained. These regions included the occipital lobe and ventral stream areas, two parietal lobe regions, the posterior parietal cortex and the precuneus lobule, and three frontal lobe regions, the frontal eye fields, dorsolateral prefrontal cortex, and the prefrontal cortex. Due to their suspected involvement in working memory and attention, the authors propose that these parietal and prefrontal regions, and occipital regions, are part of a network involved in mediating the manipulation of visual imagery. These results suggest a top-down activation of visual areas in visual imagery.

Using Dynamic Causal Modeling (DCM) to determine the connectivity of cortical networks, Ishai et al. (2010) demonstrated that activation of the network mediating visual imagery is initiated by prefrontal cortex and posterior parietal cortex activity. Generation of objects from memory resulted in initial activation of the prefrontal and the posterior parietal areas, which then activate earlier visual areas through backward connectivity. Activation of the prefrontal cortex and posterior parietal cortex has also been found to be involved in retrieval of object representations from long-term memory, their maintenance in working memory, and attention during visual imagery. Thus, Ishai et al. suggest that the network mediating visual imagery is composed of attentional mechanisms arising from the posterior parietal cortex and the prefrontal cortex.

Vividness of visual imagery is a crucial component of an individual’s ability to perform cognitive tasks requiring imagery. Vividness of visual imagery varies not only between individuals but also within individuals. Dijkstra and colleagues (2017) found that the variation in vividness of visual imagery is dependent on the degree to which the neural substrates of visual imagery overlap with those of visual perception. They found that overlap between imagery and perception in the entire visual cortex, the parietal precuneus lobule, the right parietal cortex, and the medial frontal cortex predicted the vividness of a mental representation. The activated regions beyond the visual areas are believed to drive the imagery-specific processes rather than the visual processes shared with perception. It has been suggested that the precuneus contributes to vividness by selecting important details for imagery. The medial frontal cortex is suspected to be involved in the retrieval and integration of information from the parietal and visual areas during working memory and visual imagery. The right parietal cortex appears to be important in attention, visual inspection, and stabilization of mental representations. Thus, the neural substrates of visual imagery and perception overlap in areas beyond the visual cortex and the degree of this overlap in these areas correlates with the vividness of mental representations during imagery.

Philosophical ideas

Mental images are an important topic in classical and modern philosophy, as they are central to the study of knowledge. In the Republic, Book VII, Plato has Socrates present the Allegory of the Cave: a prisoner, bound and unable to move, sits with his back to a fire watching the shadows cast on the cave wall in front of him by people carrying objects behind his back. These people and the objects they carry are representations of real things in the world. Unenlightened man is like the prisoner, explains Socrates, a human being making mental images from the sense data that he experiences.

The eighteenth-century philosopher Bishop George Berkeley proposed similar ideas in his theory of idealism. Berkeley stated that reality is equivalent to mental images—our mental images are not a copy of another material reality but that reality itself. Berkeley, however, sharply distinguished between the images that he considered to constitute the external world, and the images of individual imagination. According to Berkeley, only the latter are considered "mental imagery" in the contemporary sense of the term. 

The eighteenth century British writer Dr. Samuel Johnson criticized idealism. When asked what he thought about idealism, he is alleged to have replied "I refute it thus!" as he kicked a large rock and his leg rebounded. His point was that the idea that the rock is just another mental image and has no material existence of its own is a poor explanation of the painful sense data he had just experienced.

David Deutsch addresses Johnson's objection to idealism in The Fabric of Reality when he states that, if we judge the value of our mental images of the world by the quality and quantity of the sense data that they can explain, then the most valuable mental image—or theory—that we currently have is that the world has a real independent existence and that humans have successfully evolved by building up and adapting patterns of mental images to explain it. This is an important idea in scientific thought.

Critics of scientific realism ask how the inner perception of mental images actually occurs. This is sometimes called the "homunculus problem" (see also the mind's eye). The problem is similar to asking how the images you see on a computer screen exist in the memory of the computer. To scientific materialism, mental images and the perception of them must be brain-states. According to critics, scientific realists cannot explain where the images and their perceiver exist in the brain. To use the analogy of the computer screen, these critics argue that cognitive science and psychology have been unsuccessful in identifying either the component in the brain (i.e., "hardware") or the mental processes that store these images (i.e. "software").

In experimental psychology

Cognitive psychologists and (later) cognitive neuroscientists have empirically tested some of the philosophical questions related to whether and how the human brain uses mental imagery in cognition. 

One theory of the mind that was examined in these experiments was the "brain as serial computer" philosophical metaphor of the 1970s. Psychologist Zenon Pylyshyn theorized that the human mind processes mental images by decomposing them into an underlying mathematical proposition. Roger Shepard and Jacqueline Metzler challenged that view by presenting subjects with 2D line drawings of groups of 3D block "objects" and asking them to determine whether that "object" is the same as a second figure, some of which rotations of the first "object". Shepard and Metzler proposed that if we decomposed and then mentally re-imaged the objects into basic mathematical propositions, as the then-dominant view of cognition "as a serial digital computer" assumed, then it would be expected that the time it took to determine whether the object is the same or not would be independent of how much the object had been rotated. Shepard and Metzler found the opposite: a linear relationship between the degree of rotation in the mental imagery task and the time it took participants to reach their answer. 

This mental rotation finding implied that the human mind—and the human brain—maintains and manipulates mental images as topographic and topological wholes, an implication that was quickly put to test by psychologists. Stephen Kosslyn and colleagues showed in a series of neuroimaging experiments that the mental image of objects like the letter "F" are mapped, maintained and rotated as an image-like whole in areas of the human visual cortex. Moreover, Kosslyn's work showed that there are considerable similarities between the neural mappings for imagined stimuli and perceived stimuli. The authors of these studies concluded that, while the neural processes they studied rely on mathematical and computational underpinnings, the brain also seems optimized to handle the sort of mathematics that constantly computes a series of topologically-based images rather than calculating a mathematical model of an object.

