Mental imagery

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The following entry is excerpted and adapted from Kosslyn, Ganis, and Thompson, "Neural Foundations of Imagery," Nature Reviews Neuroscience 2.9, 635-642 (2001) by permission from Nature Reviews Neuroscience , Copyright 2005 Macmillan Magazines Ltd.

Note: This entry is meant to serve as a brief introduction to the notion of mental imagery and as a parent entry for the more specific articles visual mental imagery, auditory imagery, and motor imagery.

Mental imagery occurs when perceptual information is accessed from memory, giving rise to the experience of seeing with the mind's eye', 'hearing with the mind's ear' and so on. By contrast, perception occurs when information is registered directly from the senses. Mental images need not result simply from the recall of previously perceived objects or events; they can also be created by combining and modifying stored perceptual information in novel ways. Imagery has had a central role in theories of mental function since at least the time of Plato. It has fallen in and out of fashion, in large part because it is inherently a private affair, by definition restricted to the confines of the mind, and so it has been difficult to study. In fact, in 1913, the founder of behaviorism, John B. Watson, denied that mental images even existed. Instead, he suggested that thinking consists of subtle movements of the vocal apparatus (Watson 1913). In spite of the demonstration by Alan Paivio and his colleagues that the use of imagery greatly improves memory (Paivio 1971), many researchers were not convinced that imagery is a distinct form of thought. Indeed, Watson's position was echoed 60 years later by Zenon Pylyshyn, who championed the view that mental images are not 'images' at all, but rather rely on mental descriptions no different in kind from those that underlie language. According to Pylyshyn 1973, the pictorial aspects of imagery that are evident to conscious experience are entirely epiphenomenal, like the heat thrown off by a light bulb when you read (which has no role in the reading process).

The emergence of cognitive neuroscience has opened a new chapter in the study of imagery. An enormous amount has been learned about the neural underpinnings of visual perception, memory, emotion and motor control. Much of this information has come from the study of animal models. Unlike language and reasoning, these more basic functions have many common features among higher mammals, including humans. In addition, new neuroimaging technologies, especially positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI), allow theories of imagery to be tested objectively in humans. Researchers have taken advantage of these developments to show that mental imagery draws on much of the same neural machinery as perception in the same modality, and can engage mechanisms used in memory, emotion and motor control.

A variety of methods are used to investigate mental imagery, including studies of the effects of selective brain damage on behavior, neuroimaging and transcranial magnetic stimulation (TMS). Each approach has its strengths and weaknesses, but the methods are complementary. So, for example, neuroimaging provides only correlational data (when engaged in a particular task, a particular set of brain areas is activated), but can monitor the entire brain; TMS, by contrast, can be used to establish causal roles of distinct areas (for example, by showing that performance in a task that draws on a specific brain area is impaired following TMS to that area), but must be targeted to a specific location. To the extent that the same conclusions are reached using different methods, the conclusions drawn from these studies can be taken increasingly seriously.

Conclusions

Imagery is no longer seen as an awkward leftover from a previous, less rigorous age - a topic unfit for polite company. Rather, researchers agree that most of the neural processes that underlie like-modality perception are also used in imagery; and imagery, in many ways, can 'stand in' for (re-present, if you will) a perceptual stimulus or situation. Imagery not only engages the motor system, but also affects the body, much as can actual perceptual experience.

Nevertheless, many questions remain. For example, under what circumstances is the early sensory cortex recruited during imagery? Why is the early sensory cortex often recruited during visual mental imagery, but not during auditory imagery? Why do people differ so much in their imagery abilities? Does genetics affect some aspects of imagery more than others? How does semantic content in images engage specific mechanisms?

Unlike the situation even 20 years ago, questions such as these can now be answered. Indeed, the advent of additional technologies, such as Diffuse Optical Tomography (Obrig 2000) (DOT), promises to facilitate studies of the neural bases of imagery. This technique is portable, very inexpensive, and more forgiving than fMRI when subjects move. It is also totally silent. The drawback is that it can only monitor cortical activity, and not even all of that. However, it can assess the lion's share of cortex, and will allow large-scale individual-differences studies to be done. Such studies can relate differences in patterns of brain activation to differences in performance, which should, in turn, tell us not only about what the brain is doing, but also about how and why people differ in their modes of thinking.

Related Entries

For a more detailed discussion of research into mental imagery, please see:


Mental Imagery References

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    • This is a creative analysis of the perceptual and imagery abilities that are damaged and spared following memory. This analysis shows, without question, that imagery is not a single faculty, but is carried out by the joint operation of a set of processes.
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    • This meta-analysis and review shows that different aspects of imagery tasks predict activation in early visual cortex (for example, the need to extract highresolution details), posterior parietal cortex (used in spatial tasks) and inferotemporal cortex (used in imagery tasks that do not require the extraction of high-resolution details).
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    • auditory imagery is in many ways analogous to visual imagery, but activates only 'higher level' brain areas, not early auditory cortex. The similarities and differences between the different modalities illuminate key facets of the underlying mechanisms of imagery.
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