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Surveying the seen: one hundred years of British vision.

     All that we know of nature, or of existence, may be compared to a
     tree, which hath its root, trunk, and branches. In this tree of
     knowledge, perception is the root, common understanding is the
     trunk, and the sciences are the branches.
     Thomas Reid (1764, p. 424)


Perception provides not only the roots to the tree of knowledge, but also to the sapling of psychology. It was a dominant factor in the development of psychology and it remains one of the domains in which progress can be charted. This review briefly describes 19th century influences on perceptual research during the 20th century, before highlighting important landmarks in perceptual research during the lifetime of the British Psychological Society. The final sections describe some of the most important contemporary work in visual perception, and speculate about how these areas may develop in the next few years.

Given the enormous volume, and considerable progress, of research in visual perception, we have had to be selective in the topics we have been able to elaborate here. The motion after-effect and aspects of face perception feature more prominently than they might if this review had been crafted by others, but we hope our selectivity allows us to achieve more coherence than might be possible with a more even-handed selection of topics.

One enduring feature of the study of visual perception, well illustrated in the areas we have chosen to stress, has been its interdisciplinarity. It is difficult to discuss specifically psychological research: psychologists are among many varieties of vision scientist. One rather disturbing recent trend has been the tendency for the vision science community to set itself apart from the psychological one. We demonstrate that there remain genuinely psychological problems within the field of vision science that require contact between vision and other areas of psychology. This is certainly a lesson that can be drawn from its history.

Nineteenth century precursors

The empiricist philosophers, from Locke onwards, routed the acquisition of knowledge through the senses, and subsequent empirical psychologists have sought to sign the way in greater detail. The first stage involved developing experimental procedures that would bring some precision in stimulus control akin to that adopted successfully in the physical sciences. Natural philosophers in Britain devised the principles on which the perception of colour, motion and depth could be rendered experimentally tractable. T. Young (1802) speculated that colour perception could be based upon the detection of three primaries and Maxwell (1855) provided experimental support for this trichromatic theory. Faraday (1831; see Fig. 1a) suggested how successive images presented in close temporal sequence could result in the perception of movement, setting in train the long line of research on stroboscopic motion. Talbot (1834; see Fig. 1b), before he turned to photography, established a lawful relationship between apparent brightness and intermittent light stimulation. Wheatstone (1838, 1852; see Fig. 1c) demonstrated that depth could be synthesized from two slightly disparate images presented to separate eyes, dissociating depth perception from its object base; he also developed the electromagnetic chronoscope that subsequently was used for reaction time measurements (see Edgell & Symes, 1906).

These insights extended the scope of experimental perception, although they were not actively pursued in Britain to the extent that they were adopted and adapted within the German scientific community (see Wade & Heller, 1997). However, those who sought to control the stimulus had less regard for the response. Psychophysics developed almost independently of these instruments for stimulus manipulation, and the methods devised by Weber and Fechner were based on very simple tasks like lifting weights (see Ross & Murray, 1978). William James (1890, pp. 226-7) noted that Wheatstone's first paper:
     contains the germ of almost all the methods applied since to the
     study of optical perception. It seems a pity that England [sic],
     leading off so brilliantly the modern epoch of this study, should
     so quickly have dropped out of the field. Almost all subsequent
     progress has been made in Germany, Holland, and, longo intervallo,
     America.


[FIGURE 1 OMITTED]

Helmholtz (1867, 2000) was particularly attracted to the experimental approach and his students developed methods further (see Cahan, 1993). The dominance of German research in perception is clearly, though indirectly, reflected in what is perhaps the most thorough review of vision at the turn of the century: in Rivers's (1900) survey more than 75% of references were to German sources.

The emergency of British Psychology

Boring (1942) remarked that Helmholtz carried the torch of philosophical empiricism in a hostile Kantian climate, as did his erstwhile assistant Wundt. However, their brands of empiricism were quite different. Helmholtz borrowed the notion of unconscious inference from Berkeley to account for characteristics of colour and space perception, and the concept is still active in some theories. Wundt was more ambitious and applied empiricist and associationist ideas to account for consciousness itself. His ideas were carried to America by the likes of Titchener (see Hilgard, 1987), but his structuralist theory was not widely followed in Britain. The demise of structuralism in the second decade of the 20th century and its gradual replacement by behaviourism on the one hand and Gestalt psychology on the other also seemed to pass relatively unnoticed within British psychology. Behaviourism, with its reliance on associationism, was strongly opposed by James Ward (1886, 1918; see Fig. 2a) who maintained an introspectionist approach. In the first article published in the British Journal of Psychology, Ward wrote: 'physiological and comparative psychology must fall back on the facts and analysis of our own experience' (1904, p. 25). His widespread influence was one factor in retarding the spread of experimental psychology, and it reflected the neglect of psychology generally within British academic institutions. Relatively little research in experimental perception was conducted within Britain until well into the 20th century. Some of those who were active experimentalists were often discouraged from conducting experimental research. For example in 1904, when William McDougall (Fig. 2c) took a readership at Oxford University, the terms of his appointment denied him access to a laboratory. However, he did contribute many articles on perception (with experimental studies of afterimages, binocular rivalry, Fechner's paradox, fluttering hearts, infant colour vision, and visual persistence) to the initial volumes of the British Journal of Psychology, before his interests became more hormic. Indeed, the initial volumes of the journal were weighted heavily in favour of perception, particularly studies of illusions, but this did not continue. This probably reflected the influence of W. H. R. Rivers (Fig. 2b), who wrote an extensive review of vision at the turn of the century (Rivers, 1900). He is most noted for instigating cross-cultural investigations of perception, and he also conducted neurological research with Henry Head (see Deregowski, 1998; Whittle, 2000). Rivers delivered lectures on sensory physiology at both Cambridge and University College, London (UCL).

McDougall supervised the research at UCL of Adolf Wohlgemuth (1911) on the motion after-effect (MAE) and acted as a participant in some of the experiments. Following observation of a moving surface, like descending water, stationary objects appear to move in the opposite direction. This MAE was described graphically by Addams (1834) after observing the Falls of Foyers in Scotland, and it was called the waterfall illusion by Silvanus Thompson (1880). As was the case for stereoscopic depth perception, most of the research on this topic in the 19th century was conducted in Germany (see Wade, 1994). Wohlgemuth's doctoral dissertation was published as the first Monograph Supplement of the British Journal of Psychology: he summarized the extensive research on MAEs, added novel experimental variations of his own, and advanced a physiological model to account for its occurrence.

The conservatism and suspicion of British universities to newly established disciplines was even evident in the first editorial of the Journal, and regret at the shift from 'mental philosophy' to 'psychology' was but thinly veiled; the editors (Ward and Rivers, 1904, p. 1) wrote:

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]
     In this country half a dozen lectureships have been lately founded
     in different universities solely to promote the study of Psychology
     as a science. We have also a society of professed psychologists
     which meets frequently for the discussion of printed papers and the
     exhibition of experimental apparatus and results.


