Chromatic Borders Research Articles

Chromatic Borders, Cosmetic Bodies: Colour in the Films of Nicolas Roeg

Colour is the uncharted aesthetic terrain of Nicolas Roeg’s art. Barring a few superficial exceptions, the role and function of colour has been absent from previous writing on his work, partly due to a greater focus on Roeg’s ‘mosaic manner’ (Milne 1980: 43), and his ‘complex time games’ (Lanza 1985: 94). But we could also argue that it is also due to film theory’s lack of in-depth engagement with the chromatic. As film theorist Brian Price succinctly argues, ‘despite the centrality of color to the experience and technology of cinema, it has most often been no more than the occasional subject of the theorist, historian, or practitioner; a source more of fleeting observation than of rigorous conceptualization’ (Price 2006: 1). The intention of this article is both to contribute to the emerging debates surrounding film colour, and also to attempt to fulfil Roeg’s demand that we ‘read the images’ (Kennedy 1980: 24). While adopting an auteurist approach I do not perceive Roeg’s work with colour as unusual, but argue that through considering colour we open up Roeg’s work to alternative ideas and questions as exemplified by analyses of Performance (Cammell and Roeg, 1970) shot in Technicolor, Don’t Look Now (1973, Technicolor) and Bad Timing (1980, Eastman Kodak). Each film reveals a specific preoccupation with colour: the transition through colour from gangster to counter-culture aesthetics in Performance, the obsessive search for an elusive ethereal red in Don’t Look Now and a Klimtian-inspired femininity in Bad Timing.

Pupillary responses to coloured and contourless displays in total cerebral achromatopsia

In two patients with total acquired cortical colour blindness and in six control subjects we studied the binocular pupillary response to a variety of sharply defined coloured and grey displays that either had the same mean luminance as the background (isoluminant) or were of greater mean luminance. Despite their complete inability to identify or to discriminate between colours the patients, like the control subjects, showed a pupillary response to the structured coloured displays, even when they were masked by dynamic luminance changes. However, and unlike the control subjects, the patients showed no pupillary response when the coloured displays lacked sharp chromatic borders, as in Gabors or Gaussians. The results indicate that although chromatic processing still occurs in cortical colour blindness its function is solely to give rise to the detection of sharp boundaries which, in their case, can provide the perception of shape but not hue. In accordance with this, the patients could no longer describe the isoluminant borderless figures, which were often totally invisible to them despite their strong chromatic contrast with the background.

Microsaccades Counteract Visual Fading during Fixation

Our eyes move continually, even while we fixate our gaze on an object. If fixational eye movements are counteracted, our perception of stationary objects fades completely, due to neural adaptation. Some studies have suggested that fixational microsaccades refresh retinal images, thereby preventing adaptation and fading. However, other studies disagree, and so the role of microsaccades remains unclear. Here, we correlate visibility during fixation to the occurrence of microsaccades. We asked subjects to indicate when Troxler fading of a peripheral target occurs, while simultaneously recording their eye movements with high precision. We found that before a fading period, the probability, rate, and magnitude of microsaccades decreased. Before transitions toward visibility, the probability, rate, and magnitude of microsaccades increased. These results reveal a direct link between suppression of microsaccades and fading and suggest a causal relationship between microsaccade production and target visibility during fixation.

Chromatic edges, surfaces and constancies in cerebral achromatopsia

We tested achromatopsic observer, MS, on a number of tasks to establish the extent to which he can process chromatic contour. Stimuli, specified in terms of cone-contrast, were presented in a three-choice oddity paradigm. First we show that MS is able to discriminate the magnitude of chromatic and luminance contrast, but performance is inferior to that of normal observers. Moreover, MS can discriminate isoluminant borders of different chromatic composition. These abilities are not the result of unintended luminance differences and are abolished when chromatic borders are masked by sharp luminance change. In simple displays, local cone-contrast signals can make a significant contribution to surface colour appearance in normal observers. In more complex displays, the perception of a surface’s colour becomes largely independent of the local contrast to its background, via processes presumed to be similar to the edge integration and anchoring stages of Land’s Retinex algorithm. We show that in simple displays the percepts of both MS and normal observers are dominated by local chromatic-contrast. But, although the percepts of normal observers change in line with the predictions of retinex theory in more complex displays, those of MS do not, remaining dominated by local contrast signals. We conclude that MS has lost the ability to perform edge integration and that this loss is closely related to his absence of colour experience.

Moving stimuli define the shape of stationary chromatic patterns.

A study is reported of phenomena involved in perceptually unified organisation of a stationary chromatic pattern and a moving black outline or dot pattern. When the corners of the outline pattern were temporally oscillated on a stationary chromatic square, the chromatic border appeared to follow the moving outline, as if captured by it. This capture effect was also observed with moving dots: the chromatic border was defined by an imaginary line connecting the moving dots. Both capture effects occur over a region that becomes wider with increasing velocity of the oscillation. These observations suggest that the visual system effectively uses information from moving features to define the shape of overlapping chromatic image regions.

