THE NECKER CUBE: A NEW PERSPECTIVE.

Transcription

1 THE NECKER CUBE: A NEW PERSPECTIVE. K. CHILCOTT DEPARTMENT OF PSYCHOLOGY UNIVERSITY OF TASMANIA.

2 THE NECKER CUBE: A NEW PERSPECTIVE. A thesis submitted as a partial requirement for the Degree of Master of Psychology at the University of Tasmania at Hobart. Karen Chilcott, BA (Hons) January 1995

3 Table of Contents Acknowledgements Statement iv Literature Review Interpretation in Visual Perception Abstract 1 Section 1 Introduction and Aims 2 Outline 2 Section 2. The Neurophysiological Model: The Link Between Single Cell Activity and Perceptual Effects 3 Motion Aftereffect 3 Spatial Frequency Shift 4 Colour Contingent Aftereffects 4 Section 3. The Cognitive Model: The Link Between Perceptual Effects and Mental Processes 5 Perceptual Alternations 5 - Cyclic Decision Model 7 - Trial-and-Error 7 Reversal Rate 7 Relative Dominance 8 Colour Contingent Aftereffects 8 Section 4. A Multi Level Model 10 The Motion Aftereffect 11 The McCollough Effect 11 Relative Dominance 11

4 Section 5. A Two Stage Account 12 Top-down 12 Bottom-up 12 Section 6 The Interpretation Dilemma And The Adaptation Paradigm 13 Perceptual Alternations 13 - Monocular and Binocular Rivalry 13 Reversal Rate of Ambiguous Stimuli 13 Selective-Adaptation and Relative Dominance 14 -The Neurophysiological Approach 14 Transfer-of-Decrement and the Muller-Lyer Illusion 15 -Learning (Cognitive) Approach 15 Section 7 The Dichotomous Approach: Some Conclusions 16 Section 8 Directions For Further Research 17 Hypotheses and Experimental Strategy 18 Section 9 References 20 Journal Article The Necker Cube: A New Perspective Abstract 25 References 46 Appendices 50

5 111 ACKNOWLEDGEMENTS Firstly I must thank my supervisors Dr. Alison Bowling, Dr. Barry Mapperson and Dr. Chris Williams for their guidance and patience. Special thanks to Alison for her willingness to read numerous drafts of this thesis when only small steps were being made. Lastly thankyou to my husband Stuart for his encouragement in what has been a busy year for both of us.

6 iv STATEMENT I Karen Chilcott certify that this thesis contains no material which has been accepted for the award of any other degree or diploma in any tertiary institution and that, to the best of my knowledge and belief, the thesis contains no material previously published or written by another person, except when due reference is made in the text of the thesis. Signed Karen Chilcott Date Ct

7 LITERATURE REVIEW INTERPRETATION IN VISUAL PERCEPTION

8 1 Abstract Attempts to explain any perceptual phenomena usually do so exclusively in terms of neurophysiological mechanisms on the one hand, or in terms of inferred cognitive processes on the other. Alternative theoretical approaches offer a combination of these positions. The realisation that numerous phenomena can be accounted for by both major theories has lead to confusion of interpretation. One example is the reduction of dominance of a percept of a reversible figure after prolonged exposure to an unambiguous stimulus. Typically, the percept which resembles the unambiguous stimulus is seen for a smaller proportion of the subsequent viewing period. The neurophysiological account of this phenomenon holds that the groups of cells responding to the unambiguous stimulus become fatigued during adaptation so that during subsequent viewing of the ambiguous stimulus the most active group of cells will dominate. The alternative, cognitive explanation proposes that one of the effects of prolonged viewing or adaptation may shift the rate of accumulation of evidence for a particular perspective interpretation. Within this cognitive context one may speculate that prolonged inspection of an unambiguous stimulus simply creates a preference for whichever interpretation of the ambiguous stimulus is the more novel. Proponents of these dichotomous approaches believe that every perceptual phenomena need only be explained by one theory. However, as some phenomena can be explained by both approaches, an integrated approach in which visual processing at a physiological level is influenced by cognitive processes may be warranted. A modified experimental procedure to enable the independent manipulation of the physiological and cognitive components of this visual effect, was suggested.

