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On the Nature of Unique Hues

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On the Nature of Unique Hues Reprinted from Dickinson, C., Murray, 1. and Carden, D. (Eds) 'John Dalton's Colour Vision Legacy', 1997, Taylor & Francis, London On the Nature of Unique Hues J. D. Mollon and Gabriele Jordan 7.1 .l INTRODUCTION There exist four colours, the Urfarben of Hering, that appear phenomenologically un- mixed. The special status of these 'unique hues' remains one of the central mysteries of colour science. In Hering's Opponent Colour Theory, unique red and green are the colours seen when the yellow-blue process is in equilibrium and when the red-green process is polarised in one direction or the other. Similarly unique yellow and blue are seen when the red-green process is in equilibrium and when the yellow-blue process is polarised in one direction or the other. Most observers judge that other hues, such as orange or cyan, partake of the qualities of two of the Urfarben. Under normal viewing conditions, however, we never experience mixtures of the two components of an opponent pair, that is, we do not experience reddish greens or yellowish blues (Hering, 1878). These observations are paradigmatic examples of what Brindley (1960) called Class B observations: the subject is asked to describe the quality of his private sensations. They differ from Class A observations, in which the subject is required only to report the identity or non- identity of the sensations evoked by different stimuli. We may add that they also differ from the performance measures (latency; frequency of error; magnitude of error) that treat the subject as an information-processingsystem and have been increasingly used in visual science since 1960. Sensory observations of Class A can be interpreted if one allows merely the hypothesis that physiologically indistinguishable signals sent from the sense organs to the brain cause indistinguishable sensations. But to this day we have no secure way of interpreting Class B observations, no way of knowing what weight to place on them. Some might suppose that there exist specific cortical cells (or structures or processes) that give rise to - secrete - sensations of redness and other units that secrete sensations of blueness, and so on; and that mixed hues are seen when two types of cortical cell are concurrently secreting their proper sensations. Psychophysical linking hypotheses of this kind are not often made so unashamedly explicit, but it is a good idea to make them so. For all we really know is that in a given state of adaptation some chromaticities map on to hue sensations that typical observers describe as pure, whereas other chromaticities map on to mixed sensations. 7.1.2 THE CHROMATIC CHANNELS OF THE VISUAL PATHWAY When chromatically antagonistic signals were first recorded in the retinae of fish (Svae- tichin and MacNichol, 1958) and in the lateral geniculate nucleus of macaques (De Valois et al., 1966), it was widely assumed that Hering had been vindicated and that the neural channels of the primate LGN corresponded to the red-green and yellow-blue processes of Opponent Colours Theory. The standard zone model of the 1960s had a receptoral 'Helmholtz' stage and a second 'Hering' stage (Walraven, 1962). Such a view still survives in psychology textbooks and other secondary sources. Today, however, most colour scientists are agreed that the chromatically opponent cells of the early visual system (the 'second stage' of models of colour vision) do not correspond colorimetrically to red-green and yellow-blue processes. Two main types of neural channel have consistently been reported in Old World primates: a phylogenetically recent channel in which the signal of the long-wave cones is opposed to that of the middle-wave cones and a phylogenetically older channel in which the signal of the short-wave cones is opposed to some combination of the signals of the L and M cones (Derrington et al., 1984; Gouras, 1968; Mollon and Jordan, 1988). Figure 7.1.1 (see colour section) shows the chromaticity diagram of MacLeod and Boynton (1979), the axes of which correspond to the two chromatic channels of the early visual system. In such a diagram, a line that runs from unique yellow (c. 572 nm) to unique blue (c. 475 nm) is oblique, and not vertical as it should be if it represented a fixed, equilibrium, ratio of the quantum catches of the M and L cones. Indeed, unique blue is close to the wavelength (460 nrn) that maximises the ratio M/L (Mollon and Estevez, 1988). Recognising this discrepancy, the authors of recent models of colour vision have usually postulated a 'third stage', in which the second-stage signals are re-transformed to give channels that do correspond to those of Hering (De Valois and De Valois, 1993; Guth, 1991). It may be that a third stage of this kind does exist, but electrophysiological recording has not yet revealed it. Lennie et al. (1990) recording from neurons in the striate cortex of Macaca fascicularis, found that there was a large variation