Psychology was not accorded formal departmental status in Edinburgh and Cambridge until as late as 1931, and the incumbent professors (Drever and Bartlett; see Fig. 3) fostered experimental approaches. Both F. C. Bartlett and Drever directed psychological laboratories before these dates, and both were active in the British Psychological Society; Bartlett edited the British Journal of Psychology from 1924 until 1949, and he was assisted by Drever for the whole of that period. Nonetheless, in the words of Hearnshaw (1964, p. 208): 'The total lecturing staff in departments of psychology at the outbreak of Second World War numbered only about thirty.'

Some perceptual research was able to ride this wave of indifference because it was nurtured in a more favorable climate. While the 'New Psychology' of Wundt had been virtually ignored within Britain, a 'New Physiology' was actively pursued by Ferrier, Hughlings Jackson, Sherrington and others. Wohlgemuth's (1911) interpretations of the MAE bore closer allegiance to Sherrington's physiology than to Wundt's psychology. The continuing research on colour vision was driven by the physical control of the stimulus, and by increasing understanding of receptor function and colour anomalies (see Collins, 1925; Houston, 1932; Parsons, 1915). Indeed, it was the concept of 'schema', developed within this new physiology by Head (1920), that was applied by F. C. Bartlett to skilled tasks of memory and perception. According to Bartlett (1932, p. 201), '"Schema" refers to an active organisation of past reactions, or of past experiences, which must always be supposed to be operating in the well-adapted organic response'. The constructuve aspects of both memory and perception were emphasized at the expense of their holistic or sequential features. Gradually, perception did find a place within British psychology, and it was attached to a new type of theory linking perception to prediction and action.

The emergence of British perception

The two most prominent British perceptual psychologists throughout the 1930s were F. C. Bartlett and Maggie Vernon (Fig. 3c), although Thouless (1931a, 1931b) presented his analysis of perceptual constancy at the beginning of the decade and Craik's (1939) experiments on visual adaptation appeared at the end. The empiricist agenda initially was followed slightly differently by Bartlett and Vernon; Bartlett (1932) examined tasks in which perception goes beyond the information given in the stimulus in a search to extract meaning from it, whereas Vernon (1937a, 1937b) investigated the traditional empiricist issue of space perception. She also published several books on visual perception (see Vernon, 1937c, 1952, 1970), the second of which adopted a more Bartlettian position on perception:
    Any perceptual situation encountered could then be fitted into its
    appropriate schema or chain of schemata, by virtue of the 'effort
    after meaning', until it was understood and reacted to in the most
    appropriate and satisfying manner' [1952, pp. 258-9].


Emphasis on the constructive and individual aspects of perception contradicted approaches that stressed perceptual constancy, and equations for quantifying this had been proposed by Brunswik (1928) and Thouless (1931a). Both proposed ratios involved differences between perceived and projected values on the one hand and physical and projected on the other, although Thouless used logarithmic transformations in order to avoid anomalies that arose with the direct ratios. Thouless (1931a, 1931b) referred to perceptual constancy as 'phenomenal regression to the real object', and provided plentiful evidence to support its operation for shape, size, orientation, brightness and colour perception.

Applied perception

With the onset of war, in 1939, the approach to perception adopted by Bartlett was applied to human operators of complex systems, like flight simulation in the Cambridge cockpit (see Bartlett, 1946; Saito, 2000). The experimental research on perception in the 1940s harmonized with developments in cybernetics (Wiener, 1948), and Craik (1943; see Fig. 4a) conflated the two by considering the human operator as a complex, self-organizing system. His studies of visual adaptation had indicated that there was constant feedback from previous and concurrent stimulation, and that it could be modelled by physical processes. In the epilogue to his PhD thesis on visual adaptation he wrote:
    some of the flexibility of the perceptual process--for instance, the
    recognition of relational rather than absolute properties and of
    changes rather than constant stimulation, and a primitive type of
    abstraction--follows from the known properties of the physiological
    structure and can be imitated by physical mechanisms [1940;
    reprinted in Craik, 1966, p. 6].


The machine metaphor was to prove particularly attractive to experimental psychologists, although Craik was only able to enlist relatively simple machines. He worked with analogue devices as the digital computer was still embryonic. Nonetheless, Craik did formalize the input, processing and output components of servo-systems in a manner that could be applied to digital computers:
     The essential feature of the sensory device is its ability to
     translate the change it is to measure ... into some form of energy
     which can be amplified and used to drive the restoring cause ...
     The next part is what may be called the computing device and
     controller, which determines the amount and kind of energy to be
     released from the effector unit to restore equilibrium ... The
     final part is the power unit or effector (equivalent to the muscles
     in men and animals) which restores the state of equilibrium [1966,
     pp. 23-4].


After Craik's untimely death in 1945, computing machines increased in speed and complexity so that the tasks that they could simulate became more complex. Concepts from engineering, such as information and self-organization, were also integrated with a growing knowledge of neurophysiology resulting in the computer becoming a metaphor for the brain.

The computers mounted for this metaphorical odyssey were digital and serial, but at around the same time the ground was being laid for principles of parallel processing. McCulloch and Pitt's (1943) model of the neuron provided the foundation for later connectionist models of pattern recognition, and the networks connecting perception to its underlying physiology were further woven by Hebb (Fig. 6a) in his speculative synthesis of perception and learning. Hebb proposed that perceptual learning takes place when assemblies of cells fire together; their reverberating activity resulted in synaptic changes which further increased the probability of the nerves firing together. The functions of cell-assemblies and phase sequences were based on his neurophysiological postulate:
     When an axon of cell A is near enough to excite cell B and
     repeatedly or persistently takes part in firing it, some growth
     process or metabolic change takes place in one or both cells such
     that A's efficiency, as one of the cells firing B, is increased
     [Hebb, 1949, p. 62].


Hebb's postulate is taken as providing the foundation for current connectionist models of recognition and learning despite the fact that the principle had been enunciated over 70 years earlier by Bain (1873; see Wilkes & Wade, 1997). Hebb later applied the concepts to account for a wide range of phenomena, from stabilized retinal images to sensory deprivation.

Bartlett's emphasis on the constructive nature of perception found an echo in America in the New Look experiments, like those reported by Bruner and Postman (1947), where motivation was considered to interact with perception. Similar experimental investigations had been undertaken earlier by Brunswick (1934, 1935) who examined the perceived sizes of postage stamps of different value. Ames's many demonstrations of the ambiguities of stimulation and their perceptual resolutions (see Ittelson, 1952) were also accorded renewed attention in this cognitive climate. In these heady post-war years personality flirted with perception, but their liaison was not lasting. Certain subthreshold recognition phenomena were brought to their perceptual defence (see McGinnies, 1949), but the sober verdict was not in their favour:
     It would seem wise to regard with great caution the existence of
     limitations on speed and accuracy of perception imposed by
     personality factors, at least in normal observers [Vernon, 1970, p.
     237].


