Colour constancy and conscious perception of changes of illuminant

Abstract

A sudden change in illuminant (e.g., the outcome of turning on a tungsten light in a room illuminated with dim, natural daylight) causes a “global” change in perceived colour which subjects often recognise as a change of illuminant. In spite of this distinct, global change in the perceptual appearance of the scene caused by significant changes in the wavelength composition of the light reflected from different objects under the new illuminant, the perceived colour of the objects remains largely unchanged and this cornerstone property of human vision is often described as instantaneous colour constancy (ICC). ICC mechanisms are often difficult to study. The generation of appropriate stimuli to isolate ICC mechanisms remains a difficult task since the extraction of colour signals is also confounded in the processing of spatial chromatic context that leads to ICC. The extraction of differences in chromaticity that describe spatial changes in the wavelength composition of the light on the retina is a necessary operation that must precede colour constancy computations. A change of illuminant or changes in the spectral reflectance of the elements that make up the scene under a constant illuminant cause spatial changes in chromatic context and are likely to drive colour constancy mechanisms, but not exclusively. The same stimulus changes also cause differences in local luminance contrast and overall light flux changes, stimulus attributes that can activate different areas of the visual cortex. In order to address this problem we carried out a series of dichoptic experiments designed to investigate how the colour signals from the two eyes are combined in dichoptically viewed Mondrians and the extent to which the processing of chromatic context in monocularly driven neurons contributes to ICC. The psychophysical findings show that normal levels of ICC can be achieved in dichoptic experiments, even when the subject remains unaware of any changes of illuminant. Functional MRI (fMRI) experiments using new stimuli that produce stimulation of colour constancy mechanisms only in one condition with little or no difference in the activity generated in colour processing mechanisms in both test and reference conditions were also carried out. The results show that the processing of ICC signals generates strong activation in V1 and the fusiform colour area (V4, V4A). Significant activation was also observed in areas V2 and V3.