Motion processing in pigeon tectum: equiluminant chromatic mechanisms.

Recent psychophysical and neurophysiological studies have suggested that, in mammals, there are interactions between the P (colour processing) and M (motion processing) visual pathways, which were previously believed to be parallel and separate. In this study, the role colour information plays in the coding of object motion was determined in the tectofugal pathway of pigeons. The responses of motion-sensitive neurons in the tectum to moving stimuli formed by chromatic contrast were recorded extracellularly using standard single-unit recording techniques. A moving coloured object was presented on a uniform (opponent coloured) background (e.g. blue-on-yellow, red-on-green and black-on-white). Through systematically manipulation of the luminance contrast between object and background, an equiluminant condition was generated. It was found that, at chromatic equiluminance, the majority of cells maintain some level of response. The mean magnitude of the response at equiluminance was about one-third of the response at maximal contrast to the same chromatic border. These results suggest that tectal units can detect motion of a pattern defined by a pure colour contour, although the strength of output is considerably weaker than that for the movement of patterns formed by luminance contrast.

Cerebral achromatopsia: colour blindness despite wavelength processing

Cortical colour blindness is caused by brain damage to the ventro-medial occipital and temporal lobes. A possible explanation is that the pathway responsible for transmitting information about wavelength and its subsequent elaboration as colour has been destroyed at the cortical level. However, several signs of chromatic processing persist in an achromatopsic subject who, despite his inability to tell colours apart, can still detect chromatic borders, perceive shape from colour, and discriminate the direction in which a striped pattern moves when the determination of direction requires the viewer to 'know' which stripes have a particular colour. Perhaps only the information about wavelength that leads to conscious awareness of colour has been destroyed. It is unclear whether incomplete achromatopsia is merely a less severe form of the disorder or whether it is qualitatively different, perhaps reflecting impaired colour constancy. In monkeys, removing cortical area V4 impairs performance on colour constancy tasks but, invariably, impairs several other aspects of visual perception. If the lesion that causes total achromatopsia in human subjects corresponds to area V4 in monkeys, it is an unsolved puzzle that a totally achromatopsic subject paradoxically demonstrates certain characteristics of colour constancy, unless his residual performance reflects the much underrated retinal contribution to colour constancy.

There's more to colour than meets the eye

Patients with cerebral achromatopsia, a perceptual disorder caused by ventromedial occipital brain damage, can be completely unable to arrange colours in chromatic sequence and fail most conventional tests of colour blindness. A possible explanation for cerebral achromatopsia is that the colour-opponent parvocellular (P) channel has been selectively and totally destroyed at the level of visual cortex, leaving vision to be mediated by the broad-band magnocellular (M) channel. The persistence of normal occipital visually evoked potentials, and preserved sensitivity to isoluminant chromatic gratings indicates that if this hypothesis is correct the destruction must occur beyond the striate cortex. We have shown that an achromatopsic subject can detect chromatic borders and construct shape from colour, and that he can even perceive the apparent direction of motion of a phase shifted isoluminant chromatic grating where perceived direction depends on knowing the sign of the colour diffence, i.e., which colour is which in the stripes. This and other evidence suggests that perhaps only one part of the cortical P channel has been destroyed. Does the critical area involved in achromatopsia correspond to cortical area V4 of monkeys, often implicated in processing wavelength? When Visual Area 4 is totally ablated in monkeys they have onlya mild colour discrimination impairment and easily solve the colour ordering and colour selection tasks that an achromatopsic patients finds impossible. However, monkeys with ventromedial damage rostral to Area V4 do perform like achromatopsic patients, suggesting that the role of V4 in the perception of colour is still unclear and that the colour area of the human brain does not correspond to area V4.

On the role of parvocellular (P) and magnocellular (M) pathways in cerebral achromatopsia.

We assessed the ability of an achromatopsic patient to detect and discriminate colour and form concealed in a static or dynamic checkerboard display where the luminance differences among adjacent squares were randomly assigned. There were no conditions under which he could discriminate two very different saturated colours from each other. Nevertheless, he could discriminate chromatic from luminance boundaries in static displays when the colour defining the boundary was saturated and the achromatic boundaries all had similar luminance contrast, i.e. varied over a narrow range. However, he could not readily detect chromatic boundaries from among many achromatic boundaries that differed widely in luminance contrast. In addition, he was able to detect chromatic boundaries even when they were concealed by dynamic random luminance masking. His ability to pick out chromatic borders was abolished when desaturated colours were used. However, he was singularly proficient at detecting coloured forms in static or dynamic displays even when the saturation of the colours of which the form was composed were such that they were rendered invisible when concealed as a single square in a checkerboard. This implies that signals about chroma are still available in extracting shape. The patient performed flawlessly when asked to indicate the direction of motion of a horizontal red/green isoluminant grating which was phase shifted by 90 degrees in either direction, demonstrating unequivocally that he has access to the sign of colours that he nevertheless does not perceive.