9 2 1. Introduction and Aims Since the 1950's a large body of research has been developed to enhance the understanding of the physiology thought to underpin visual perception. A further approach has developed an equally valid theoretical account of perception in terms of inferred mental processes or cognitive factors. In addition to these dichotomous viewpoints, several alternatives have been put forward, generally proposing a combination of the two major approaches, including a multilevel and a two stage account. Although not directly stated, a theoretical rift has developed between the proponents of the two major approaches. The literature also reveals that the conflict has been enhanced by a commonly adopted experimental procedure. This procedure, is variously termed selective-adaptation or transfer of decrement and is generally referred to as the adaptation paradigm. This paradigm reveals that contrary to the expectations of those who use it, it does not allow verification or falsification of either of the two major theoretical positions. The objectives of this paper therefore are to examine the evidence in support of the major theoretical approaches in vision research and their alternatives. Specifically, the aim is to investigate the various interpretations of one phenomenon, a special case of perceptual alternations. The prolonged exposure of an unambiguous stimulus affects the relative dominance of the percepts in subsequently presented ambiguous figures. This adaptation effect will be detailed for several reasons. Firstly because it has been explained by both major theoretical positions and, secondly, it is generated by the contentious paradigm. Lastly, it is intended to consider the adaptation paradigm and detail how it may be modified to provide a resolution to the theoretical impasse in the interpretation of relative dominance changes. Outline The vast body of research in support of the dominant neurophysiological approach to visual perception will be briefly outlined in section 2. In section 3, the cognitive approach will be introduced, as this is generally considered to be the major theoretical position opposing the neurophysiological approach. Explanations of several phenomena including a special case of perceptual alternations will demonstrate this position. Research findings based on two alternative theoretical

10 3 approaches will be discussed in sections 4 and 5. In an attempt to determine which of the theoretical approaches to accept some researchers have looked to the experimental paradigms for answers. The adaptation paradigm is discussed in section 6. Section 7 will consider the approaches to visual perception and the adaptation paradigm. This section aims to clarify the issue of interpretation for the change in relative dominance that occurs when an ambiguous stimulus is presented after the prolonged inspection of an unambiguous stimulus. Finally, Section 8 will conclude by proposing an experimental strategy to resolve the issue of interpretation for relative dominance changes. 2. The Neurophysiological Model: The Link Between Single Cell Activity and Perceptual Effects Throughout the recorded history of visual perception there have been repeated attempts to provide comprehensive theories of perceptual phenomena in terms of neural interaction (e.g., Kohler, 1940; McDougall, 1902, 1903a, 1903b, 1906). The current era of discussion of sensory phenomena in neurophysiological terms began with the pioneering work of Hubel and Wiesel in the 1950's. Recordings of single cell activity in the striate cortex of anaesthetised cats confirmed the existence of cells specific for position, size and movement (Hubel & Wiesel, 1968). Subsequently single cell activity was linked to perceptual effects in human vision. Motion Aftereffect Barlow and Hill (1963) were possibly the first to take Hubel and Wiesel's findings further. They confirmed the existence of cells selective for the direction of motion after observing that the firing rate of cells diminished with the repeated stimulation of a stimulus moving in one direction. Later studies reported that the prolonged viewing of a stimulus moving in one direction induced a strong sensation of opposite motion when a stationary test stimulus was subsequently viewed (Masland, 1969; Mayhew & Anstis, 1972). This motion aftereffect (MAE) is explained by the fatiguing of directionally specific cells in humans.

11 4 Spatial Frequency Shift A further example of the stimulus specificity of visual cells was demonstrated by the 'spatial frequency shift'. Prolonged inspection of a striped pattern by an observer who then views a pattern of the same orientation with narrower stripes, perceives the stripes as narrower than they actually are, and broader stripes seem even broader. Blakemore and Sutton (1969) claimed that this effect implied evidence of a system of sizedetecting channels in human vision. The motion aftereffect and the spatial frequency shift are examples of perceptual phenomena that can be accounted for by the activity of neurones 'tuned' to one stimulus characteristic e.g., motion and size. Neurones sensitive to multiple stimulus characteristics have also been discovered. Colour Contingent Aftereffects McCollough (1965) reported a colour aftereffect (CAE) contingent on orientation. This effect was originally produced by exposing subjects to a grating of vertical black stripes on an orange background, alternated every few seconds with a horizontal grating on a blue background. When subjects were asked to view a test display of black and white vertical and horizontal gratings side by side, most reported seeing a desaturated blue green on the background of the vertical lines and orange on the horizontal portion. The generally accepted explanation of this effect, referred to as the McCollough Effect (ME) suggests that the colouration of the achromatic horizontal or vertical grid is the result of the adaptation or fatiguing of sensory cells sensitive to both colour and orientation. For two of these effects, the MAE and the spatial frequency shift, a neuronal basis has been confirmed by recording the activity of single cells. An unfortunate consequence of this technological advance is that the majority of researchers now assume that all visual effects are explicable in neurophysiological terms even when evidence is provided to the contrary. For example, Uhlarik et al. (1977) showed that the McCollough hues faded when observers intentionally caused the subjective organisation of the test stimulus to change, inferring that cognitive factors affect the hues in the ME. Broerse and Crassini (1981) had difficulty