Recent studies in neurology and neuropsychology on mental imagery have further questioned the "mind as serial computer" theory, arguing instead that human mental imagery manifests both visually and kinesthetically. For example, several studies have provided evidence that people are slower at rotating line drawings of objects such as hands in directions incompatible with the joints of the human body, and that patients with painful, injured arms are slower at mentally rotating line drawings of the hand from the side of the injured arm.

Some psychologists, including Kosslyn, have argued that such results occur because of interference in the brain between distinct systems in the brain that process the visual and motor mental imagery. Subsequent neuroimaging studies showed that the interference between the motor and visual imagery system could be induced by having participants physically handle actual 3D blocks glued together to form objects similar to those depicted in the line-drawings. Amorim et al. have shown that, when a cylindrical "head" was added to Shepard and Metzler's line drawings of 3D block figures, participants were quicker and more accurate at solving mental rotation problems. They argue that motoric embodiment is not just "interference" that inhibits visual mental imagery but is capable of facilitating mental imagery.

As cognitive neuroscience approaches to mental imagery continued, research expanded beyond questions of serial versus parallel or topographic processing to questions of the relationship between mental images and perceptual representations. Both brain imaging (fMRI and ERP) and studies of neuropsychological patients have been used to test the hypothesis that a mental image is the reactivation, from memory, of brain representations normally activated during the perception of an external stimulus. In other words, if perceiving an apple activates contour and location and shape and color representations in the brain’s visual system, then imagining an apple activates some or all of these same representations using information stored in memory. Early evidence for this idea came from neuropsychology. Patients with brain damage that impairs perception in specific ways, for example by damaging shape or color representations, seem to generally to have impaired mental imagery in similar ways. Studies of brain function in normal human brains support this same conclusion, showing activity in the brain’s visual areas while subjects imagined visual objects and scenes.

The previously mentioned and numerous related studies have led to a relative consensus within cognitive science, psychology, neuroscience, and philosophy on the neural status of mental images. In general, researchers agree that, while there is no homunculus inside the head viewing these mental images, our brains do form and maintain mental images as image-like wholes. The problem of exactly how these images are stored and manipulated within the human brain, in particular within language and communication, remains a fertile area of study.

One of the longest-running research topics on the mental image has basis on the fact that people report large individual differences in the vividness of their images. Special questionnaires have been developed to assess such differences, including the Vividness of Visual Imagery Questionnaire (VVIQ) developed by David Marks. Laboratory studies have suggested that the subjectively reported variations in imagery vividness are associated with different neural states within the brain and also different cognitive competences such as the ability to accurately recall information presented in pictures Rodway, Gillies and Schepman used a novel long-term change detection task to determine whether participants with low and high vividness scores on the VVIQ2 showed any performance differences. Rodway et al. found that high vividness participants were significantly more accurate at detecting salient changes to pictures compared to low-vividness participants. This replicated an earlier study.

Recent studies have found that individual differences in VVIQ scores can be used to predict changes in a person's brain while visualizing different activities. Functional magnetic resonance imaging (fMRI) was used to study the association between early visual cortex activity relative to the whole brain while participants visualized themselves or another person bench pressing or stair climbing. Reported image vividness correlates significantly with the relative fMRI signal in the visual cortex. Thus, individual differences in the vividness of visual imagery can be measured objectively. 

Logie, Pernet, Buonocore and Della Sala (2011) used behavioural and fMRI data for mental rotation from individuals reporting vivid and poor imagery on the VVIQ. Groups differed in brain activation patterns suggesting that the groups performed the same tasks in different ways. These findings help to explain the lack of association previously reported between VVIQ scores and mental rotation performance.

Training and learning styles

Some educational theorists have drawn from the idea of mental imagery in their studies of learning styles. Proponents of these theories state that people often have learning processes that emphasize visual, auditory, and kinesthetic systems of experience. According to these theorists, teaching in multiple overlapping sensory systems benefits learning, and they encourage teachers to use content and media that integrates well with the visual, auditory, and kinesthetic systems whenever possible. 

Educational researchers have examined whether the experience of mental imagery affects the degree of learning. For example, imagining playing a 5-finger piano exercise (mental practice) resulted in a significant improvement in performance over no mental practice—though not as significant as that produced by physical practice. The authors of the study stated that "mental practice alone seems to be sufficient to promote the modulation of neural circuits involved in the early stages of motor skill learning".

Visualization and the Himalayan traditions

In general, Vajrayana Buddhism, Bön, and Tantra utilize sophisticated visualization or imaginal (in the language of Jean Houston of Transpersonal Psychology) processes in the thoughtform construction of the yidam sadhana, kye-rim, and dzog-rim modes of meditation and in the yantra, thangka, and mandala traditions, where holding the fully realized form in the mind is a prerequisite prior to creating an 'authentic' new art work that will provide a sacred support or foundation for deity.

Substitution effects

Mental imagery can act as a substitute for the imagined experience: Imagining an experience can evoke similar cognitive, physiological, and/or behavioral consequences as having the corresponding experience in reality. At least four classes of such effects have been documented.

1. Imagined experiences are attributed evidentiary value like physical evidence.
2. Mental practice can instantiate the same performance benefits as physical practice.
3. Imagined consumption of a food can reduce its actual consumption.
4. Imagined goal achievement can reduce motivation for actual goal achievement.