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A mental image is the representation in a person's mind of the physical world outside of 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.
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 mental imagery) can occur in the form of any sense, so that we may experience auditory images, olfactory images, and so forth. However, the vast majority of philosophical and scientific investigations of the topic focus upon visual mental imagery. It has been assumed that, like humans, many types of animals are capable of experiencing mental images. Due to the fundamentally subjective 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 the William 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 how 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.
How mental images form in the brain
Common examples of mental images include daydreaming and the mental visualization that occurs while reading a book. Another is of the pictures summoned by an athlete 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 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.
Philosophical ideas about mental images
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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.
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.[why?]
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,[who?] 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").
Mental imagery in experimental psychology
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 recently 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.
These 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[who?] 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."
Mental imagery, 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.
- Animal cognition
- Creative visualization
- Fantasy (psychology)
- Fantasy prone personality
- Guided imagery
- Mental event
- Mental rotation
- Motor imagery
- Visual space
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