The nature of information in the perception of colours
D. Philipona1 & A. Bompas1
1. Laboratoire de Psychologie Expérimentale
71, avenue Edouard Vaillant
92774 Boulogne-Billancourt Cedex, FRANCE
Colour perception is a long studied human ability. Much is known about the lights
that normal observers can and cannot discriminate, about the way subjects arrange chips
based on their colour, about the categories they draw among colours, about their ability to
detect a coloured item among distractors and about various colour illusions. Likewise a
fair number of the photochemical processes, neuronal circuitry and cortical areas that
come into play during colour perception have been identified and described at length.
Therefore one might think that, for the main part, colour perception is explained and only
details remain to be filled in.
Troubles start however as soon as psychophysicists step beyond a cold behavioural
report or neurophysiologists step beyond a simple correlation account, for the trouble is
to define what colour is, and to define what colour perception is. In this brief article we
outline the opposition that exists between the concepts of colour within the orthodox, the
ecological and the sensorimotor theories of perception, and briefly outline for the latter
two the issue of information.
A common, often implicit, theory of perception among scientists supposes that
perception involves the acquisition of knowledge about physical properties of objects
mediated by mental representations (Locke, 1690; Jackson, 1977). With the rise of
neuroscience, mental representations have then been identified with neuronal encoding,
so that a majority of distinguished scientists argues today that perceptual experience
should be explained with respect to the cortical activity triggered by sensory inputs (Zeki
et al., 1999; Rees, Kreiman & Koch, 2002; Crick & Koch, 2003). Nevertheless, ever
since its early formulation this theory has been embroiled in numerous issues that
undermine the logical consistency of the theory itself and its epistemic power (Reid,
1764; Ryle, 1949; Levine, 1983; Dennett, 1991): in particular, it seems that no logical
equation will ever be given by science to relate cortical properties and the character of
perceptual experience. We will not elaborate on this view of perception, but simply note
that for this form of indirect realism “color is created somewhere in the brain, although
the exact location of this process is still unknown, and we even have no idea what entities
the sensations called color are.” (Backhaus & Menzel, 1992). The psychophysical
experiments conceived by proponents of this viewpoint aim to establish the relationship
between a proximal stimulus (the light that impinges on the retina) and the colour
sensation “created” by the brain, as reported by subjects. This is commonly considered to
be at the heart of the issue of colour perception, and will be contrasted with two other
positions presented in the remaining of this paper.
The ecological approach attempts to avoid the contradictions of the position just
presented by holding that perception is the direct and active pick-up of information in the
temporally extended stimulus array faced by the organism, denying any sense-datum
mediation (Gibson, 1969). This brings into the centre of focus the organism-environment
interaction instead of mental representations, and redefines what there is to perceive. As
regards colour perception for instance, it has been noted for long that the “greatest
significance” of colours (von Helmholtz, 1896) is the extent to which they provide cues
about the identity of fruits, flowers, creatures, and so on. A purely Gibsonian approach of
colour perception would therefore claim that to perceive colours is to pick up information
about the identity of an object based on its reflection properties, by tuning the organism’s
behaviour to some invariants of the optical array. What there is to explain about colour
perception is then the expression of these invariants. Or the explanatory perspective can
be broadened so much as to become phylogenetic. In that case a lax ecological
explanation of colour perception might be envisioned to describe how trichromatic vision
in primates coevolved with coloured fruits (Mollon, 2000) since it is mutually
advantageous for fruits to be seen by primates. This would be an example demonstrating
that perceiver and environment are inherently interdependent and that the act of
perceiving must be accounted for with respect to the self-organisation of the overall
system.
Even when the phylogenetic perspective is put aside, the ecological conception of
colour perception leads straightforwardly to concrete methodological and ontological
oppositions with indirect realism. Methodologically ecologists disagree with the kind of
experiments that have been conducted by psychophysicists, experiments based on the
reduction of colour perception to elementary stimuli. The retinal stimulus produced by a
light directed to the eye is an extreme impoverishment of the stimulus array explored by
the organism in natural conditions, and demonstrates a singular mode (rather than
elementary limit) of colour perception. And indeed this experimentation of colour
perception is hardly compatible with the fact that in most conditions, colour perception is
about surface colours rather than about the spectral composition of the light constituting
the proximal stimulus, as demonstrated by colour constancy (Jameson & Hurvich, 1989;
Kraft & Brainard, 1999). The point shifts from a purely methodological issue when
indirect realists appeal to compensatory mechanisms to explain the stability of the
conjectured mental representation. For ecologists, to perceive colour is to be tuned to
some invariant, and therefore colour constancy is naturally a foundational part of colour
perception, not an auxiliary mechanism that compensates for something whose
conception they criticize (sensations).
This leads to the more salient ontological opposition: the ecological approach has to
conceive colour as information in the stimulus array and not as a product of the brain1.