The affair did focus attention on the lability of the limen, which was scrutinized in the theory of signal detection (Tanner & Swets, 1954).

Fechner's (1860) psychophysics had survived for almost a century but it did not provide a secure platform for predicting target detection in life-threatening situations. Radar operators could report signals below their classically determined thresholds, but they also made more mistakes. Accordingly, the task of detecting signals within background noise was analysed as a decision process. The principal factors affecting the decisions reached were the payoffs for hits and false alarms and the signal probability (see Green & Swets, 1966). The other assault was on Fechner's logarithmic law linking sensory and stimulus intensities. It had been supported by indirect category scaling of sensory magnitude, but novel direct methods, such as magnitude estimation and cross-modality matching, cast doubt on its validity. An alternative formulation in terms of a power function was proposed by Stevens (1951), although a variety of models can be applied to data from magnitude estimation (see Poulton, 1968). Dissatisfaction with classical psychophysics was also voiced by those who tried to apply it to skilled tasks like flying (Gibson, 1950). Gibson also poured scorn on the artificiality of the laboratory for studying vision in comparison to the natural world, where objects and observers do not retain the same spatiotemporal relations to one another. Gibson's 'psychophysical theory of perception' had little in common with Fechner, and he discarded the distinction between sensation and perception.

Information and pattern processing

Information theory was developed in the context of telecommunications, and the mathematical measurement of information was formalized by Shannon and Weaver (1949); its powerful impact on perception was felt in the 1950s. Miller (1957) linked the concept of limited information capacity to absolute perceptual judgments. Attneave (1954) devised procedures to determine the locations of highest information in simple patterns. They corresponded to boundaries of brightness (contours) and particularly to abrupt changes in contour direction (corners). Support for the significance of contours in perception derived from two other sources: single unit recordings from various levels in the visual pathway, and scanning eye movements. Indeed, early attempts to stabilize the retinal image by compensating for any involuntary eye movements resulted in disappearance of the target (Ditchburn & Ginsborg, 1952; Riggs & Ratliff, 1952).

However, it was the qualitative concept of information processing rather than quantitative information measures that was to have lasting appeal. The perceiver was conceived of as a limited capacity information processor and the information could be filtered, filed or reformulated on the basis of stored events. Broadbent's (1958) model was among the first to formalize and represent pictorially the putative processing stages. He stated that the 'advantage of information theory terms is ... that they emphasize the relationship between the stimulus now present and the others that might have been present but are not' (1958, pp. 306-7). Thus, Broadbent (Fig. 4b) combined Bartlett's approach of examining skilled tasks with Craik's modelling metaphor.

Theoretical attention was shifted towards pattern recognition by both humans and computers because they were both thought of as information processors or manipulators of symbolic information. The patterns were typically outline figures or alphanumeric symbols, and rival theories, based on template matching and feature analysis (see Uhr, 1966), vied for simulated supremacy at recognition and one result was pandemonium (Selfridge, 1959)! Sutherland (Fig. 6b) sounded a cautionary note on this endeavour that was not generally heeded then nor has been subsequently:
     Patterns are of importance to animals and man only in so far as
     they signify objects. It is the recognition of objects that is
     vital for survival and as a guide to action, and the patterned
     stimulation of our receptors is of use only because it is possible
     to construct from it the nature of the object from which it
     emanated [1973, p. 157].


[FIGURE 4 OMITTED]

Pattern recognition was the theme that unified physiologists, psychologists and computer scientists, many of whom were assembled for a seminal symposium, held in 1959, on sensory communication (Rosenblith, 1961).

While there were dangers in the oversimplification of the stimulus, the approach also allowed important tools to be developed to probe discrete visual achievements. One example was the random dot stereogram (RDS) developed by Julesz (1960). Wheatstone (1838) had employed outline figures for his stereoscope in order to reduce any monocular cues to depth, but he was acutely aware that some remained. Julesz employed the dawning power of the computer to produce pairs of matrices of black and white dots, the central areas of which were displaced with respect to the common backgrounds, and hence disparate. The displays looked amorphous when viewed by each eye alone, but when viewed binocularly patterns gradually arose or descended from the background. This not only spawned a new area called 'cyclopean perception' (Julesz, 1971), but the technique was adopted in the clinic as a test for stereoscopic depth perception. Analogous developments in the temporal domain produced random dot kinematograms which were used by Braddick (1974) and others to make important discoveries about different types of motion processing.

Developments in visual neuroscience

Research on patterned stimulation at the receptor level had proceeded throughout the first half of the century, but its pace quickened thereafter. The glimmerings of pattern processing beyond the receptors emerged in the 1950s, and were amplified in the 1960s. When recordings of nerve impulses could be made from individual cells in the visual pathway their adequate stimuli could be determined. Adrian (1928) coined the term 'receptive field' and Hartline (1938) applied it to describe the region of the receptor surface over which the action of light modified the activity of a neuron. It came as something of a surprise that retinal ganglion cells of frog responded to quite complex features of stimulation (like moving dark regions of a specific visual angle, resembling a bug), and stimulus properties that excited or inhibited neurons were generally called 'trigger features' (Barlow, 1953; see Fig. 4c). Retinal ganglion cells of cat, on the other hand, were excited by rather simpler stimulus arrangements. Kuffler (1953) found that they were concentrically and antagonistically organized; if the centre was excited by light the surround was inhibited, and vice versa. Such an arrangement served the detection of differences in luminance well, but steady states would have little effect, since excitation nullified inhibition. This pattern of neural activity was retained in the lateral geniculate, but it underwent a radical change at the level of the visual cortex. Hubel and Wiesel (1962, 1968) found that single cells in primary visual cortex (V1), first of cat then of monkey, responded to specifically oriented edges; they had different receptive field properties which were called simple, complex and hypercomplex.

Physiologists refined the stimulus characteristics of trigger features throughout the 1960s, while psychologists sought their phenomenal counterparts. Almost any experiment involving contours paid lip service to Hubel and Wiesel, despite the tenuousness of the links between particular phenomena and their underlying physiology. At least an appeal to trigger features was considered preferable to reliance on the speculative neurophysiology advanced by Kohler (1940). Spatial illusions, for example, attempted to rise above their enigmatic status by adopting this reductionist path; despite its attractions the greatest success was found for contour repulsion (Blakemore, Carpenter, & Georgeson, 1970). The alternative lure of illusions was to relate them to the traditional empiricist concept of constancy (Gregory, 1963). The links between perception and physiology were made explicit for the motion after-effect (Barlow & Hill, 1963; Sutherland, 1961) resulting in an explosion of empirical studies examining their consequences. Barlow (1963) also investigated the link between visibility and retinal image motion using afterimages and optically stabilized retinal images.