Colour inputs to random-dot stereopsis.

Recently it has been claimed by Livingstone and Hubel that, of three anatomically and functionally distinct visual channels (the magnocellular, parvocellular interblob, and blob channels), only the magnocellular channel is involved in the processing of stereoscopic depth. Since the magnocellular system shows little overt colour opponency, the reported loss of the ability to resolve random-dot stereograms defined only by colour contrast seems consistent with this view. However, Julesz observed that reversed-contrast stereograms could be fused if correlated colour information was added. In the present study, 'noise' (non-corresponding) pixels were injected into random-dot stereograms in order to increase fusion time. All six subjects tested were able to achieve stereopsis in less than three minutes when there was only correspondence in colour and not in luminance, and three when luminance contrast was completely reversed. This ability depends on information about the direction of colour contrast, not just the presence of chromatic borders. When luminance and chromatic contrast are defined in terms of signal-to-noise ratios at the photoreceptor mosaic, chromatic information plays at least as important a role in stereopsis as does luminance information, suggesting that the magnocellular channel is not uniquely involved.

Responses of macaque ganglion cells to movement of chromatic borders.

1. We have measured responses of macaque ganglion cells to moving borders under conditions designed to simulate the minimally distinct border (MDB) task. 2. Extending previous results, we show that minimization of responses of phasic ganglion cells of the magnocellular (MC)-pathway obey the photometric laws of transitivity and additivity. 3. To equal luminance borders, a residual response was present in MC-pathway cells analogous to the second harmonic response seen in these neurones with temporal chromatic modulation. It was proportional to the tritanopic purity difference (magnitude of delta Pt, the rectified middle- to long-wavelength cone opponent signal) between the two colours on either side of the border. For a delta Pt of one, the mean residual response was equivalent to the response evoked by achromatic borders of about 14% luminance contrast. Both these properties of the MC-pathway closely resemble psychophysical estimates as to the distinctness of equal luminance borders. 4. We show how MC-pathway cell responses could be used centrally to support the MDB task. It was difficult to generate a model from responses of tonic ganglion cells of the parvocellular (PC)-pathway which would support the task. 5. The MDB task is still possible psychophysically after blurring the retinal image. Although blurring the border spatially smeared the responses of MC-pathway ganglion cells and reduced their amplitude, responses still went through a minimum close to equal luminance. Thus, blurring the image did not affect the ability of MC-pathway cells to support the task. Blurring the retinal image decreased the 'sharpness' of the border response of tonic, PC-pathway ganglion cells, but response amplitude was unaffected. Response features indicative of centre-surround organization were attenuated. A central mechanism reliant on centre-surround field structure of PC-pathway cells would thus not be able to support the task after blurring. 6. Taken together, these results strongly suggest that the MC-pathway forms the sole physiological substrate of the MDB task, and any contribution of the PC-pathway is, indeed, minimal.

The discrimination of blur in peripheral coloured borders

Our interest is in measuring the threshold for blur, for long borders at well-defined retinal eccentricities between large fields of colour. Our method is to precisely match a centrally fixated sharp-edged disk of one colour against a surround of another, using the minimum distinct border criterion. We then measure the amount of added Gaussian blur that makes the sharp step-edged border just noticeably different. The selected colours are small fixed chromatic deviations of 500 msec duration from an adaptive low photopic or mesopic white. The effects of retinal eccentricity are striking. Blur thresholds are large for yellow-blue borders centrally (at 1.25° eccentricity) and extremely large for red-green borders peripherally (at 20°). Blur thresholds are generally larger for isoluminous chromatic borders than for a low contrast achromatic border. Accurate Rovamo-Virsu M-scaling is limited to photopic achromatic borders.

Visual segmentation of a heterochromatic field into center/surround components

The process of visual segmentation was studied by quantitatively estimating the apparent difference (i.e. the perceptual contrast) between two segments of a heterochromatic field. A chromatic test bar (Munsell 5R6/6, 5BG6/6, 5Y6/6, or 5PB6/6) was centered in an achromatic surround (Munsell N6) and the luminance contrast between the test bar and surround was varied between 0 and 39%. At equiluminance (0% luminance contrast) the chromatic border between center and surround was vague and perceptual contrast was minimal. When luminance contrast was increased slightly (5–15%) the border between center and surround was more distinct and perceptual contrast was almost two times greater than predicted by a model that combines spatial differences in chromaticity and luminance as orthogonal vectors. A luminance border may enhance perceptual contrast by increasing the salience of the center/surround hue difference. A vector model is consistent with perceptual contrast only at relatively high luminance contrasts, where hue differences play a relatively minor role in visual segmentation.