12 accepting this interpretation noting a 'bias in direction of causation' in Uhlarik et al.'s (1977) work. They commented that if the perceived organisation alters the appearance of the colours then it is equally likely that the colours may alter perceptual organisation, and pointed out that this possibility was not investigated. This may be correct but Broerse and Crassini (1981) appear to have overlooked a footnote added by Uhlarik et al. (1977) suggesting that explanations in terms of cognitive factors do not exclude nor are incompatible with neurophysiological descriptions. Broerse and Crassini (1981) nonetheless reject all cognitive interpretations of colour contingent aftereffects (CCAEs) and consider these explanations to be 'inadequate' and concluded that neurophysiological accounts are more 'parsimonious'. Whilst a neurophysiological explanation may be appropriate to account for CCAEs it is questionable whether only one explanation is tenable for every perceptual phenomenon. The most prominent alternative explanation of visual effects assumes the involvement of mental processes The Cognitive Model: The Link Between Perceptual Effects and Mental Processes Perceptual Alternations The cognitive approach can account for the characteristics of ambiguous stimuli and the perceptual effects that are reported when this type of stimulus is used. An ambiguous stimulus is one that, although physically stable, can be perceived in more than one way. When there is a perceived change from one version of the stimulus to the other a perceptual alternation occurs. A well known ambiguous picture is the rabbit/duck which can be seen as either a rabbit, on the left, or a duck on the right (Figure la). The most common types of ambiguous figures in vision research include figure ground ambiguity (Maltese Cross, Figure lb), and perspective ambiguity (Necker cube, Figure lc). According to a cognitive model perceptual alternations depend on two logically separable processes. They may be attributed to a 'cyclic decision process' (Vickers, 1972), or a process of continuous trial-and-error on the part of the perceptual system (Rock, 1975).

13 a Figure 1. Examples of (a) ambiguous pictures, co) figure-ground reversals, and (c) perspective reversals.

14 7 Cyclic Decision Model Vickers (1972) concluded that simple judgements such as would be involved in indicating the longer of two lines, seem to be mediated by a decision process. He argued that the process underlying any perceptual discrimination involves the accumulation of evidence from a series of inspections until some criterion is reached. This process recommences each time a decision is reached. When viewing any potentially ambiguous stimulus one percept will be periodically replaced by another when the decision process produces a different result. Trial-and-Error Rock (1975) conceptualised the alternating perceptions of the reversible figure as reflecting an active though non conscious trial-anderror problem solving process within the visual system. He proposed that as the two possible solutions of the reversible figure are essentially equivalent, the governing decision process oscillates between the two, effectively replacing one alternative with the other equally acceptable alternative. These two explanations reflect the activity of spontaneous, generally active processes and conceptualises perceptual reversals as alternating solutions. Studies in support of the cognitive viewpoint have also shown that subjects not only are able to control the reversibility of ambiguous figures but also that the knowledge of reversibility, if withheld, affects the observers ability to perceive the alternative view of the stimulus (Girgus, Rock & Egatz, 1977; Reisberg & O'Shaughnessy, 1984). Reversal Rate One commonly reported finding is that perceptual reversals tend to occur more rapidly over an extended viewing period (Brown, 1955; Cohen, 1959; Long, Toppino & Kostenbauder, 1983; Price, 1967a, 1967b). From a cognitive viewpoint, the increase in reversals is attributed to the learning of alternate percepts, resulting in a shortening of the decision cycle. Further support for this approach is provided by the asymptotic curve depicting the number of reversals as a function of viewing time which is assumed to reflect a standard learning curve.

15 8 Relative Dominance A widely reported phenomenon of reversible figures is the reduction of dominance of a percept after prolonged exposure to an unambiguous stimulus. Typically the percept which resembles the unambiguous stimulus is seen for a smaller proportion of the subsequent viewing period (Carlson, 1953; Hochberg, 1950; Virsu, 1975; Von Grunau, Wiggin & Reed, 1984). This affect is generally attributed to the fatigue of neurophysiological mechanisms. Harris (1980), however, proposed that one of the effects of prolonged viewing or adaptation may be to shift the rate of accumulation of evidence for a particular perspective interpretation. Within this cognitive context one might speculate that prolonged inspection of an unambiguous stimulus simply creates a preference for whichever interpretation of the ambiguous stimulus is the more novel. Colour Contingent Aftereffects Ambiguous stimuli have also been incorporated into studies of colour contingent aftereffects including the McCollough effect (ME) (Uhlarik, Pringle 8z. Brigell, 1977). Jenkins and Ross (1977) found that the ME was contingent on subjective changes in the organisation of a test pattern. As the organisation of the test pattern was consciously changed from one state to the other, the colour aftereffect switched in and out. Unlike the standard test pattern to measure the ME, Jenkins and Ross (1977) used a test figure with no contours separating regions of vertical and horizontal gratings. This figure could be seen as a set of concentric square 'channels' or as four large triangles converging to a point in the centre of the pattern (see Figure 2). Figure 2. The pattern developed by Jenkins and Ross (1977) and used to test for the McCollough effect.

