ON THE NATURE OF SIMULTANEOUS COLOUR CONTRAST Dissertation zur Erlangung des Doktorgrades der Philosophischen Fakultat¨ der Christian-Albrechts-Universitat¨ zu Kiel vorgelegt von Vebjørn Ekroll Kiel 2005 Erstgutachter: ........................................ Prof. Dr. Rainer Mausfeld Zweitgutachter: ........................................ PD Dr. Johannes Andres Tag der Mundlichen¨ Prufung:¨ ............................................. 27.06.2005 Durch den zweiten Prodekan, Prof. Dr. Norbert Nubler¨ zumDruckgenehmigtam: .................................. .................. 13.07.2005 Our journey from the psychophysics of simple displays toward the perception of nat- ural scenes may call to mind the following well-known story that appears in many guises. Benighted space traveller in impenetrable rain forest on unknown planet, on meeting small person with long ears: ”Sir, can you tell me the way to Alderon?” Per- son with ears: ”If to Alderon going were I, not from here start would I.” -PAUL WHITTLE 4 Contents 1 Introduction 7 I Theoretical background 11 2 Basic colour theory 13 2.1 Trichromatic theory . 13 2.2 Trichromatic theory and colour appearance . 32 2.2.1 Hue, saturation, brightness, and Helmholtz coordinates . 33 2.2.2 Complementarity of colours . 37 2.3 Opponentcolourstheory .............................. 39 3 Models of simultaneous contrast 49 3.1 ThevonKriesmodel ................................ 50 3.2 Thetwo-processmodel ............................... 53 3.3 The contrast-coding model . 54 3.3.1 Walraven’s experiment . 55 3.3.2 Whittle’s experiment . 56 3.3.3 Other evidence for contrast-coding . 58 3.3.4 Thezero-contrastproblem . 58 3.3.5 The scope of the zero-contrast problem . 59 3.3.6 Contrast-coding and everyday colour perception . 61 3.3.7 Evidence for contrast-coding under ordinary viewing conditions . 63 II Experiments 67 4 The convergence paradox 69 4.1 Basic logic of the experiments . 70 4.2 Generalmethods .................................. 71 4.3 Experiment1 .................................... 72 4.4 Experiment2 .................................... 73 4.5 Experiment3 .................................... 76 4.6 Discussion...................................... 81 5 Kinds of simultaneous contrast 93 5.1 Experiment4 .................................... 93 5.2 Experiment5 .................................... 100 5.3 Discussion...................................... 105 5 6 CONTENTS 5.3.1 How uniform surrounds are special . 106 5.3.2 Saturation scale extension and truncation . 107 5.4 Experiment6 .................................... 108 6 General Discussion 113 6.1 Accounting for Meyer’s effect . 114 6.2 The role of contrast-coding in colour vision . 115 6.3 On the functional equivalence of surrounds . 118 6.4 The validity of grey settings and matching data . 119 6.5 The representativity of grey settings . 120 6.6 The relation to colour constancy . 120 6.7 The role of perceptual transparency . 121 6.8 Relation to simultaneous brightness contrast . 122 6.9 Conclusions..................................... 123 6.10Outlook ....................................... 124 6.11 Acknowledgements................................. 125 References 127 A Colour Plates 137 B Zusammenfassung (German abstract) 151 C Lebenslauf (Curriculum Vitae) 165 Chapter 1 Introduction The two central squares in Colour Plate I (see page 137) are physically equal, yet they appear rather different since they are embedded in surrounds of different colour. This phenomenon, which is referred to as simultaneous colour contrast, has been known since antiquity, and particularly in the two last centuries a rather impressive amount of scientific effort has been directed towards de- veloping a deeper understanding of it (Tschermak, 1903; Whittle, 2003). The motivations behind these investigations are and have been manifold, ranging from immediate practical concerns to pure intellectual curiosity and philosophical questions. In the sciences of vision and perception the major impetus stems from the notion that the study of perceptual illusions is likely to teach us something about the mechanisms which transform the highly ambiguous raw input imping- ing on our sensory organs into useful and reliable information about the external world (Mach, 1866; Hoffman, 1998; Eagleman, 2001). In accordance with this general approach, it has been proposed that the simultaneous contrast ‘illusion’ is intimately linked to biologically important mechanisms of colour constancy. According to Hering’s (1920) classical theory, for instance, both colour constancy and simultaneous colour contrast are the results of the same neural mechanism of lateral inhibition. Similarly, Helmholtz (1911) proposed that the simultaneous colour contrast ‘illusion’ can be observed