The concept of channels or spatial filters emerged during the decade, and it was applied with particular rigour by Fergus Campbell (Fig. 6b) and his colleagues to the detection of and adaptation to sine-wave gratings (see Campbell & Robson, 1968). The attraction of gratings was that they provided at one and the same time a definition of the stimulus and theory of the response to it. Craik (1966; pp. 44-5) characteristically foresaw the principle behind these developments:
      the action of various physical devices which 'recognize' or
      respond identically to certain simple objects can be treated in
      terms of such [mathematical] transformations. Thus the essential
      part of physical 'recognizing' instruments is usually a filter--
      whether it be a mechanical sieve, an optical filter, or a tuned
      electrical circuit--which 'passes' only quantities of the kind it
      is required to identify and rejects all others.


Establishing the trichromacy of vision also proved amenable to this approach, both psychophysically (Rushton, 1964; Stiles, 1959) and physiologically (MacNichol, 1964).

Visual development

The cortical mapping of visual receptive fields had an unexpected influence on the age old nativist/empiricist debate, providing fuel for both sides. Hubel and Wiesel (1963) demonstrated that receptive fields were present prior to visual experience but that they could be modified by it. This applied to both binocularity and orientation selectivity. For example, the responsiveness of cortical cells to stimulation by either eye, present at birth, could be modified by monocular deprivation from birth. The timing of such modification was critical; Blakemore and van Sluyters (1974) demonstrated that the cortical monocularity could be reversed if the eye occluded was reversed in the fourth week of the kitten's life. Similar sensitive periods have been shown to operate for exposure to orientation (Blakemore & Mitchell, 1973). The relevance of this research to developmental abnormalities in humans (like strabismus and astigmatism) has been stressed by many (see Blakemore, 1978, for a review of the early research).

Contour extraction was also considered to be one of the first tasks tackled by the visual systems of newborns, and many novel methods were devised to study them. For example, Fantz (1961) inferred infant perceptual discrimination from the patterns of differential fixation, and Bower (1966) used operant conditioning techniques for investigating the emergence of perceptual constancies. Fantz even suggested that there was an innate preference for viewing human faces: although the outline figures he used as stimuli had very little ecological validity, his suggestions have been confirmed in recent work (e.g. see Johnson, Dziurawiec, Ellis, & Morton, 1991). Salapatek and Kessen (1966) recorded eye movements of infants only a few days old and found that they were concentrated on contours or corners. Habituation to repeated presentations of a stimulus provided another source of inference regarding discrimination (Bridger, 1961). These methods were refined and the course of perceptual development began to be charted (see Cohen & Salapatek, 1975). From the late 1960s the principal stimulus for vision research became the sine wave grating, and these were presented to infants, too (e.g. Atkinson, Braddick, & Braddick, 1974; Atkinson, Braddick, & Moar, 1977). (Other, more recent British contributions to the study of visual development are mentioned by Goswami in this issue.)

Perception, cognition and action

The information-processing agenda was at the heart of the 'cognitive revolution' which engulfed experimental psychology in the 1960s (see Gardner, 1987; Neisser, 1967), although this had more marked repercussions for perceptual research in America than in Britain. The distinctions between perception and thought became even more blurred, problems of pattern recognition were considered paramount, representational processes proliferated, and support for particular theories derived increasingly from computer simulations. The trend was epitomized by Gregory's (1963; see Fig. 5a) analysis of visual illusions, and it was expressed eloquently in the many editions of his Eye and brain (1966; pp. 222-3):
    Perception becomes a matter of suggesting and testing hypotheses ...
    The continual searching for the best interpretation is good evidence
    for the general importance of augmenting the limitations of the
    senses by importing other knowledge.


This hypothesis testing theory was applied to a range of visual phenomena like ambiguous figures, size illusions and subjective contours. Gregory was critical of many attempts to link perception with its underlying physiology, as this reflected a passive rather than an active theory. All perceptions were considered to be cognitive fictions:
      Though generally predictive, and so essentially correct,
      cognitive fictions may be wrong--to drive us into error. On this
      Active view, both veridical (correct-predictive) and illusory
      (false-predictive) perceptions are equally fictions. To perceive
      is to read the present in terms of the past to predict and control
      the future [1974, p. xix].


He likened perception to the operation of programs in computers with the same basic units performing many different tasks. This, in turn, was used as an attack on attempts to localize brain functions by means of lesions, and in so doing he drew a distinction between serial and parallel systems:
    In a serial system the various identifiable elements of the output
    are not separately represented by discrete parts of the system ...
    If the brain consisted of a series of independent parallel elements
    with separate output terminals for each, like a piano, it might be
    possible to identify behavioural elements with particular parts of
    the system {1961, p. 321}.


It is precisely this distinction that has fuelled the modular approach outlined below.

Sutherland (1968, 1973; see Fig. 5b) developed the cognitive agenda in his advocacy of an alliance between perception and language. With regard to object recognition he remarked that:
     there must exist a set of rules that makes it possible to map from
     one domain onto another ... For example, in the case of solid
     objects composed of plane surfaces, each surface must be joined
     at its edges to other surfaces and at a corner at least three
     surfaces must meet. A similar type of process underlies the
     recognition of speech {1973, p. 159}.


[FIGURE 5 OMITTED]

Broadbent's model encompassed ever more processes, like decision and stress (Broadbent, 1971) and spawned offspring, like logogens (Morton, 1969). Driver (this issue) provides further discussion of Broadbent's work, and Altmann (this issue) elaborates Morton's model.

However, almost all cognitive topics were systematically eschewed by Howard and Templeton (1966) in their integrative survey of human spatial orientation. They placed the spatial senses in an external framework and examined their interactions; particular importance was given to the many links between the visual and vestibular systems. Howard (Fig. 5c) continued his analytic approach to perception with detailed commentaries on visual orientation (Howard, 1982) and binocular vision (Howard & Rogers, 1995).

Gibson (1966) sought to stem the cognitive current and developed a novel theory that owed more to Thomas Reid (1764) than to his own contemporaries:
    When the senses are considered as perceptual systems, all theories
    of perception become at one stroke unnecessary. It is no longer a
    question of how the mind operates on the deliverances of sense, or
    how past experience can organize the data, or even how the brain can
    process the inputs of the nerves, but simply how information is
    picked up [Gibson, 1966, p. 319].


Despite Gibson's pejorative purview of conventional perceptual experiments, the strongest support for his position derived therefrom: simplified dynamic dot patterns could be recognized far more easily than static ones (Johannson, 1964). Gibson's ideas established a new field of 'ecological' optics which has been tilled by many. For example, Lee (1976, Lee & Reddish, 1981) has examined perception in complex natural conditions, like driving and diving. (See Haggard, this issue, for more on this topic.)

Thus, the 1960s saw the beginnings of a split between a 'cognitive' approach, where the goal of vision could be seen as an abstract categorization of the objects of vision, and an 'action' approach, where vision was part of an integrated system allowing manipulation of and navigation through the world. This distinction has matured in contemporary approaches to vision, both through the influence of David Marr and through further developments in neuroscience and neuropsychology. The remainder of this article describes the past alongside present-day achievements and, where appropriate, dares to speculate about the future.

