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