16 9 The ME was generated with vertical green and horizontal red gratings or the reverse. When the test pattern appeared as separate triangular regions the ME was observed with white stripes tinted with the hue complementary to that of the induction grating of the same orientation. When the pattern appeared as concentric square 'channels' and regionless, the McCollough hues faded. Thus when the switch occurred even though the same physical edges were present the ME diminished, a finding not consistent with a neurophysiological account of the effect. The alternative explanation of the standard ME proposes an underlying cognitive process in which the pairing of a colour with a specific grid orientation comes to evoke a learned response in the visual system to the colour. Specifically, the red horizontal produces adaptation for red in conjunction with horizontal lines, so that after a number of pairings the line grid alone evokes the adaptive response (Murch, 1972). According to this approach mental processes are implicated, in which learning is assumed to have taken place. Explanations of CCAEs as a learning process or conditioned response has met with much resistance from proponents of the neurophysiological approach. This shows no sign of relenting if recent studies of the ME, arguing in favour of this viewpoint are any indication (Savoy & Gabrieli, 1991; Bedford & Reinke, 1993). The extreme application of researchers favouring the cognitive position was recently demonstrated by Horlitz and O'Leary (1993) who repeated an experiment by Hochberg (1950). Hochberg introduced the duration of the percepts of an ambiguous stimulus, a Maltese Cross (see Figure 1), as an indicator of neural fatigue. He found that after adaptation to the unambiguous stimulus that the first percept that was experienced in the test phase was the non inspected alternative. He also found that this percept that was reported for a longer duration than the other. This was taken as evidence that adaptation had resulted in neural fatigue. Using a figure ground reversible pattern of their own design Horlitz and O'Leary (1993) found that after adaptation to the unambiguous stimulus that each percept in the test phase was viewed for approximately fifty percent of the viewing period. Based on the assumption that all reversible figure phenomena can only be accounted for by cognitive processes, non

17 confirmation of Hochbergs finding was automatically accepted as evidence favouring the cognitive viewpoint. 10 Horlitz and O'Leary (1993) also speculated that a preference for novelty or a process of habituation may account for many of the phenomena of reversible figures including the duration of percepts, the acceleration of reversals and relative dominance changes. According to this position relative dominance changes occur when an unambiguous stimulus is viewed because the more novel alternative percept is preferred in the ambiguous test stimulus. They additionally claimed that habituation in vision: "is not a process that is color-specific....or locationspecific..." (p.680). No reason is given for this caveat and they appear not to have taken the evidence into account. For example studies have shown change in relative dominance to be colour contingent (e.g., Hochberg, 1950; Mapperson & Lovegrove,1989). Further, since location constancy is known to be acquired in infancy (see Keating & McKenzie,1986), it is unclear why Horlitz and O'Leary (1993) would assume that visual effects would not habituated to location. Location specificity was demonstrated in one of a series of experiments by Von Grunau et al. (1984) relative dominance changes were measured when an adaptation stimulus was presented in 0.5 degree steps above and below a centrally fixated test stimulus. At 1.5 degrees above and below the fixation point no significant change in relative dominance was reported. Von Grunau et al. (1984) interpreted this from the neurophysiological perspective concluding that the experiment had demonstrated the activity of retinally localised 'channels' for perspective orientation. Although Horlitz and O'Leary (1993) cite Von Grunau et al.'s (1984) study they appear to have overlooked the findings of this experiment. 4. A Multi Level Model An additional theoretical position holds that a phenomenon may be attributed to multiple causes. As long ago as 1897 Chamberlain suggested that the scientific research of his era had become biased in favour of the theoretical perspective of the researcher. He warned that if this approach is adhered to researchers would ultimately shape their findings to confirm

18 11 the theory. Instead of a singular approach, with one hypothesis, Chamberlain proposed multiple hypothesis testing which puts forward and develops every tenable hypothesis. According to the literature it would appear that vision research has developed through single hypothesis testing with a bias in favour of singular accounts of visual phenomena. The equally likely possibility that multiple factors may contribute to a phenomenon has been overlooked in most cases in favour of more 'parsimonious' explanations. A reductionist approach, as this has inadvertently become, may be unnecessary. Three phenomena the MAE, ME and relative dominance changes provide evidence for a multiple factor approach. Evidence drawn from both major theoretical perspectives demonstrate this position. The Motion Aftereffect The MAE is generally believed to be attributed to the fatigue of directionally sensitive cells (Barlow & Hill, 1963; Favreau & Corballis, 1976; Sekuler & Ganz, 1963; Sutherland, 1961). However mental processes have also been shown to influence this effect. Meyer and Sherman (1981) found that by 'effort of will' subjects were able to alter the magnitude of the aftereffect. The McCollough Effect The widely accepted explanation of the ME postulates that it may be attributed to adaptation or fatigue of neural structures that are feature specific (McCollough, 1965). The experiments of Jenkins and Ross (1977) reviewed previously, however, argue that the effect can be produced and controlled by mental processes. Relative Dominance Several studies have shown that change in relative dominance, under certain conditions (e.g., in monocular rivalry when one eye views alternating patterns), may at least partially be attributed to neurophysiological mechanisms. Harris (1980) however provided an equally plausible account of the effect in terms of cognitive factors. Clearly, neither model can provide a complete explanation of these three effects and a finding in favour of one position does not necessarily preclude, or is incompatible with any other position. In the case of the