Computers and computational theory

Craik, as well as Turing (see Millican & Clark, 1996), anticipated that the computer would be a powerful tool to simulate theories of perception, as well as providing a metaphor for the processes of perception and cognition themselves. Since the late 1960s the study of visual perception has been influenced profoundly by computers. As well as allowing scientists to collect or to analyse data more quickly, the digital computer provided a tool for the laboratory scientist to develop new ways of testing the visual system with novel kinds of visual displays. The move away from reliance on oscilloscopes to present sine wave and other simple patterns facilitated the increasing use of more naturalistic patterns, as well as those which can be constructed and manipulated in controlled ways. Computer developments also enabled the better recording of eye movements and the linkage of eye movements to changes in display features, allowing a number of groups to conduct ingenious experiments into the control of eye movements in reading (e.g. Rayner, 1978). Computer developments also created new problems. Unknown non-linearities in screen display properties or their temporal characteristics present problems that were not initially foreseen (Kennedy, Brysbaert, & Murray, 1998), in addition to a variety of problems that have been termed 'visual stress' (Wilkins, 1995).

[FIGURE 6 OMITTED]

During the 1970s David Marr (Fig. 6c) worked with a number of outstanding collaborators, including Ellen Hildreth, Tommy Poggio and Shimon Ullman, on research which has significantly shaped our understanding of--and approach to--vision. Marr set out to develop a complete framework for vision, spanning the very lowest level processes within the retina (Marr, 1976; Marr & Hildreth, 1980) up to the process of visual object recognition (Marr & Nishihara, 1978). Sadly, Marr died very young, and his overall theory was published posthumously (Marr, 1982).

The key feature of Marr's theory was that vision can be understood at different levels. The first 'computational' level is a theory of the task that the visual system is to solve, and an understanding of the constraints that can enable solution of that task. The second level, of 'representation and algorithm', is a means of achieving the task, and the final 'hardware implementation' level describes how the brain, or a computer, actually implements these algorithms in neural tissue or silicon. Marr (1982, p. 5) argued that:
    For the subject of vision, there is no single equation or view that
    explains everything. Each problem has to be addressed from several
    points of view--as a problem in representing information, as a
    computation capable of deriving that representation, and as a
    problem in the architecture of a computer capable of carrying out
    both things quickly and reliably.


The distinctions between Marr's three levels are not always quite as clear as Marr proposed, but there are important examples which make the distinctions extremely clear. For instance, Marr and Poggio's original (1976) approach to stereopsis tackled the question of how the visual system can solve the correspondence problem in perceiving forms in depth from RDSs. Somehow the brain is able to pair up the dots in each eye's view of an RDS so that the three-dimensional solution is revealed, but it is not at all clear how this process can work given the enormous number of different potential solutions. Marr and Poggio's computational theory specified three simple constraints on the matching process which cut down the number of potential matches, and which are sensible assumptions to make in the world of natural objects. In the natural world, the same surface patch looks similar to each eye (so in an RDS, black dots must match black dots); a surface patch can only be in one place at a time (so in an RDS, each dot can match only one other), and discontinuities in depth at the edges of objects are rare (so in an RDS disparity shifts should be rare--disparity should vary smoothly almost everywhere except at the rare boundaries between objects). Starting from this computational level Marr and Poggio (1976, 1979) produced two different algorithms for solving RDSs based upon rather different representations of images. Marr and Poggio's first algorithm (1976) was cast in terms of a neural network model in which the steady state for an RDS emerged from the balance of excitation and inhibition between simple processing units. Their second algorithm (1979) accommodated observations on the way in which stereopsis is guided by information in distinct spatial frequency bands, making links with both psychophysics and physiological evidence for the separate analysis in early vision of information from different spatial scales.

In addition to presenting a unified approach to different topics within vision, Marr and his colleagues also presented a complete theory of the different stages of representation (primal sketch, 2.5D sketch, 3D models) involved in the interpretation of retinal images. In so doing, Marr distinguished a stage which made explicit the three-dimensional layout of the world with respect to the viewer (the 2.5D sketch), potentially useful for action in the world from the more abstract 3D models which allowed object recognition. Despite this distinction, however, his framework fails to anticipate the more recent idea of a separation of visual pathways for action and for object categorization, which we discuss later.

Marr's framework for visual object recognition was developed and extended by Biederman and colleagues (Biederman, 1987; Hummel & Biederman, 1992), and in the UK Humphreys and Riddoch made important experimental and neuropsychological tests of many of the claims about representation and sequence (e.g. Humphreys & Riddoch, 1984, 1987). Much recent theory about object recognition, however, departs from Marr's economical object-centred representations, to develop representational ideas based around the representation of multiple viewpoints (e.g. Bulthoff & Edelman, 1992; Tarr, 1995; see Biederman, Subramaniam, Bar, Kalocsai, & Fiser, 1999, for a recent discussion). Thus it is not the details of Marr's theory which have so far stood the test of time, but the approach itself. In this spirit, a great deal of important British research built upon this approach to develop new analyses of early visual processing (e.g. Watt & Morgan, 1985), stereopsis (e.g. Mayhew & Frisby, 1981) and particularly motion processing, where important contributions have been made by a large number of UK scientists.

Modular visual processing

Another key feature of Marr's theory (1982, p. 102) was the principle of modularity:
    Computer scientists call the separate pieces of a process its
    modules, and the idea that a large computation can be split up and
    implemented as a collection of parts that are as nearly independent
    of one another as the overall task allows, is so important that I
    was moved to elevate it to a principle: the principle of modular
    design.


If a complex task like 'vision' comprises many discrete modules, more-or-less mutually independent, then failure in one aspect of vision (e.g. the stereo 'module') is not fatal for other aspects of the task. This means that modular designs make sense, both for the construction and debugging of computer programs and also for brains in the course of evolution. Indeed, there is good evidence from single cases of brain-injured patients increasingly studied by cognitive neuropsychologists since the 1970s that visual perception may indeed be modular in the manner proposed by Marr.

Evidence for modularity of visual processing systems has been around since the 19th century, but the neurological theory that there was a single area of primary visual cortex for the reception of retinal inputs, leading to a subsequent stage of association with stored knowledge in the association cortex, held sway until the 1970s. Zeki (1993) provides a fascinating description of how and why evidence for specialized subsystems of cortical visual processing was ignored for so long.

The 1970s and 1980s saw a revival in the use by perceptual theorists of patterns of perceptual and cognitive impairment observed in individual patients who had suffered brain injury. The single case method of cognitive neuropsychology was pursued particularly in Europe. Marr himself was influenced greatly by findings described by Elizabeth Warrington on dissociations between the perception and understanding of visual shapes (Warrington & Taylor, 1978; see Marr, 1982). Interest was revived in much earlier reported cases of apparent dissociations between the perception of form and colour (e.g. Verrey, 1888, cited in Zeki, 1993) and between form and motion (Riddoch, 1917). Zeki (1993) provided a detailed history of how early cases of acquired colour blindness (cerebral achromatopsia) were described and how, eventually, the idea became accepted that there was an area of cortex where colour was analysed (now conventionally labelled V4) beyond the primary visual cortex (V1).

