C H A P T E R 5
THE RELATION BETWEEN IMAGERY AND PERCEPTION
“The study of mental imagery is a process which involves a marriage between the poet and the scientist “. Dan Tomasulo (1978)
Imagery is, “the miraculous quality that human beings use to re‑evoke and reorganise perception”, Shorn et al (1980). Imagery is no longer considered idiosyncratic. It is an absolutely integral part of human cognition which gives substance to subjective memory and realistic abstract thought. When we imagine seeing an object, the sensation seems to be visual in nature. Given this sensation, a reasonable question is: to what degree is visual imagery analogous to perception and what is the overlap between the two processes?. This review examines the question.
Considerable time and effort were spent in the early psychological laboratories pursuing the problem of the distinction between imagery and perception (Titchener, 1921). Imagery first became a topic of serious interest in psychology with Fechner‘s pioneering experimentation in his book ‘Elemente de Psychophysik'(1860). Twenty years later Sir Francs Galton conducted his investigation of imagery with a thoroughness and scope which few investigations of imagery after him have approached.
In the past two decades, work on imagery has steadily accelerated. As the data have piled up it has become plain not only that imagery is an attractive research area but that imagery itself constitutes a process which provides a powerful tool, both for the theoretician of human nature and for the practitioner who seeks to bring about behavioural change.
Galton (1883) describes how he became interested in imagery and his method of investigating the phenomenon in “Inquiries into human faculty and its development” (1883). Galton had heard of people whose visual memory was so clear and vivid they could examine their mental pictures as thoroughly as if they had been real objects. This inspired Galton to conduct an extensive inquiry into visual representation in different people. He began by questioning scientist friends, since he thought they were more likely to respond accurately about their experience of visualizing. However, he was a little surprised when most of these scientists protested that mental images were unknown to them. But Galton persisted and asked the same questions of people in general society and this time found that men, and a larger proportion of women and also many children, habitually saw mental images and were able to describe them in fine detail. This prompted him to look again amongst scientists and this time he found “a scattering of images“. He then circulated his questions among friends and through them to other people of both sexes and various ages. He found a range among his subjects from a low‑through a medium‑ to a high‑order of visualizing ability.
Many researchers have attempted to consider the effect of the external stimuli in generating imagery in experiments with hypnotized subjects in which they could mime a perceptual experience under hypnosis.
Binet and Féré (1891), writing on hypnosis, revealed a variety of situations in which hypnotized subjects able to reinstate a perceptual‑ like experience. Their experimental data reported, in one task, the subject hallucinated a coded form, and then reported subsequent afterimages.
Binet and Féré (1891) directed subjects to ‘hallucinate’ a red square on a sheet of white paper; when a fresh sheet of white paper was placed before them, subjects reported “seeing” a green square. More recently, Hibler (1938), in a complex design in which colours were imaged under hypnosis and subjects immediately awakened and asked to report afterimages, obtained essentially negative results. Hibler asserted that positive findings were probably due to suggestion.
Barber (1959, 1964) reviewed the findings on this topic and concluded that the results were equivocal. In a series of experiments, Barber found that only two out of six hypnotized subjects were able to hallucinate the colour blue within a pencilled circle; these two subjects were also able to hallucinate other colours, and had negative afterimages to all. Post hypnotic amnesia was induced six subjects and the same procedure was followed in the waking state.
Again, only two subjects were able to image the primary colour, but both reported afterimages to a coloured display.
The study of imagery began its return to favour during the 1950’s and 1960’s. In some situations a perception is defined as an experience occurring in response to a physical stimulus, but an image or hallucination is defined as a qualitatively similar subjective experience which occurs when there is no physical stimulus. Attempts to compare imagery to perception have frequently focused on the most extreme situations. If an image is the same as a perception, then it should show aftereffects, Mach bands, illusions and so on.
Although imagery was studied seriously as long ago as 1860, as a topic in psychology it is still relatively young. It lost favour with the advent of the behaviourist school during the early part of this century.