between cells in their preferred direction in colour space, with some bias towards the 'second stage' axes; only a few cells behaved as would be expected of the putative 'red-green' and 'yellow-blue' mechanisms of Hering. In the prestriate region V4, Zeki (1980) reported cells with narrow spectral sensitivities, but the wavelengths of peak sensitivity were distributed through the spectrum, with some avoidance of the yellow region; and extraspectral purples were well represented. Komatsu et al. (1992) examined the colour selectivity of neurons in the inferior temporal cortex and found that the population of cells together covered most of the chromaticity diagram. There were, for example, cells that gave their most vigorous response to a desaturated pink. Lennie et al. (1990) speak of the 'red-green and yellow-blue mechanisms whose existence is so firmly established by psychophysics'. Yet what is this psychophysical evidence? The psychophysical experiments most commonly invoked to support a third stage are the chromatic cancellation measurements of Jameson and Hurvich (1955; see also Werner and Wooten, 1979): in these experiments the strength of, say, the green chromatic response was established by finding at each wavelength the amount of a fixed, reddish, wavelength that needed to be added to yield a light that looked neither reddish nor greenish. These measurements are certainly quantitative, but they too are Class B observations and they are in effect only an extension of the basic determination of the unique hues. This is clear when one considers that it is not necessary to perform the measurements as cancellations. It is completely equivalent to ask the subject to identify directly the sets of non-spectral chromaticities that are neither reddish nor greenish or are neither bluish nor 520 55C 590 66" Unique yellow Figure 7.1.1 MacLeod and Boynton chromaticity diagram. The two ordinates of the diagram correspond to the two chromatic channels that have been identified in the early visual system. In this space the line running from unique-yellow to unique-blue is not vertical but oblique. (We are grateful to Ben Regan for preparation of the original colour figure.) l" yellowish: in a chromaticity diagram these sets form (often curved) loci that connect the wavelengths of the unique hues to the white point (Bums et al., 1984). Conventional colonmetry will then allow the reconstruction of cancellation curves in the form presented by Jameson and Hurvich. So the cancellation experiments amount to the extended determination of unique hues. They show us that the topology of chromaticity space is preserved in our phenomenological colour space, but they remain Class B observations and, as evidence for a third stage, they add nothing to the original observation that some hues are unique and some are phenomenal mixtures. 7.1.3 DOCTRINE OF COINCIDENT CATEGORIES In the hypothesis that each unique hue represents the activity of a discrete class of cortical cell, we can recognise a modem form of Miiller's Doctrine of Specific Nerve Energies. And in judging this hypothesis we should place it in its broader context: one of the chief unsolved questions of brain science is that of whether the elements of perception and thought are represented by the activities of individual cells (Barlow, 1972, 1995). It may be useful to identify a more general form of the hypothesis, in which we replace 'cell' by a term that can refer to any discrete 'structure' or 'process' or 'neural signal'. Let us speak of 'neural primitives'. It may also be useful to make explicit the distinct question of whether these neural primitives always map exactly on to our phenomenological categories. In the case of colour, then, we can ask: does the existence of unique and non- unique hues tell us that the neural representation of colour is discontinuous, and do the phenomenally unique hues correspond to the discrete primitives of this neural representa- tion whereas mixed hues correspond to more than one kind of neural primitive? We might use the term Doctrine of Coincident Categories for the idea that phenomenological categories correspond to neural primitives in the above way. Suppose that the Doctrine of Coincident Categories were wrong in the following sense. Suppose that colours were represented centrally only by neural primitives that corre- sponded to the axes of the MacLeod-Boynton space, i.e. primitives that did not correspond to redness, greenness, yellowness and blueness. This would imply that the transformation between the two categorical organisations (the transformation from the second to the third stage in current theories) arose in the relationship between the neural representation and the phenomenological, whatever that relationship might be. But now we are on radical ground. For it is easy for the normal trichromat to base his behaviour, verbal or otherwise, on the categories red, green, yellow and blue - much more easily than he can base his behaviour on the categories of the MacLeod-Boynton space.
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