One reason why it took so long for the idea of a 'colour centre' to be accepted was that it can sometimes be argued that selective deficits arise because some tasks are just too difficult for a mildly injured brain to perform. Cerebral achromatopsia, for example, often accompanying selective inability to recognize faces (prosopagnosia), might just show that injury to the visual cortex makes complex processing of colours and fine detail of faces impossible. However, where a double dissociation is seen, where two different patients show opposite patterns of deficient and spared ability, it is difficult to argue that one or other task is intrinsically more difficult (see Shallice, 1988).

During the 1970s, cognitive neuropsychologists began to understand the power of the double dissociation in highlighting potentially distinct modules. A particularly dramatic example is provided in the area of motion perception. In 1917, George Riddoch (grandfather of Jane Riddoch, herself a neuropsychologist of distinction; e.g. see Humphreys & Riddoch, 1984, 1987) noticed that brain-injured soldiers from the Great War could sometimes see movements in otherwise blind visual fields. His observations led him to suggest that 'the elementary visual perceptions, of light, of movement, and of an object, are dissociated' (p. 15). This idea was well ahead of its time, yet even in G. Riddoch's day such an argument could have been supported by examples of the opposite condition, in which a person could see reasonably normally when things were still, but was blind to movement (see Zihl, von Cramon, & Mai, 1983). In passing, Riddoch also noted among his cases one patient who was unable to appreciate depth in or solidity of objects which he could otherwise see quite well in his intact fields, thus demonstrating a further selective deficit the physiological origin of which remains poorly understood.

The theoretical climate had changed significantly by the time that the akinetopsic patient LM was studied by Zihl and colleagues in Germany (Zihl et al., 1983). Following brain injury as a result of thrombosis, LM reported that she could no longer see movement, making it difficult for her to cope with a number of everyday tasks including pouring drinks and crossing the road safely. Testing showed that she was blind to fast movements but retained some ability to see slower movements of 10 deg/s or less. She did not report MAEs after prolonged viewing of a moving spiral, and reported such effects only rarely, and briefly, after viewing a slowly moving pattern of stripes. However, she did report seeing the spiral and the stripes move in the adaptation phase, suggesting that different processes are involved in adaptation and test. A similar conclusion has been applied to the MAE in normal observers (Wade, Spillmann, & Swanston, 1996): adaptation is to local motion whereas test involves global structure. Consistent with residual ability to see only certain kinds of motion, McLeod, Dittrick, Driver, Perrett, and Zihl (1996) reported that LM is able to identify the moving human figures in Johannson stimuli, though unable to describe how the figures are moving, and unable to see them at all if they are presented in textured noise.

Patient LM illustrates the convergence of evidence obtained from humans with that found using non-human animals in visual neuroscience. By the time that LM was studied, there was a substantial body of neurophysiological work on the monkey showing the functional specialization of an area labelled V5 (also known as MT) of prestriate cortex for movement. Marcar, Zihl, and Cowey (1997) compared LM's performance on tasks for which movement was either essential or inessential with the performance of monkeys with a lesion in the same area of the brain (V5/MT) damaged in LM. The performances of lesioned monkey and brain-injured human were identical.

The use of converging evidence from a wide range of different methodological sources is now a widespread feature of contemporary cognitive neuropsychology, and gets over many of the problems of deducing normal functions from observation of single cases of brain damage (e.g. see Humphreys, Price, & Riddoch, 1999 for a good example of the application of converging evidence to object recognition). This convergence of evidence from a range of human and animal work has increased rapidly during the 1980s and 1990s with the use of imaging techniques to probe the workings of the human brain in the new interdisciplinary area of cognitive neuroscience.

Neuroimaging

The activity of the human brain has been studied using external measurements of electrical and/or magnetic activity for some 70 years. In 1929, Hans Berger discovered that electrical activity could be measured by placing electrical conductors on the human scalp and amplifying and transcribing the resulting signals. This electroencephalogram (EEG) was an early and important tool for diagnosing brain damage, but also provided a research tool for examining electrical responses to events (event-related potentials, or ERPs). Productive research using ERPs to map cognitive activity in the brain has been conducted for over 30 years (see Kutas & Dale, 1997; Rugg & Coles, 1995), but there have always been problems of interpretation owing to limited information about the spatial origins of ERP components. However, recent developments of dense-mapped ERP and the related technique of using a magnetometer to record the magnetoencephalogram (MEG) and detect magnetic event-related fields (ERFs) have attracted much more attention, in part because the precise temporal information gained by these techniques can now be seen to complement (and perhaps be combined with) the spatial precision which can be achieved with newer techniques of brain imaging.

Another technique based on magnetic fields generated in the cortex is transcranial magnetic stimulation (TMS). Following the lead of Silvanus Thompson (1910), alternating magnetic fields can be applied to restricted regions of the head in order to stimulate or to disrupt neural activity in some way. In TMS a magnetic coil is positioned over a particular area of a participant's head and a current is passed briefly through the coil. The magnetic field so produced induces an electrical current in a specific part of the participant's brain (see Walsh & Cowey, 1998). The timing of such TMS is very precise and so it can be applied at known intervals after some visual stimulation has taken place. It is as if the technique produced virtual patients because the disruption is temporary.

At the forefront of recent work on the neuroimaging of visual function in normal human brains has been Semir Zeki and colleagues at UCL and Hammersmith (e.g. Frackowiak, Friston, Frith, Dolan, & Mazziotta, 1997; Zeki, Watson, Lueck et al., 1991). They initially used positron emission tomography (PET) scans to examine the regional cerebral blood flow when volunteers looked at different kinds of visual patterns. A coloured pattern produced activation in regions corresponding to monkey V1, V2 and V4. When the same pattern was shown in shades of grey, the activation in V4 was much reduced, suggesting that 'human V4' was an area for the analysis of colour. Similarly, a moving compared with a static pattern produced specific activation of 'human V5', and illusory motion seen in static patterns has also been attributed to this area (Zeki, 1993).

The increasingly sophisticated ways of examining human brain activity during perceptual processing hold exciting prospects for the future, though all techniques have drawbacks, and experiments must be designed with great care if they are to be clearly interpretable. In comparison to PET scans, magnetic resonance imaging (MRI) yields more precise spatial resolution, but it is only more recent developments in functional MRI (fMRI) that allow activity to be temporally as well as spatially mapped, and it will be work using fMRI combined with developments in other technologies with more temporal precision such as TMS and MEG which will hold the key to understanding in detail the neural processing of visual information by people. One example comes from the phenomenon of storage of the MAE. If observers exposed to a moving pattern close their eyes for a period after adaptation, then the after-effect is still seen when the original pattern is examined again (Wohlgemuth, 1911). Culham et al. (1999) have been able to show using fMRI that activity in and around area V5 is highest when the MAE is experienced most strongly, immediately after adaptation, or after the storage period, and is lower during the storage interval, providing a stronger linkage between the perception of illusory as well as real motion and area V5. Indeed, the MAE has been used increasingly as a tool for localizing motion processing at various stages in the visual system, either on the basis of inferences related to stimulus features or as a correlate with neuroimaging (see He, Cohen, & Hu, 1998; Mather, Verstraten, & Anstis, 1998).