The behaviourists were not prepared to devote any time to studying internal processes including imagery. Watson (1930) did not deny that there were internal processes such as thinking or imagery, but, like all the behaviourists of that time, he believed that “thinking” was a justifiable inference but that there was no purely objective method of investigating it.
5.1 The definition of the image
The problem of defining images is not essentially different from the problems commonly faced by psychologists in defining many concepts like cognitive dissonance or intelligence. Precisely, what is imagery ?
Unfortunately imagery as topic in psychology is still relatively young. There is very little consensus of opinion as to what it is. It is still not possible to give a categorical and all‑ encompassing definition of imagery. Each investigator has tended to give his own interpretation of the phenomenon and has discussed it within his own definition.
Zenon W. Pylyshyn (1973) has listed a few of these show some of which are reproduced below.
Images are; “… indirect re‑activation of former sensory or perceptual activity ” ( Bugelski 1970); “… the occurrence of perceptual processes in the absence of stimulation which normally gives rise to perception” (Hebb 1966); “… the ability of a subject to generate or synthesise a sensory‑like datum in the absence of physical stimulation … a faint representation of a sensation or perception without an adequate sensory input“, (Holt 1964). There is a common concept linking these different definitions but these definitions are not generally used in empirical research, rather ” both images and verbal processes are operationally defined and the concern is with their functional significance”, Paivio (1971). In these terms the most succinct operational definition of imagery would be “the rating assigned to a stimulus“. The reason for this operational definition of imagery will become clear in due course.
Richardson (1969) reviews some classical definitions and offers his own definition: ” Mental imagery refers to (a) all those quasi‑perceptual experiences of which (b) we are self‑consciously aware, and which (c) exist for us in the absence of those stimulus conditions that are known to produce their genuine sensory or perceptual counterparts, and (d) may be expected to have different consequences from their sensory or perceptual counterparts”. This definition specifies that the external stimuli are not present during the conscious experience, but the reaction is in some way similar to what it would have been if the external stimuli were present.
The next sections provides a selective review of empirical data attesting to the functional similarities between imagery and perception.
A core assumption of cognitive theories of visual imagery is “seeing with the mind’s eye”; “Seeing” in the absence of the appropriate immediate sensory input is referred to mean employing cognitive representations and processes which are used in visual perception but not in non‑imagined thinking (Bisiach, and Berti (1990 ); Farah (1989); Finke (1985), Kosslyn (1983, 1988), Paivio (1979), Pinker (1984). Implicit in these theories is the consequence that visual imagery is based on the activity of modality‑specific visual cortex.
There are numerous sources of evidence that imagery shares processing mechanisms with like‑modality perception. Segal and Fusella (1970) asked subjects to hold an image while trying to detect a faint auditory or visual stimulus. They reported that holding a visual image impairs visual perception more than auditory perception, but holding an auditory image has the reverse effect. Segal and Fusella suggested that imagery and like‑modality perception share common mechanisms that are not used in other modality processing.
Peterson (1975) explored the relation between imagery and perception within a memory paradigm . He suggested that if imagery is dependent upon or arises from reconstruction of perception, the pattern of recall of material imagined should resemble the pattern of material actually seen. Peterson (1975) performed an experiment to test this suggestion with three groups of normal subjects by requiring recall of letters arranged in 4 x 4 matrices. The first group of subjects saw the matrices, the second were told their contents. The third group, like the second, were told the letters, but were instructed to form an image of the matrix as it was described. The results indicates that both seen and imagined matrices were stored in similar spatial formats, resulting in similar recall patterns. The results support the view that visual perception and visual imagery share the same mechanisms. As Block (1983) has pointed out, the relation of imagery to perception and the formation of mental imagery are issues which are often confabulated but are in principle independent.
The finding that imagery shares representations with perception would not imply that imagery is pictorial; both imagery and perception might be descriptive; furthermore, perceptual representations and mental images could have the same format (pictorial or descriptive) and yet be distinct representations.
