Philosophers such as Berkeley, and Hume, and early experimental psychologists, such as Wundt and James, understood ideas in general to be mental images, and today it is very widely believed that images function as mental representations (or mental models), playing an important role in memory and thinking (Paivio, 1986; Egan, 1992; Barsalou, 1999; Prinz, 2002). Indeed, some have gone so far as to suggest that images are best understood as by definition a form of inner, mental or neural representation (Block, 1983; Kosslyn, 1983). Others, however, reject the view that the image experience may be identical with (or directly caused by) any such representation in the mind or the brain (Sartre, 1940; Ryle, 1949; Skinner, 1974; Thomas, 1999; Bartolomeo, 2002; Bennett & Hacker, 2003).
According to some psychologists our experiences of the world are stored as mental images. These mental images can then be associated and compared with other mental images, and can be used to synthesize completely new images. Some believe that this process allows us to form useful theories of how the world works based on likely sequences of mental images, without having to directly experience that outcome, for example through the processes of deduction or simulation. Whether other creatures have this capability is debated.
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 uses the metaphor of a prisoner in a cave, bound and unable to move, sitting with his back to a fire and watching the shadows cast on the wall in front of him by people carrying objects behind his back. The objects that they are carrying are representations of real things in the world. The prisoner, explains Socrates, is like a human being making mental images from the sense data that he experiences.
More recently, Bishop Berkeley's 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 (while out on a walk in Scotland) 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 was just another mental image and had no material existence of its own, was 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 these philosophers, scientific realists cannot explain where the images and the perceiver of them exist in the brain or its functions. To use the analogy of the computer screen, these critics argue that cognitive science and psychology has been unsuccessful in identifying the component in the brain (e.g. 'hardware' such as a computer graphics card) or the mental processes that store these images (e.g. 'software' such as a graphics device driver).
One related theory of the mind that was examined in these experiments was the "brain as serial computer" philosophical metaphor of the 70s. Psychologist Zenon Pylyshyn theorized that the human mind processes mental images by decomposing them into an underlying mathematical proposition. Roger Shepard and Jacqueline Metzler (1971) challenged that view by presenting subjects with 2D line drawings of groups of 3D block "objects" and asking them to determine whether that "object" was the same as a second figure, some of which were 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" (Gardner 1987) assumed, then it would be expected that the time it took to determine whether the object was the same or not would be independent of how much the object was 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. Kosslyn and colleagues (1995; see also 1994) 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 were 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 is both visually and motorically embodied. For example, several studies 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 (Parsons 1987; 2003), and that patients with painful injured arms are slower at mentally rotating line drawings of the hand from the side of the injured arm (Schwoebel et al. 2001).
Some psychologists, including Stephen Kosslyn, have argued that such results occur because of interference in the brain between distinct systems in the brain that process the visual and motoric mental imagery. Subsequent neuroimaging studies (Kosslyn et al. 2001) showed that the interference between the motoric 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. However, Amorim et al. (2006) have recently showed 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. Researchers generally 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 (Rohrer 2006). The problem of exactly how these images are stored and manipulated within the human brain, particularly within language and communication, remains a fertile area of study.
One of the longest running research topics on the mental image has been 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 (Marks, 1973). Rodway, Gillies and Schepman (2006) 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. (2006) 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 by Gur and Hilgard (1975).
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. Cui et al. (2007) used functional magnetic resonance imaging (fMRI) to study the association between early visual cortex activity relative to the whole brain while participants visualised 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.
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 which 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. Examples of these teaching methods include spoken components with a whiteboard or overheads.
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 and 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." (Pascual-Leone et al 1995).
Mental images, and particular images from dreams, are the basis for the theories of Sigmund Freud about human behavior. His basic thesis was that our childhood experiences strongly influence the mental images that we make in later life. He believed that humans form mental images in the unconscious according to their "latent" desires and they are not aware of them in their conscious mind although, according to Freud, they have a major influence on human behavior.