Two visual pathways

As the functional specialization of visual areas of the cortex became better understood, so there have also been suggestions that different regions of the visual brain are organized into two rather different kinds of processing pathway or stream (see Lennie, 1998; Milner & Goodale, 1995, for reviews). An early idea was that of 'two visual systems' (Ungerleider & Mishkin, 1982) which developed the distinction drawn in the 1960s between the 'what' and 'where' systems of the cortex and superior colliculi, respectively (e.g. Humphrey & Weiskrantz, 1967; Schneider, 1968, 1969, Trevarthen, 1968). Ungerleider and Mishkin's proposal was that there were two distinct cortical streams of processing: the inferotemporal pathway or 'ventral' route allowing the detailed perception and recognition of an object (its size, shape, orientation and colour), with the posterior parietal or 'dorsal' route allowing the perception of an object's location.

Goodale and Milner (1992; Milner & Goodale, 1995) produced an important develoment of this theory by suggesting that these two parallel streams of visual processing are actually separately specialized for action (dorsal stream) and for visual experience of the world (ventral stream). The dorsal route is said to be the evolutionarily older visual system which enables a creature to navigate through the world and catch prey. The ventral route is developed particularly in primates to allow the detailed perception and interpretation of objects and, it seems, a conscious awareness of these. Among the evidence for their theory was the performance of a patient DF who, as a result of brain injury, was almost completely unable to recognize or describe the shapes of objects, but was able to orient shapes appropriately in order to do things with them (Goodale, Milner, Jakobson, & Carey, 1991). For example, DF could not match the orientations of a card with a slot placed in different orientations in front of her, but her performance was almost normal when the task was changed to reaching out and posting the card into the slot.

The distinction between conscious form perception and perception for action assists the interpretation of the puzzling phenomena of 'blindsight'. Human patients who had damage to the visual cortex which left them apparently blind could nevertheless respond much better than chance when asked to make certain kinds of visual judgment--particularly about the locations of lights which moved or had abrupt onsets (Sanders, Warrington, Marshall, & Weiskrantz, 1974; Weiskrantz, 1986). It seemed that not only was there residual visual capacity in areas of the visual system outside visual cortex, but that this activity apparently did not reach consciousness. The blindsight patients were not aware of the lights they pointed to, but felt as though they were guessing, or using some 'feeling' about the target. The mechanisms by which neural processing give rise to conscious awareness are the subject of much current debate (see Young & Block, 1996) and explanations of visual consciousness in particular seem likely to occupy neuroscientists for some years to come (cf. Crick & Koch, 1990).

The theoretical separation of pathways for action and conscious reflection has been just one of several developments in recent years placing renewed emphasis on understanding how vision may be designed for action (see also Haggard, this volume). Some perceptual research echoes Gibson in taking issue with the idea that perception furnishes elaborate representations of the world which can underlie conscious reflection and memory. 'Change blindness' demonstrations have shown that in a wide variety of situations observers find it extremely difficult to detect major differences in a scene from one moment to the next (see e.g. Simons, 1999, for an edited collection of recent work on this topic). Such observations have influenced some researchers to take the view that rather little is represented explicitly from a visual scene--perhaps just enough to guide the eye to the next required location in order to proceed with the task at hand (see e.g. Ballard, Hayhoe, Pook, & Rao, 1997). This 'active vision' approach may be of increasing interest in the years to come.

Natural images: the human face

Much of the computational research in vision prior to Marr made use of simplified images, such as the blocks world of Guzman (1968) and Clowes (1971). Marr's work on early visual processing particularly stressed the perception of, and constraints on processing, natural images. Psychophysics, however, has tended to continue studying the perception of simple dots, lines and gratings. Yet during the 1970s one particular class of visual pattern became of increasing interest to both perceptual and cognitive psychologists: the human face.

The first review of face recognition research was published in this journal by Hadyn Ellis (1975). During the mid-1970s there was considerable public concern about legal cases of mistaken identity, and the fallibility of the human eyewitness to a crime (see Devlin, 1976). Much of the early work on face recognition addressed this applied problem, and recently there has been a renewed emphasis on questions arising in the context of face identification from CCTV images (Bruce et al., 1999; Burton et al. 1999) and photo-bearing identity cards (Kemp, Towell, & Pike, 1997). However, in the intervening years theoretical understanding of the cognitive processes involved in face identification has also developed (e.g. Bruce & Young, 1986; Hay & Young, 1982; Valentine, 1991; Young, Hay & Ellis, 1985). During the same period, research into the perceptual processing of face patterns was also growing, particularly emphasizing how face processing seemed to be based upon some special sensitivity to the configuration or holistic pattern of the upright face, with particular importance to relatively coarse-scale information (Harmon & Julesz, 1973; Diamond & Carey, 1986; Yin, 1969). Thompson's ingenious creation of the Margaret Thatcher illusion (P. Thompson, 1980) provided a new tool with which to probe the sensitivity of face configural processing to orientation (Bartlett & Searcy, 1993). There have been attempts to integrate computational models of perceptual processing of faces with the later stages of retrieving information about personal identity (see e.g. Burton, Bruce, & Hancock, 1999).

The topic of face perception was revolutionized by developments in computer graphics. For example, the technique of 'morphing' allowed researchers in the 1990s to produce photographic quality caricatures of faces (Benson & Perrett, 1991), and to age faces or give them characteristics of the opposite sex (Burt & Perrett, 1995). Such developments have affected not only our understanding of how faces are perceived (see Rhodes, 1996) but also have allowed novel explorations of the basis of facial attractiveness (e.g. Perrett, May, & Yoshikawa, 1994; Perrett et al., 1998). It has become easier to build and manipulate realistic high quality computer-based face models, and to animate these to produce speech or expressive movements. These further developments mean that psychologists will be able to develop ever more ingenious ways to probe the processes of face perception in the future, and to consider the respective roles of static form and time-varying information in face perception (cf. Knight & Johnston, 1997; Lander, Christie, & Bruce, 1999). There will also be increasing applications for such research, since the ability to manipulate faces in psychologically convincing ways is relevant to a range of medical, cosmetic and entertainment industries.