19 12 MAE, ME, and relative dominance any explanation must consider that, in addition to sensory input, cognitive factors have been shown to influence both the nature and extent of the processing of visual information. 5. A Two Stage Account Based on the discovery that information transfer in the human visual system is multidirectional one further approach to vision research has developed. This '2 stage' or information processing account of perceptual phenomena argues against models of perception being exclusively in favour of either of the two major theoretical viewpoints (Chaundhuri, 1990; Mapperson & Lovegrove, 1984). According to this approach information transfers from higher levels to influence the first steps in visual processing (top-down) or from lower sensory levels to higher cognitive levels (bottom-up). Top-down Recently the MAE has been discussed in terms of cognitive (attentive) processes to account for a 'bottom-up' model of information processing in which input from sensory mechanisms influences cognitive processes. Braddick (1990) and Chaundhuri (1990) found that the MAE could be considerably reduced in duration when an observer was attending to another task. This finding in addition to the neurophysiological explanation of the MAE was taken as support for the 'top down' model of information processing in which input from cognitive mechanisms influences low level sensory mechanisms. Oottom-up Research findings have also reported the compatibility of the neurophysiological and cognitive approaches to account for a bottom-up model of information processing in which input from sensory mechanisms influences cognitive processes. For example Long, Toppino and Kostenbauder (1983) explain the switching of the percepts of an ambiguous stimulus, as a cognitive phenomenon and the reduction in dominance as due to the fatigue of the neurophysiological mechanisms. A similar account of the perceptual alternations and relative dominance changes of monocular rivalry, was put forward by Mapperson and Lovegrove (1984).

20 13 6. The Interpretation Dilemma and the Adaptation Paradigm Sections 2 to 5 have outlined several theoretical positions currently adopted in vision research. With such varying viewpoints a rift has developed between researchers concerning the 'ownership' of some phenomena (e.g., reversal rate, relative dominance). In an attempt to determine which theoretical approach to accept, some researchers have looked to the experimental paradigms for answers (Long, 1988; Porac, 1989). The numerous theoretical accounts of perceptual alternations illustrates the dilemma. Perceptual Alternations An alternative to the cognitive account of perceptual alternations argues that any potentially ambiguous stimulus excites different sets of cells corresponding to different percepts, and that only the percept corresponding to the most active groups will be experienced (e.g., Kohler, 1940; McDougall, 1906; Orbach, Ehrlich, & Heath, 1963). This group of cells will become fatigued and their firing rate will decrease until some critical level is reached. At this critical level an unfatigued set of cells corresponding to an alternative percept will take over. This set will in turn become fatigued (while the original set recovers) until a further alternation occurs. This neural fatigue explanation reflects the activity of a passive process in which the stimuli share common physical features. Under certain conditions this interpretation appears to be justified. Monocular and Binocular Rivalry This explanation may be valid when a pattern of crossed gratings is perceived as alternating monocularly or binocularly. In the case of binocular rivalry it is known that the inspection of a single grating will reduce the dominance of a later grating only when the gratings are similar in both orientation and spatial frequency (Blake & Overton, 1979). Reversal Rate of Ambiguous Stimuli According to the neurophysiological model the increase in reversals over time, that occurs during the prolonged viewing of an ambiguous stimulus, is due to the build up of residual fatigue in channels/neurones that do not fully recover. The result of this fatigue is

21 14 an increasingly shorter time between reversals. However not all phenomena can be explained on this basis. A review of numerous studies by Long (1988) revealed that proponents of both major theoretical positions based many of their findings on identical experimental procedures. Closer inspection of the paradigm and its use may demonstrate how multiple interpretations have been generated. Selective-Adaptation and Relative Dominance During the adaptation period of the selective-adaptation paradigm an unambiguous inspection figure is viewed for a period of two to three minutes. This is followed by an ambiguous test figure which shares certain features with the adaptation figure. Most frequently, any percept that resembles the unambiguous stimulus in the test phase becomes less dominant, that is it is seen for a smaller proportion of the viewing period. Thus the effect of adaptation changes the relative dominance of the percepts. The Neurophysiological Approach The neurophysiological approach necessitates that the adapting and test stimuli have features in common such as perspective orientation or colour. Adaptation effects have nevertheless been reported with stimuli that were in many aspects not similar to the test stimulus. Von Grunau, Wiggin and Reed (1984), in an attempt to provide evidence for channels for perspective orientation, adapted subjects to black/white photographs of a wooden box, positioned and illuminated to emphasize two perspective orientations (pointing up or down). Observers reported that the test stimulus (a line drawing of an ambiguous Necker cube) appeared for a longer time in the orientation opposite to the one experienced during adaptation. As they noted "it is remarkable that adaptation effects occurred with stimuli that were in many basic aspects not similar to the test stimulus, that is in contrast, orientation and presence of contours"(p.321).