5.2 Images as analogue representations
Most of the evidence points to images being analogue representations. The most famous demonstration is the rotation experiments of Shepard and Metzler (1971). Subjects were asked to judge whether two block structures were the same or different when their relative orientation was varied. It was found that decision time was a simple linear function of the orientation difference between the two shapes, suggesting an analogical representation.
Another famous experiment is that of Paivio (1975). Subjects were asked which was the larger of two objects and it was found that this was answered more quickly if the objects were pictured rather than labelled and if the physical difference between the objects was large. This also suggests that representations of the objects are analogically stored.
There is compelling evidence then that images share properties with pictures and that scanning mental images share properties with time visual scanning. Pylyshyn (1973, 1981), however has criticized the view that images are analogue representations.
Pylyshyn (1981) explains the evidence pointing towards analogical representations of images by saying that images do not reflect properties of the object but knowledge of the world. Images can be influenced by a subject’s beliefs and desires. When subjects appear to rotate mental images they are utilizing their knowledge of the physical act of rotation. The reason it takes longer to “rotate” an image 270 degrees than 90 degrees is that the subject believes that it should.
Pylyshyn concludes that all information, verbal and pictorial, is represented in the form of modality free propositions. Mental images are simply symbolic descriptions of the objects. This debate between analogue or propostional representations remains unsolved. Anderson (1978) claims that not only is the representation important in any explanation but also for the processes which act upon it. Therefore, for this debate to be resolved Anderson claims it is necessary to wait for evidence from the study of brain physiology.
Johnson‑Laird (1983) rejects the debate. He states that what is important is not what mental images really are but what function they serve (i.e; the function being to facilitate retrieval of information stored in memory). An awareness of the two theories is useful however when looking at individual differences in image ability (Baddeley, 1990).
Paivio (1972) suggested that “imagery variables are among the most potent memory factors ever discovered”.
Paivio (1971) suggested that the two major modes of coding experience are verbal and imaginal. Verbal processes would involve a functional symbolic system and would be auditory‑motor in nature. Imaginal processes would be used for non‑verbal thought, involving processes similar to active perception. Paivio’s work implies that combined verbal and imaginal representations may constitute the representation of an object concept.
Visual imagery has received more attention than imagery within other modalities. Pinker & Kosslyn (1983) state that it has long been held that images are like “pictures in the head” and that while no recent theorist has claimed that images literally are pictures, many have treated images metaphorically as if they are pictures.
The term image is ambiguous, referring both to phenomenological experience and to an internal representation (code) that give rise to this perceptlike experience. Most of the definitions or attributes of imagery hinge on its similarity to like‑modality perception.
In the above sections I have attempted to illustrate the relation between visual perception and visual perception. Having a visual mental image produces the conscious experience of “seeing” but with the “mind’s eye” rather than with real ones. In summary, the purposes of visual imagery are: firstly; imagery relies on the use of recognition processes to make explicit information stored implicitly in memory. In order to make explicit a particular aspects of a remembered pattern, one may form an image and “internally recognize” that aspect of it. Secondly; visual imagery is a way of anticipating what would happen if a person, or an object or objects, were to move in a particular way. That is, imagery can be used to perform mental simulations, in which one looks to “see” what would happen in the analogous physical situation. Finally, visual imagery can be used in the service of learning and thinking. In learning, memory may be improved by having an imaged object simulate the actual object, allowing one to associate a perceptual representation of the object with the relevant context (see Paivio, 1971).
The observation that imagery shares mechanisms with perception implies a remarkable amount about the structure of the information‑processing system underlying high‑level vision.
Thus, in the following sections of the present chapter, I review selectively some of the recent empirical and theoretical work that has focused on the apparent parallels between the purposes of imagery and perception. A number of studies has demonstrated that imagery and like‑modality perception recruit some common processing mechanisms. These studies are reviewed in section of 5.4. Following this section, I describe a neuropsychological model of visual imagery. In the last section, I review very briefly some evidence from brain damaged subjects and discuss how the theory of visual imagery can be used to integrate a wide range of empirical and theoretical work pertaining to the recognition of disoriented objects.