One enduring theme in the study of face processing has been the question of whether faces are processed by special mechanisms. British scientists have contributed important studies revealing the special status of the face pattern in the visual world of the neonate (Bushnell, Sai, & Mullin, 1989; Johnson & Morton, 1991). Moreover, there is strong evidence for specialization of areas within monkey and human cortex for processing faces (e.g. see Allison, Puce, & Spencer, 1999; Gross, 1992; Heywood & Cowey, 1992; McCarthy, Puce, Gore & Allison, 1997; Perrett, Hietenan, Oram, & Benson, 1992; Rolls, 1992), and neuropsychological investigations have shown that brain injury can selectively impair, or spare, face processing abilities (e.g. see Young, 1992, for a review). However, an important contribution of British researchers in particular has been to emphasize that the face is used for many different social purposes, and to furnish evidence that these different uses made of facial information may be handled by distinct neurological pathways (e.g. see Young, Newcombe, de Haan, Small, & Hay, 1993).

Computer graphics alongside cognitive neuroscience developments have led to fascinating studies of how different brain areas are involved in the perception of different emotional expressions (Adolphs, Tranel, & Damasio, 1998; Calder et al., 1996; Phillips et al., 1997, 1998; Sprengelmeyer et al., 1997). The study of face gaze has become a topic for cognitive neuroscientists too (e.g. Baron-Cohen, Campbell, Karmiloff-Smith, Grant, & Walker, 1995; Hood, Willen, & Driver, 1998; Langton & Bruce, 1999; see Langton, Watt, & Bruce, 2000, for a review) having previously been studied extensively by developmental and social psychologists but curiously neglected by perceptual and cognitive ones. This increasing emphasis on the cognitive neuroscience of 'social' perception is likely to continue in the future, and is one area where it is important that the study of vision remains firmly connected with other areas of psychology.

Multimodality and virtuality

Our understanding of visual neuroscience reveals an increasingly complex, though elegant, picture, and in the future our diagrams of visual areas, their interconnections and their microstructure are likely to become increasingly complicated. For example, the route map of primate visual systems charted by van Essen, Anderson, and Felleman (1992) would place great demands on a navigator. Evidence for the analytic separation of different aspects of the visual scene--motions, forms, colours, etc--additionally raises the question of how these elements become associated or 'bound' (see e.g. Bartels & Zeki, 1998). Driver (this issue) describes the more cognitive approach of Anne Treisman (1988, 1998) to this topic. In addition to the problem of binding within the visual domain, however, there is the problem of how, and when, different modalities (vision, touch, hearing) become integrated or otherwise influence each other. It is likely that there will be still more emphasis on multimodality in future research.

One classic demonstration of multimodal integration was discovered by McGurk and MacDonald (1976). If a videotape is dubbed so that a person's voice says 'ba' but their face movements correspond to 'ga', most people will hear something which is neither of these two speech sounds but a blend of the two, such as 'da'. This shows that speech perception is not based solely on acoustic features, but involves visual phonetic markers such as whether lips meet. Neuroimaging work has demonstrated that visual speech activates auditory areas in the brain as well as ones to do with visual shape and movement analysis (Calvert et al., 1997). There are other intriguing demonstrations of cross-modal influence. In the ventriloquism effect (e.g. Bertelson & Aschersleben, 1998; Witkin, Wapner, & Leventhal, 1952), an auditory voice is mislocated towards the apparent visual location of the speaker. Ventriloquism and McGurk effects can be dissociated--for example, they are differentially affected by the orientation of the speaker's face (Bertelson, Vroomen, Wiegeraad, & de Gelde, 1994), and there have been some very interesting investigations of these links between cross-modal perceptual effects and selective attention by Driver and his colleagues (e.g. Driver, 1996; Driver & Spence, 1994). Cross-modal attention is itself an exciting current topic (e.g. Driver & Spence, 1998a, 1998b). Even that apparently most visual effect, the MAE, is not immune to influence from other modalities, since it has been demonstrated that a moving sound source can moderate the MAE (Garrod, Houghton, & Hammett, 1998), as can attention (see Mather et al., 1998). The relationship between initially separable modules of visual processing and cross-modal (and cross-module) integrative processes is likely to remain an important research topic for some time.

Coordination across modalities is also a fundamental aspect of virtual reality systems in which artificial scenes, viewed in a head-mounted display, change as a consequence of head and body movements made by the observer (see Peli, 1999). Associated auditory and tactile stimulation can enhance the impression of exploring a novel environment. Thus, the developing power of computers has been the engine for work on creating the geometry of virtual environments: not only can more naturalistic images be presented but they can also be transformed in real time and viewed stereoscopically. The techniques can be applied to simulating virtual worlds or it can have more practical applications involving remote sensing.

Developments in this area have been driven by advances in technology rather than an understanding of sensory-motor control. Indeed, many of the theoretical issues raised above find expression in virtuality research. These include Gregory's emphasis on active exploration, which is evident in the transformations of scenes as a consequence of the perceiver's movements, the use of dynamic and naturalistic images in preference to static and simplified ones, and the optic flow follows Gibson's guidance in encompassing most of the visual field, so that apparent changes in body posture can be induced visually. However, the conflation of such issues inevitably generates some conflict. For example, Gibson's flows are essentially monocular, whereas the virtual systems make a virtue of their binocularity. In this context, the use of stereoscopic devices has often overlooked the conflicts that they have generated between different sources of depth information. Wheatstone (1838, 1852) could have provided adequate evidence of the ill-advised reliance on disparity alone as indicating relative depth.

In the next few decades an increasing number of industrial and medical tasks will be performed virtually. Physical presence during performance

of some intricate task will no longer be necessary as the relevant information can be relayed and viewed on some appropriate device with corresponding remote control of the instruments involved in the task. This had occurred already in minimal access (endoscopic) surgery where the surgeon does not see the instruments manipulated, but a displayed image of them. That is, the visual guidance is determined indirectly. Some of the problems associated with such indirect control of behaviour have been described (see Wade, 1996), and more attention will be paid to reducing the differences between the displayed and physical systems under examination. The rewards associated with the successful resolution of these perceptual problems will be enormous.

Concluding remarks

The experimental study of perception has progressed apace over the century celebrated by the British Psychological Society. The progress has not, however, been evenly paced. Following a very slow start, a distinctive brand of British perception was developed by Bartlett and amplified by his students Craik, Broadbent and Gregory. Their cognitive approaches to perception were given impetus by the application of computers to experiments and computer metaphors to theories. Marr officiated at the computational marriage between these, and the union continues to bear fruit. Experiments using dynamic naturalistic stimuli can now be conducted, virtual scenes can be constructed, and images of brain activity while viewing these can be captured in a way that would have been difficult to envisage a century ago. However, the simulated lure of the screen (or even a pair of screens) should not blind experimenters and theorists to the differences that exist between the virtual and the real.

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Nicholas J. Wade*

Department of Psychology, University of Dundee, UK

Vicki Bruce

Department of Psychology, University of Stirling, UK

* Requests for reprints should be addressed to Professor Nick Wade, Department of Psychology, University of Dundee, Dundee DD1 4HN, UK (e-mail: N.J.WADE@dundee.ac.uk).
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Author:Wade, Nicholas J.; Bruce, Vicki
Publication:British Journal of Psychology
Geographic Code:4EUUK
Date:Feb 1, 2001
Words:15363
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