22 Transfer-of-Decrement and the Muller-Lyer Illusion The adaptation paradigm is refered to by proponents of the cognitive approach as transfer-of-decrement. Transfer-of-decrement therefore indicates that there is a decrease in the magnitude of an illusion that occurs within a prolonged inspection period. This is demonstrated most often using the Muller-Lyer figure. 15 Learning (Cognitive) Approach Observers' measured illusion magnitudes decrease if they scan the Muller-Lyer stimulus for a short time. Five minutes of inspection of the standard Muller-Lyer figure invariably produces reductions in the original illusion magnitude estimation. Illusion decrement has also been found to respond to traditional learning variables. For example massed as compared to spaced intervals of viewing produces illusion decrement (Dewar, 1968). Further, subjects asked to estimate the magnitude of the Muller-Lyer figure days or weeks after adaptation retain the previously learned underestimation (Girgus, Corm, Durant & Porac, 1975). A further observation is that illusion reduction depends on the judged similarity between the adapting and test figures. Coren andgirgus (1974) showed that transfer of decrement in the Muller-Lyer figure varied as a function of perceptual similarity. Observers viewed the classic form of the Muller-Lyer illusion for 5 minutes to induce illusion decrement. They tested whether the decrement tranferred to other variants of the illusion, all of which had been rated for their similarity to the standard form. They reported that there was transfer to the other configurations, with the figures that had been rated most similar to the inspection figure showing the largest effects and those that were most dissimilar showing the least transfer. These findings indicate the involvement of learning in illusion reduction. Adequate explanation concerning precisely how perceptual judgements of similarity are made and how learning takes place are not provided in the literature. As the explanations of these phenomena show, the adaptation paradigm can be used to provide evidence in favour of either of the two major theoretical positions: a neurophysiological account of relative dominance and a cognitive account of the Muller-Lyer illusion. Adopting

23 16 this experimental approach has also resulted in some researchers, even when faced with 'remarkable' findings that do conform with the commonly reported data, to conclude in favour of an assumed theory (Von Grunau et al., 1984). The continued application of this paradigm unfortunately maintains the dichotomy of approaches to vision research according to which a finding excluding one approach automatically provides evidence in favour of the other. 7. The Dichotomous Approach: Some Conclusions The preceding discussion has provided a brief overview of the status of vision research with particular emphasis on the various interpretations of the characteristics of perceptual alternations. The search for explanations of some perceptual phenomena has unfortunately lead to a situation in which the conflict has come to embody a debate between researchers who hold dichotomous viewpoints. For example Broerse and Crassini (1981) explain all complex perceptual effects in terms of one neurophysiological mechanism or process at the exclusion of any other explanation. Other perception reseachers adopt an exclusively cognitive interpretation of all phenomena (Horlitz & O'Leary, 1993). One notable consequence of this conflict is an impasse in the intepretation of visual effects produced by ambiguous stimuli. Much of the impasse may be due to the nature of the evidence that supports the two major theoretical positions. On the one hand is the neurophysiological or 'hard evidence' e.g., psychophysiological recordings of cell activity. On the other is cognitive evidence which by its nature is largely subjective. For example in the case of the ME cognitive influence is demonstrated by an observers ability to consciously switch the organisation of the test stimulus and cause the hues to fade. Criticisms of the cognitive approach based on the absence of parametric measures, data and statistical analyses would appear warranted. Criticism however based on any alternative explanation being more 'parsimonious' is unfounded. The dominance of the neurophysiological model led Harris (1980) to draw attention to the extreme application of the approach by advocating

24 the existence of yellow volkswagon detectors to explain how we perceive yellow volkswagons. A continuation of this approach is apparent with Von Grunau et al's (1984) proposal of channels for perspective orientation to account for the perception of boxes. 17 The two major theoretical approaches in vision reseach have developed valid theoretical acounts of many perceptual phenomena. Generally researchers have adopted the singular hypothesis testing approach with proponents of these dichotomous approaches believing that perceptual effects can be adequately explained by one theory and without rational consideration of any alternative explanations. As there is clearly evidence for both theoretical approaches and both are able to explain the same phenomena, e.g., reversal rate of perceptual alternations, an integrated approach, in which visual processing at a physiological level is influenced by cognitive process, appears to be warranted. 8. Directions For Further Research Long (1988) noted that the one experimental paradigm adopted by the proponents of both major theoretical approaches is unable to verify or falsify either position. Clearly the adaptation paradigm has enhanced the difficulty of interpretation for some researchers. In the case of relative dominance changes there appears to be little consensus of interpretation. Additionally researchers have attempted to provide evidence to support a single hypothesis. For example Von Grunau et al's (1984) experiment, in which the adapting stimulus was placed in varying locations above and below the test stimulus, was based on the hypothesis that only neurophysiological mechanisms account for changes in relative dominance. This experiment was undertaken without considering that changing the perceived location of the stimulus would have an impact on the experimental outcome. Close inspection of Von Grunau et al.'s (1984) findings show that it was only in one condition, when the adapting and test stimuli were both centrally located and fixated, that the stimuli occupied the same retinal and physical location. Further it was in this condition that the largest adaptation effects were reported. On the basis of