whenever the physical situation is at odds with assumptions that the visual system relies on in order to obtain colour constancy, i.e. to provide the organism with an illumination-independent representation of object colours. Under either hypothesis it is assumed that simultaneous colour contrast and colour constancy are basically two sides of the same coin. Hence, the study of simultaneous contrast can be expected to yield insights into the nature of colour constancy, and vice versa. Indeed, experiments designed to investigate colour constancy are often virtually indistinguishable from experiments performed in order to learn more about simultaneous colour contrast. By virtue of the postulated connection to colour constancy, understanding simultaneous con- trast is regarded an important goal of vision science, and accordingly there is an impressive amount of experimental data available. Yet, although significant insights have been gained, simultaneous colour contrast is still far from a well-understood phenomenon; A reasonably general and com- monly accepted quantitative model accounting for the psychophysical data has yet to emerge. The reasons for these difficulties are probably manifold. It is well known that the simultaneous contrast effect depends on a number of parameters, such as viewing conditions, stimulus size, perceptual organisation, and how the observers are instructed. In the present investigations, though, the main focus is on a more basic conceptual problem. As will be discussed in chapter 3, most quantitative models of simultaneous contrast implic- itly rest on the seemingly natural and innocuous presumption that the colour changes induced by a coloured surround are, in principle, of the same nature as those obtained by changing the physical color-coordinates of the target patch. Accordingly, it is assumed that the latter kind of 7 8 CHAPTER 1. INTRODUCTION colour change may compensate the former, or vice versa. Though this ‘compensation assump- tion’ is seldom explicitly discussed, it is of profound theoretical significance. The hard facts of Young-Helmholtz trichromatic theory demonstrate that human colour vision is trivariant, both in a well-defined psychophysical sense (Krantz, 1975b) and a corresponding physiological one (Sharpe & Stockman, 1999). If the perceived colour of a local stimulus were independent of the context in which it is embedded, this would imply that the perceived colour corresponding to a location of the visual field can vary along no more than three dimensions. The context-dependence of perceived colour demonstrated by such phenomena as simultaneous contrast, however, opens up the theoret- ical possibility that perceived colour may vary along more than three dimensions, simply because context-dependence means that perceived colour depends on more triplets of cone-excitation val- ues than that of the target itself (Evans, 1974). Thus, that perceived colour should be trivariant does not follow from trichromatic theory alone. Rather, the trivariance of perceived colour would only follow from trichromatic theory provided that the compensation assumption is valid (W. S. Stiles, 1961). That most quantitative models of simultaneous contrast are based on the compensation as- sumption is evidenced by the fact that they represent the perceived colour of the target by a triplet of numbers. That this may be inappropriate, though, is suggested by the work of several authors: Based on a number of different arguments, it has been conjectured that more than three numbers are necessary in order to represent the perceived colour of the target adequately (e.g. Katz, 1911; Gelb, 1929; Kanizsa, 1966; Evans, 1964, 1974; Mausfeld, 1998; Niederee,´ 1998). In chapter 4, I present experimental evidence lending strong support to these claims. The basic finding is that experimental estimates of the neutral point obtained with the conventional method of grey settings differ, in a clear and lawful manner, from estimates of the neutral point obtained with some novel methods, in which the neutral point is measured according to indirect criteria. It is found, for instance, that when targets are viewed embedded in a uniform coloured surround, the convergence point for lines of constant hue does not, as one may intuitively expect, coincide with the chromaticity of a patch which is judged to appear achromatic, but instead with the chromaticity of the coloured surround. Though this finding may appear paradoxical, it is shown that it can be accounted for in quite rational terms by giving up the compensation assumption and instead assuming that simple centre-surround stimuli sometimes evoke phenomena reminiscent of perceptual transparency, which is traditionally thought to occur only in more complex stimulus displays