Indeed as any form of direct realism the ecological approach holds that what is perceived
(in the tuning sense) are the properties of the world, even though it concedes that the
organism’s body, in particular its sensory apparatus, restricts (but does not participate in
the definition of) the properties that the organism is able to perceive.
The sensorimotor approach is another attempt to develop a theory of perception
more consistent than indirect realism. It starts with the same diagnosis as the ecological
approach that part of the trouble comes from conceiving perceptual experience as a state
1
Compare with the earlier quotation of Backhaus & Menzel.
rather than a way of acting. Hence the sensorimotor approach also suggests restating both
what it means to perceive and what there is to perceive, in an active perspective. For that
purpose it introduces the notion of sensorimotor contingency: the constraints satisfied by
variations in sensory input as a result of motor activity, and argues that perception is
cognitive access to the structure of the sensorimotor contingencies that the organism is
engaged in. This definition provides the means of a logical categorization of sensory
modalities and intra-modal differences (O’Regan & Noë, 2001; Myin & O’Regan, 2002).
This is a substantial improvement with respect to indirect realism, which does not even
possess the resources for a positive specification2 of these categories because there is no
logical link between neuronal representations and the perceptual experience they are
correlated with (Levine, 1983).
The sensorimotor approach only relies on the existence of physical laws implying
sensorimotor contingency, but nonetheless it differs importantly from direct realism (e.g.
the ecological approach) or at least of any form of naïve realism. The structure of the
contingencies that an organism experiments is indeed constrained together by the
environment, the particularities of that organism’s body and the sensorimotor skills that
have evolved in that body and nervous system. Further, the organism's cognitive access to
this structure of contingencies is constrained by the various abilities of categorization,
abstraction and inference that the organism has evolved. These factors therefore all
combine in determining the organism’s perception rather than merely drawing the
borders of a window over an “objective” (i.e. canonically described and conceived
without reference to any sensorimotor apparatus of any kind) reality. In this approach it is
the set of possible sensorimotor contingencies that make sensory data to be information,
as compared to the ecological approach that considers information to exist prior and
independently of the organism.
The position of the sensorimotor approach on this point is exemplified in its account
of unique hues (Philipona & O’Regan, 2005). Starting from a standard description of the
physical laws of interaction between lights and surfaces as well as between light and
human photogiments, one can derive a description of the contingencies associated with
the human sensorimotor apparatus. As our eye fixation point moves along a surface, or as
2
i.e. not build upon perceivers’ subjective report.
our gaze angle over a given area changes, or even as we manipulate a coloured object, we
are engaged in a particular sensorimotor contingency. Since changes of viewpoint on the
surface, or displacements of the surface, are associated with changes of the incident light
producing the reflected light that impinges on our retina, these sensorimotor
contingencies are overall constrained by the reflection properties of the surface, that is by
the way the surface alters the spectral composition of the incident light. Actually it is
constrained only by those reflection properties that correspond to modifications of the
spectral composition of light that can be measured by human photopigments. Also, it is
reasonable to focus on the statistically most significant part of these reflection properties,
i.e. on the way elements of a statistically representative set of natural incident lights are
altered (rather than any mathematico-physically conceivable light). These reflection
properties can be straightforwardly computed from physical data about the spectral
composition of natural illuminants, the reflectance function of coloured surfaces, and the
spectral sensitivity of human photopigments. They provide a biological analogue of the
physicists’ reflectance function, taking into account only those properties that are relevant
with respect to the human sensory apparatus.
As humans only have three types of photopigments, this biological analogue
describes the attenuation of only three elementary components of the light instead of the
attenuation of the infinite number of monochromatic components considered by the
physicist. Further study then reveals that there exist four singular types of reflection
properties, hence of sensorimotor contingency, and that these singular reflection
properties quantitatively correspond to the surface colours most often given a name
across cultures (Berlin & Kay, 1969; Kay & Regier, 2003): red, green, blue and yellow
surfaces. The origin of the well documented perceptual singularity of these colours is a
long standing question in colour science, and so far no account purely based physical and
physiological data exits. Numerical results are provided showing that these surfaces are
singular in the sense that instead of reflecting all three components of the incident light in
arbitrary proportions, they reflect essentially one or essentially two components. Results
about unique hues (Kuehni, 2003) can then be shown to be consistent with the hypothesis
that observers in psychophysical studies try to infer what would be the surface that
provides the stimulation deriving from the light being looked at.
The upshot is that this account is based only on physical laws governing the
environment and (a part of) the organism, and constitutes a logical account3 of why the
perceptual experience of a unique hue is different from the perceptual experience of a socalled mixed colour: the organism is engaged in the former case in a sensorimotor
contingency that is indeed singular with respect to the generic case. Further, while the
approach is developed in a purely realistic framework about the organism-environment
system and yields quantitative predictions about a fact of our perceptual experience, it
does not claim that perception is grasping of “real” properties of the physical world. On
the contrary, the nature of human photopigments strongly determines the definition of the
biological notion of surface reflection properties considered. Hence the sensorimotor
approach stands apart from both indirect and direct realism.
