Imagery

The term imagery is inherently ambiguous -- it can refer to iconography, visual effects in cinematography, mental events, or even to some types of prose. This article focuses on mental imagery. For example, when asked to recall the number of windows in one's living room, most people report that they visualize the room, and mentally scan over this image, counting the windows. In this article we consider not the introspections themselves, but rather the nature of the underlying representations that give rise to them. Because most research on imagery has addressed visual mental imagery, we will focus on that modality.

Unlike most other cognitive activities (such as language and memory), we only know that visual mental image representations are present because we have the experience of seeing, but in the absence of the appropriate sensory input. And here lies a central problem in the study of imagery (the "introspective dilemma"): there is no way that another person can verify what we "see" with our inner eye, and hence the phenomenon smacks of unscientific, fanciful confabulation.

Nevertheless, imagery played a central role in theories of the mind for centuries. For example, the British Associationists conceptualized thought itself as sequences of images. And, WILHELM WUNDT, the founder of scientific psychology, emphasized the analysis of images. However, the central role of imagery in theories of mental activity was undermined when Kulpe, in 1904, pointed out that some thoughts are not accompanied by imagery (e.g., one is not aware of the processes that allow one to decide which of two objects is heavier).

The observation that images are not the hallmark of all thought processes was soon followed by the notion that images as ordinarily conceived (and, indeed, thoughts themselves) may not even exist! John Watson (1913) -- the founder of BEHAVIORISM -- argued that images actually correspond to subtle movements of the larynx. Watson emphasized the precept that scientific phenomena must be publicly observable, and imagery clearly is not. This line of argument led to diminished interest in imagery until the early 1960s.

Behaviorism ultimately had a salutary effect on the study of imagery; it made it rigorous. Whereas researchers in Wundt's lab were trained in INTROSPECTION, modern researchers are trained in measuring subtle aspects of behavior. In such cases, the behavior is like the track left by a cosmic ray passing through a cloud chamber: It is a kind of signature, a hallmark, that allows us to draw inferences about the phenomena that produced it.

Imagery has effects on the accuracy of LEARNING and MEMORY. In the early 1960s the behaviorist approach to language led to the study of "verbal learning": words were treated as stimuli that could be learned like any other. Paivio and his colleagues (e.g., Paivio 1971) showed that imagery is a major factor that affects the ease of learning words. Not only is a picture worth a thousand words, it is also easier to remember. The same is true for "mental pictures." Words are better learned and remembered if they name an object that can be visualized, such as "boat" or "cat," than they are if they name an abstract idea, such as "justice" or "kindness." The way objects are visualized can, however, affect how easily the words they correspond to are remembered; Bower (1972) found that forming an image of the interaction between a pair of objects made it easier to remember the names of those objects than was the case by simply forming an image of the objects existing separately.

Imagery also affects the detection of perceptual stimuli. Studies of imagery benefited from new techniques developed to investigate perception. For example, SIGNAL DETECTION THEORY (e.g., Green and Swets 1966) has been used to show that forming a visual mental image interrupts vision more than it does hearing, but forming an auditory image interrupts hearing more than vision. Such results reveal that imagery draws on mechanisms used in like-modality perception. However, Craver-Lemley and Reeves (1992) report, this interference occurs only if the image overlaps the target. It is also worth noting that images can be mistaken for percepts, both at the time and in memory (e.g., Johnson and Raye 1981; see Kosslyn 1994 for a review).

One reason that behaviorism failed to hold sway was that a viable alternative was introduced: cognitive psychology. Cognitive psychology likened the mind to software running on a computer, and imagery -- along with all other mental processes -- was conceptualized in terms of such "information processing." This approach stressed that different sets of processes are used to accomplish different ends. For example, to plan how to arrange furniture in an empty room, one could generate an image of the furniture and then mentally transform it (rotate it, perhaps imagine stacking a shelf on a crate or desk); one must maintain the image as it is being inspected. Sometimes imagery involves only some of these processes; for instance, image transformation is irrelevant in remembering the way to get to the train station. Information processing is often studied by measuring response times.

Consider tasks that require image inspection. Do you know the shape of a Saint Bernard's ears? Subjects will require more time to perform this task if they are first told to visualize the dog far off in the distance; before they can answer the question, they will "zoom in" on the part of their image containing the dog's head. Similarly, subjects will require more time to answer this question if they are first told to visualize a Saint Bernard's tail; they will now mentally "scan" across the image of the dog in order to "look" at its ears. Indeed, the time to respond increases linearly with the amount of distance scanned -- even though the subject's eyes are closed. Such results suggest that the image representations embody spatial extent (see Kosslyn 1994).

Now consider image generation. Images are generated by activating stored information. Images are created "piece by piece," and thus an image with more parts takes longer to form. Indeed, the time to form an image often increases linearly with the number of parts that are assembled.

Finally, consider image transformation. When objects in images are transformed, they often mimic the movements of real objects. For example, rotating objects appear to shift through a trajectory; the farther one rotates an imaged object, the longer it takes to do so (see MENTAL ROTATION). Shepard (1984) suggests that this occurs because, due to natural selection, certain laws of physics have been internalized in the brain and act as constraints on the imagery process. Alternatively, it is possible that motor programs guide imagery and, consequently, objects are mentally manipulated in the same manner that real objects would be physically manipulated. In fact, motor parts of the brain have been shown to be activated during some image transformation tasks (e.g., Georgopoulos et al. 1989).