25 Von Grunau et al.'s (1984) interpretation of localised channels for perspective orientation and an examination of their findings several hypotheses can be made. 18 Hypotheses and Experimental Strategy Three experiments are proposed to investigate whether Von Grunau et al.'s (1984) interpretation of relative dominance changes in terms of localised channels for perspective orientation is appropriate or whether a cognitive account may be more valid. As will be recalled, cognitive explanations in terms of habituation (Horlitz & O'Leary, 1993) assume that learning of the unambiguous percept occurs in the adaptation phase. When the ambiguous test stimulus is subsequently viewed a preference is created for the unlearned and more novel alternative percept. Further, if the adaptation effect is location dependent this would account for the diminished effect reported by Von Grunau et al. (1984) when the adapting stimulus was moved away from the test stimulus. The first experiment hypothesises that if only localised channels are involved then the magnitude of the change would be expected to be the same irrespective of whether the adapting and test stimuli are in the same perceived positions. Conversely if relative dominance can be attributed to factors under cognitive influence such as location, then it is also hypothesised that the magnitude of the change will be significantly different when the perceived (or spatiotopic) location of the stimulus is manipulated, irrespective of the retinal (or retinotopic) location. Von Grunau et al's (1984) interpretation can be evaluated in the first experiment if an eye movement is introduced in the adaptation/test paradigm between the adapting and test periods when the test stimulus is located below the adapting stimulus. When the observer changes fixation from the centre of the adapting stimulus to the centre of the test stimulus it can then be ensured that the stimuli will fall on the same retinal area even though the perceived locations differ. This condition in addition to one used by Von Grunau et al. (1984) with retinally and spatially coincident stimuli tests the hypothesis that only localised channels are involved in relative dominance changes. If this is correct there will be no difference in the adaptation effect when the physical location is changed in the eye movement condition.

26 19 Further, if relative dominance changes can be attributed to factors under cognitive influence then it is hypothesised that the magnitude of the change will be significantly different when the perceived location of the stimuli is manipulated, irrespective of the retinal location. In this experiment an eye movement will be made between the adapting and test period. When the observer changes fixation from below the adapting stimulus to the centre of the test figure it can then be ensured that the stimuli occupy different retinal locations when the perceived location remains the same. This condition in addition to one used by Von Grunau et al (1984) with stimuli that were neither retinally nor spatially coincident tests the hypothesis that cognitive factors such as location may be involved in relative dominance changes. It is expected that there will be a significant difference in the adaptation effect when the physical location is changed, even though the retinal location differs in both conditions. A third experiment proposes that if relative dominance is dependent upon location, then a more appropriate explanation may be in terms of a cognitive or learning process such as habituation. In this experiment the adapting and test stimulus occupy the same physical location. When the observer changes fixation from below the adapting stimulus to the centre of the test stimulus this ensures that the test stimulus occupies a different retinal location. Because of the established phenomenon of dishabituation (Thomson & Spencer, 1966) it is hypothesised that if a novel stimulus is presented before the test stimulus there will be an increase in reporting the adapted percept. It is also hypothesised that when this stimulus is presented on the next trial that it will have little effect on the observers' reporting of the adapted percept indicating that learning of the novel stimulus occurred on the first presentation. Further, as the stimuli in this experiment are in different retinal locations, any change in relative dominance can only be attributed to cognitive factors and not neurophysiological mechanisms such as localised channels.

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29 Jenkins, B., & Ross, J. (1977). McCollough effect depends upon perceived organisation. Perception. 6, Keating, M.B., & MacKenzie, B.E. (1986). Spatial Localization in Infancy: Position Constancy in a Square and Circular Room with and without a landmark. Child Development, 57, Kohler, I. (1940). Dynamics in Psychology. Liveright Publishing Corporation: New York. Long, G. (1988). Selective Adaptation vs. Transfer of Decrement: The Conjoint Effects of Neural Fatigue and Perceptual Learning. Perception and Psychophysics, 43, Long, G.M., Toppino, T.C., & Kostenbauder, J.F. (1983). As the Cube Turns: Evidence for Two Processes in the Perception of a Dynamic Reversible Figure. Perception and Psychophysics, 34, Mapperson, B., & Lovegrove, W. (1984). The dependence of monocular rivalry on spatial frequency: some interaction variables. Perception, Mapperson, B., & Lovegrove, W. (1989). Color-Contingent Aftereffects on Relative Dominance. Bulletin of the Psychonomic Society. 27, Masland, R.,S. (1969). Visual Motion Perception: Experimental Modification. Science, 165, Mayhew, J.E.W., & Anstis, S.M. (1972). Movement Aftereffects Contingent on Color, Intensity and Pattern. Perception and Psychophysics, 12, McCollough, C. (1965). Colour Adaptation of Edge-Detectors in the Human Visual System. Science, 149, McDougall, W. (1902). The Physiological Factors of the Attention Process. Mind