Colors are a genuine quandary to science because they forcibly instantiate the
difficulty of explaining the particular quality of perceptual awareness. While the
dominant view today is that perceptual qualities are determined by specificities of cortical
representations, a different stance about the role of nervous activity can be taken. Rather
than saying that nervous activity generates sensation, alternative approaches such as the
ecological or sensorimotor approaches propose instead that this activity merely enables
the interaction of the organism with its environment. What determines the perceived
quality of sensations can then be sought in intrinsic features of the laws that constrain this
interaction.
There is a long history of arguments considering colours as the demonstration of the
limits of such a viewpoint. The famous inverted spectrum argument explicitly assumes
for instance that colour sensations cannot be determined by our sensorimotor interactions,
as opposed to shape perception (Locke, 1689). Yet it turns out at closer investigation that
the perceptual structure of colour corresponds to the theoretical structure of our
sensorimotor interaction with surfaces, and further that a dependence can be
experimentally exhibited between the eyes' sensorimotor contingencies and the
3
The account sketched is grossly naïve. But the point is that the general framework does not rely on
notions about sensations as sense-datum still to be scientifically defined, while contending that there is
something to explain about the perceptual experience of red and actually providing a quantitative
explanation of it (as over-simplified as can be a first account).
perception we have of colours (Bompas & O’Regan, 2005). This sheds a new light on
colour sensations and on perceptual experience in general.
References
Berlin, B. & Kay, P. (1969) Basic color terms: Their universals and evolution. Berkeley:
Univ. of California Press
Bompas, A. & O’Regan, J.K. (2005) Evidence for a role of action in color perception.
Perception (in press)
Crick, F. & Koch, C. (2003) A framework for consciousness Nature Neuroscience
(6)2:119-126
Dennett, D. C. (1991). Consciousness Explained. Boston, Little Brown and Co.
Backhaus, W. and R. Menzel (1992). Conclusions from color-vision of insects.
Behavioral and Brain Sciences 15(1): 28.
Gibson, J. J. (1979). The Ecological Approach to Visual Perception. Boston, Houghton
Mifflin
von Helmholtz, H. (1896/1962) Helmholtz’s Treatise on Physiological Optics (ed.
Southall, J.P.C.). Dover, New York.
Jameson, D. and L. M. Hurvich (1989). Essay concerning color constancy. Annual
Review of Psychology 40: 1-22.
Kay, P. & Regier, T. (2003) Resolving the question of color naming universals.
Proceedings of the National Academy of Science 100(15): 9085-9089
Kraft, J.M. & Brainard, D.H. (1999) Mechanisms of color constancy under nearly natural
viewing Proc Natl Acad Sci U S A. 96(1): 307–312.
Kuehni, R.G. (2004) Variability in Unique Hue Selection: A Surprising Phenomenon.
Color Research & Application (29)2:158-162
Jackson, F. (1977). Perception: A Representative Theory. Cambridge, Cambridge
University Press.
Levine, J. (1983) Materialism and qualia: the explanatory gap. Pacific Philosophical
Quarterly, 64.
Locke, J. (1689/1975). An Essay Concerning Human Understanding. New York, Oxford
University Press.
Mollon, J. D. (2000). "Cherries among the leaves": The evolutionary origins of color
vision. In: Color Perception: Philosophical, Psychological, Artistic, and Computational
Perspectives, Ed. S. Davis. Vol. 9: 10-30. New York, Oxford University Press.
Myin, E. & O'Regan, J.K. (2002) Perceptual consciousness, access to modality and skill
theories: A way to naturalise phenomenology? Journal of Consciousness Studies 9(1),
27-45.
O'Regan, J.K. & Noë, A. (2001) A sensorimotor account of vision and visual
consciousness Behavioral and Brain Sciences 24(5), 939-1011
Philipona, D. & O’Regan, J.K. (2005) The perceptual structure of colors corresponds to
singular reflecting properties. Under review.
Rees, G., Kreiman, G. & Koch, C. (2002) Neural Correlates of Consciousness in Humans
Nature reviews Neuroscience (3):261-270
Reid, T. (1764) An Inquiry into the Human Mind, on the Principles of Common Sense
Ryle, G. (1949) The Concept of Mind. Hutchinson, London.
Thompson, E. (1995). Colour Vision. New York, Routledge.
Varela, F. J., E. Thompson, et al. (1991). The Embodied Mind : Cognitive Science and
Human Experience. Cambridge, MA, MIT Press.
Zeki, S., Aglioti, S., McKeefry, D. & Berlucchi, G. (1999) The neurological basis of
conscious color perception in a blind patient Proceedings of the National Academy of
Sciences USA (96)24