The computer metaphor encouraged a kind of "disembodied mind" approach, which ignored certain classes of data and constraints on theories. Recent years have seen a sharp increase in research on the neural bases of imagery. Such research has addressed each of the processes noted earlier, as well as the issue of how images are internally represented.

It has long been known that some parts of the brain are spatially organized (see Felleman and Van Essen 1991); the pattern of activity on the retina is projected onto these areas (albeit distorted in several ways). Studies reported by five laboratories have now shown that some of these areas, such as primary VISUAL CORTEX, are activated when people visualize. Moreover, the pattern of activation is systematically altered by spatial properties of the image, in a way similar to what occurs in perception. Such results suggest that imagery relies on "depictive" representations, which use space to represent space. This result bears directly on the "imagery debate" of the 1970s and 1980s, which focused on the question of whether a depictive representation is used during imagery. However, this result is not always obtained, and the crucial differences between the tasks that do and do not engender such representations have not yet been identified (Mellet et al. 1998).

Image generation is most often disrupted by damage to the posterior left hemisphere (Farah 1984). However, recent studies have shown that images can be generated in at least two ways, one of which uses stored descriptions to arrange parts and relies primarily on left-hemisphere processes, and the other of which uses stored metric spatial information to arrange parts and relies primarily on the right hemisphere (for a review of the literature on this topic, see Behrmann, Kosslyn and Jeannerod 1996).

Patients with brain damage that impairs visual perception sometimes also have corresponding deficits when they inspect imaged objects. For example, some patients who have suffered damage to the posterior right parietal lobe display a phenomenon known as "unilateral VISUAL NEGLECT"; they ignore objects to the left side of space. These patients may display the same behavior when visualizing -- they ignore objects on the left half of their images (e.g., Bisiach and Luzzatti 1978).

Image transformations are accomplished by a complex set of brain areas. However, studies with brain-damaged patients suggest that the right hemisphere plays a crucial role in the transformation process itself (e.g., Corballis and Sergent 1989; Ratcliff 1979).

In conclusion, the recent research on visual mental imagery reveals that imagery is not a unitary phenomenon, but rather is accomplished by a collection of distinct processes. These processes are implemented by neural systems, not discrete "centers." These processes can be combined in different ways (with each combination corresponding to a distinct method or strategy) to allow one to accomplish any given imagery task.

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-- Stephen M. Kosslyn and Carolyn S. Rabin

References

Behrmann, M., S. M. Kosslyn, and M. Jeannerod, Eds. (1996). The Neuropsychology of Mental Imagery. New York: Pergamon.

Bisiach, E., and C. Luzzatti. (1978). Unilateral neglect of representational space. Cortex 14:129-133.

Boring, E. G. (1950). A History of Experimental Psychology. 2nd ed. New York: Appleton-Century-Crofts.

Bower, G. H. (1972). Mental imagery and associative learning. In L. Gregg, Ed., Cognition in Learning and Memory. New York: Wiley.

Corballis, M. C., and J. Sergent. (1989). Hemispheric specialization for mental rotation. Cortex 25:15-25.

Craver-Lemley, C., and A. Reeves. (1992). How visual imagery interferes with vision. Psychological Review 99:633-649.

Farah, M. J., (1984). The neurological basis of mental imagery: a componential analysis. Cognition 18:245-272.

Felleman, D. J., and D. C. Van Essen. (1991). Distributed hierarchical processing in the primate cerebral cortex. Cerebral Cortex 1:1-47.

Georgopolous, A. P., J. T. Lurito, M. Petrides, A. B. Schwartz, and J. T. Massey. (1989). Mental rotation of the neuronal population vector. Science 243:234-236.

Green, D. M., and J. A. Swets. (1966). Signal Detection Theory and Psychophysics. New York: Wiley.

Johnson, M. K., and C. L. Raye. (1981). Reality monitoring. Psychological Review 88:67-85.

Kosslyn, S. M. (1994). Image and Brain: The Resolution of the Imagery Debate. Cambridge, MA: MIT Press.

Logie, R. H., and M. Denis. (1991). Mental Images in Human Cognition. Amsterdam: North Holland.

Mellet, E., L. Petit, B. Mazoyer, M. Denis, and N. Tzourio. (1998). Reopening the mental imagery debate: Lessons from functional neuroanatomy. NeuroImage 8:129-139.

Paivio, A. (1971). Imagery and Verbal Processes. New York: Holt, Rinehart and Winston.

Ratcliff, G. (1979). Spatial thought, mental rotation and the right cerebral hemisphere. Neuropsychologia 17:49-54.

Shepard, R. N. (1984). Ecological constraints on internal representation: resonant kinematics of perceiving, imagining, thinking, and dreaming. Psychological Review 91:417-447.

Watson, J. B. (1913). Psychology as the behaviorist views it. Psychological Review 20:158-177.

Imagery

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