30 23 McDougall, W. (1903a). The Physiological Factors of the Attention Process. Mind McDougall, W. (1903b). The Physiological Factors of the Attention Process. Mind McDougall, W. (1906). The Physiological Factors of the Attention Process. Mind Meyer, G., & Phillips, D. (1980). Faces, vases, subjective contours and the McCollough effect. Perception. 9, Meyer, G., & Sherman, R. K. (1981). Reversible figures and the motion after-effect. Vision Research. 21, Murch, G.M. (1972). Binocular Relationships in a Size and Colour Orientation Specific Aftereffect. Journal of Experimental Psychology, 93, Orbach, J., Ehrlich, D., & Heath, H.A. (1963). Reversibility of the Necker Cube: 1. An Examination of the Concept of 'Satiation of Orientation'. Perceptual and Motor Skills, 17, Porac, C. (1989). Is Visual Illusion Decrement Based on Selective Adaptation? Perception and Psychophysics. 46, Price, J.R. (1967a). Two Components of Reversal Rate of a Rotating Skeletal Cube: 'Conditioned Satiation'. Australian Journal of Psychology Price, J.R. (1967b). Perspective Duration of a Plane Reversible Figure. Psychonomic Science. 9, Reisberg, D, & O'Shaughnessy, M. (1984). Diverting subjects' concentration slows figural reversals. Perception. 13, Rock, I. (1975). An introduction to perception. Macmillan: New York

31 Savoy, R.L., & Gabrieli, J.D.E. (1991). Normal McCullough Effect in Alzheimer's Disease and Global Amnesia. Perception and Psychophysics, 49, Sekuler, R.R., & Ganz, L. (1963). Aftereffect of Seen Motion with a Stablised Retinal Image. Science, 139, Sutherland, N.S. (1961). Figural After-Effects and Apparent Size. Quarterly Journal of Experimental Psychology, 13, Thomson, R.F., & Spencer, W.A. (1966). Habituation: A Model Phenomenon for the study of the Neuronal substrates of Behavior. Psychological Review, 73, Uhlarik, J., Pringle, R., & Brigell, M. (1977). Color aftereffects contingent on perceptual organisation. Perception and Psychophysics, 22, Vickers, D. (1972). A cyclic decision model of perceptual alternation. Perception, 1, Virsu, V. (1975). Determination of perspective reversals. Nature, 257, Von Grunau, M.W., Wiggin, S., & Reed, M. (1984). The Local Character of Perspective Organisation. Perception and Psychophysics, 35,

32 JOURNAL ARTICLE THE NECKER CUBE: A NEW PERSPECTIVE

33 25 Abstract A widely reported phenomenon of reversible figures is the reduction in dominance of a percept after exposure to an unambiguous stimulus. Von Grunau, Wiggin and Reed (1984) measured relative dominance changes when an adaptation stimulus was placed above and below a test stimulus. The adaptation effect diminished at 1.5 degrees from the test stimulus and it was proposed that reduction in dominance may be attributed to retinally localised channels for perspective orientation. In this study however it was taken for granted that only the retinal position of the stimulus was important. They were unaware that changing the position of the adapting stimulus manipulated both the retinal and the perceived location. The present study hypothesises that if only localised channels are involved in relative dominance changes then the magnitude of the change would be expected to be the same irrespective of whether the stimuli are in the same perceived positions. Conversely if relative dominance can be attributed to factors under cognitive influence, such as location, then it is also hypothesised that the magnitude of the change will be significantly different when the perceived location of the stimulus is manipulated, irrespective of the retinal location. Von Grunau et al.'s (1984) interpretation of relative dominance changes was evaluated in experiment 1 with the introduction of an eye movement between the adapting and test periods. It was then ensured that the stimuli will fall on the same retinal area even though the perceived locations differ. Further, in experiment 2 an eye movement to a different location ensured that the stimuli occupied different retinal locations when the perceived location remains the same. Experiment 1 showed negligible reduction in dominance when the perceived location differed although the retinal location remained constant, a finding contrary to that predicted by Von Grunau et al. (1984). Experiment 2 showed a large reduction in dominance when the perceived location remained the same and the retinal location differed. This was taken to demonstrate that the effect was dependent on the coincidence of perceived location.

34 26 It was further hypothesised that if relative dominance changes could be accounted for by a cognitive process, such as habituation, then a phenomenon such as dishabituation may be demonstrated. Support for this hypothesis was provided in experiment 3. The wider implications of the procedure utilised in this study were discussed.