Introduction

Colour constancy represents an important, fundamental aspect of our visual experience and can involve both rapid processes, almost instantaneous with the change of illuminant, that rely largely on spatial computations of changes in chromatic context (Land, 1986; Land & Daw, 1962), as well as slower processes that may involve photoreceptors and retinal adaptation mechanisms (Kamermans, Kraau, & Spekreijse, 1998). These processes, together, yield almost perfect functional colour constancy, i.e., the colour of objects is recognised as belonging to the same category, even when the same objects are viewed under illuminants that differ significantly in spectral composition (Arend, Reeves, Schirillo, & Goldstein, 1991; Craven & Foster, 1992). Functional colour constancy also relies on the internal representation of different colour categories and the ability to match perceived object colours in context with remembered colours (Ling & Hurlbert, 2005). There are significant advantages in employing both rapid and slow processes to achieve colour constancy. A change of illuminant carries useful information that is easily detectable during the first few seconds, but less so, following a period of adaptation to the new illuminant. This information would not be available to the subject if the rapid colour constancy processes produced complete constancy. Once chromatic adaptation is established, the perception of object colours matches well our expectations of colour categories based on their internal representation. When special viewing arrangements eliminate all chromatic context, the “instantaneous” colour constancy mechanisms become ineffective and the perceived colour of an isolated object viewed against a black background is determined almost entirely by its wavelength radiance distribution. This is again of advantage since under such conditions neither illuminant nor spectral reflectance can be estimated accurately and any assumptions about either of these two variables can lead to erroneous observations. The presence or absence of surround information has been used to define two modes of perceived object colour (Moutoussis & Zeki, 2000). The “void mode” (when the object, viewed in isolation against a dark background field, takes the colour of the illuminant) and a “natural mode” (when the object, viewed in context, surrounded by other objects, under more natural conditions, tends to preserve its colour under changes of illuminant). Many studies have attempted to localise neural mechanisms with the properties needed to achieve ICC. Although some investigations suggest that, at least in some species, von Kries type transformations of cone photoreceptor signals (West & Brill, 1982) can be achieved rapidly within the retina (Kamermans et al., 1998), the majority of studies investigated mainly the large field chromatic organisation of neurons in extrastriate visual area V4 and beyond (Bartels & Zeki, 2000; Lueck et al., 1989; Wild, Butler, Carden, & Kulikowski, 1985). Experiments carried out in patients with homonymous hemianopia caused by unilateral lesions to the primary visual cortex reveal the complete absence of ICC, when the change of illuminant is restricted to the cortically blind hemifield, but normal ICC in the sighted hemifield (Barbur, de Cunha, Williams, & Plant, 2004). The experiments involved measurement of instantaneous colour constancy using techniques similar to those described in this study, but with the change of illuminant restricted to either the sighted or the blind hemifields. The results showed normal colour constancy in the sighted hemifield, but no constancy when the change of illuminant was restricted to the blind hemifield. These findings were somewhat surprising since subjects with unilateral cortical lesions show normal, long-range chromatic interactions in colour induction/adaptation experiments, even when the coloured stimuli are restricted to the cortically blind regions of the visual field (Barbur et al., 2004, Poppel, 1986). Other studies have also demonstrated residual chromatic processes in cortically blind hemifields in both man (Barbur, Weiskrantz, & Harlow, 1999; Stoerig & Cowey, 1992) and monkey (Cowey & Stoerig, 2001). There is therefore little doubt that retinal mechanisms and subcortical pathways can mediate some form of chromatic processing in the absence of direct geniculostriate projections. These results suggest that, at least in human vision, retinal processing of chromatic signals is not sufficient to achieve ICC. There is little doubt that lesions in the lingual and fusiform gyri can cause selective impairment of colour vision (Verrey, 1888, Zeki, 1990), even when other visual functions such as luminance defined motion perception and contrast acuity remain relatively unaffected (Barbur, Harlow, & Plant, 1994). This functional specialisation (as inferred from clinical studies) also extends to the processing of colour categories and the different uses one makes of colour signals (Rizzo, Smith, Pokorny, & Damasio, 1993). Some patients show greater loss of sensitivity when required to detect changes in the colour of an object defined by luminance contrast whilst others are unable to construct spatially structured objects defined only by colour (Barbur et al., 1994; Heywood et al., 1998a, Heywood et al., 1998b). These findings suggest that although, in some patients, the processing of colour signals in V1 can remain relatively unaffected by the lesion, the specific use one makes of these colour signals in areas upstream to V1 can be impaired selectively as a result of extrastriate lesions. The extent to which the primary visual cortex and extrastriate areas such as V4 and V4A contribute to ICC in human vision therefore remains unclear. Loss of chromatic sensitivity and ICC has been reported in patients with postgeniculate lesions (Kennard, Lawden, Morland, & Ruddock, 1995; Vaina, 1994). Other studies have demonstrated functioning ICC mechanisms in patients with lesions in the lingual and fusiform gyri, even when the patients were unaware of the effects of colour constancy because of significant loss of chromatic sensitivity (Barbur et al., 2004). Since the normal functioning of ICC mechanisms can be made completely ineffective by the loss of chromatic sensitivity, the possibility remains that the processing of chromatic signals in V1 also contributes significantly to ICC (Barbur et al., 2004, Hurlbert, 2003). Recordings from the primary visual cortex of the macaque monkey reveal single cells with chromatic tuning properties that reflect the contribution of the immediate surround (Conway & Livingstone, 2005; Wachtler, Sejnowski, & Albright, 2003). Since the machinery for the generation of some comparisons of adjacent chromatic borders exists in V1, it is of great interest to establish the extent to which V1 contributes to ICC in human vision. To answer this question we designed a number of visual psychophysical experiments to examine the extent to which ICC relies on signal processing within monocularly driven neurons. Novel stimuli for fMRI studies were also designed to produce selective stimulation of colour constancy with little or no difference in the activity generated in colour processing mechanisms.