Chromatic Discrimination in a Cortically Colour Blind Observer.

We tested the ability of a subject with cerebral achromatopsia to discriminate between colours and to detect chromatic borders. He was unable to identify colours or to arrange them in an orderly series or choose the odd colour out of an array or even to pick out a colour embedded in an array of greys. Nevertheless, he could select the odd colour when the colours were contiguous, even when they were isoluminant, and could discriminate an ordered from a disordered chromatic series as long as the colours in each row abutted one other. His verbal replies showed that he did so by detecting an edge between two stimuli that were, to him, perceptually identical. Introducing a narrow isoluminant grey stripe between adjacent colours abolished or greatly impaired this ability. As long as isoluminant colours were contiguous the patient could identify the orientation of the chromatic borders. Photopic spectral sensitivity showed evidence both for activity of three cone channels and for chromatic opponent processing, indicating that postreceptoral chromatic processing is occurring despite the absence of any conscious awareness of colour. The results indicate that both parvocellular colour opponent and magnocellular broad-band channels are active and that the cortical brain damage has selectively disrupted the appreciation of colour but not the ability to detect even isoluminant chromatic borders, which would be invisible to a retinal achromat. The subject's performance on non-colour tasks involving the discrimination of shape, texture, greyness and position was excellent. His disorder is therefore not like that of macaque monkeys in which cortical area V4 has been removed, and which are much more severely impaired at discriminating shape than colour.

The gap effect revisited: Slow changes in chromatic sensitivity as affected by luminance and chromatic borders

Chromatic discrimination was studied with two half-fields that were either precisely juxtaposed or were separated by a narrow gap. When present, the gap was in some conditions filled with light isoluminant to the test fields. When the fields were juxtaposed, chromatic sensitivity declined with viewing duration. For a discrimination based solely on S cone activity, separating steadily-viewed fields by either a luminance or a purely chromatic gap caused similar enhancements of sensitivity. Neither type of gap had much effect when the fields were flashed. The results may be interpreted as showing that either a luminance or chromatic contour can spatially delimit the two half-fields, thus preventing a slow spatial integration from reducing the discriminability of the two sides of the field.

Chromatic border distinctness: Not an index of hue or saturation differences*

Some investigators have suggested that the distinctness of chromatic borders (i.e., borders visible in photic arrays of uniform luminance) can be used as an index of hue and saturation differences between lights. However, recent evidence indicates that only two types of cones in the trichromatic eye contribute to chromatic border perception. A series of experiments are reported that were designed to discriminate between these alternatives, utilizing mainly the short-wavelength visible spectrum. The results support the notion that only R and G cones in the trichromatic eye mediate the perception of chromatic borders; thus the distinctness of such borders alone cannot be used as an index of either hue or saturation differences, because both of these aspects of color involve contributions from B cones.

Chromatic border perception: The role of red- and green-sensitive cones

Where the perception of borders is concerned, the normal visual system is found to be tritanopic. The strength of a border depends on the relative activity of only R and G cones, no matter what the B cones are doing. Since normal color vision depends on the activity of all three cone types, chromatic border strength alone cannot be used as an index of color differences. Borders produced by spatial luminance steps or chromatic steps appear similar, except that Mach-type lateral inhibitory mechanisms, which enhance the visibility of low-contrast luminance borders, are unnecessary to predict chromatic border strength. Two functions are presented, using only the responses of R and G cones, which provide a way of expressing chromatic border strength in terms of either a subjective rating scale or a luminance step that appears to be equivalent in its distinctness.

Monkey superior colliculus: properties of single cells and their afferent inputs

1. Receptive-field (RF) properties of 212 single cells in the superior colliculus of paralyzed macaque monkeys were studied with microelectrodes. Units were divided into superficial (0-1 mm) and deep (1-2.5 mm) layers. Orthodromic action potentials were evoked in these cells by shocking optic chiasm. 2. The vast majority of superficial cells responded to stationary or moving stimuli with transient bursts of activity and were nondirectionally selective. Moving stimuli were most effective and three main cell groups, based on response patterns to leading and trailing stimulus edges, were identified. 3. All cells had chromatically nomopponent RFs, as judged by their spectral response functions in the presence of neutral and chromatic backgrounds and on their lack of response to moving, equal-luminance chromatic borders. 4. With the exception of some very short and very long values, orthodromic latencies were unimodally distributed with a mean of 7.8 ms. The prime determinant of a cell's latency was its depth below the collicular surface rather than a specific RF feature. 5. Cells with shorter latencies (located in superficial layers) were able to reliably signal high-velocity stimulus movement; those with longer latencies (located in deeper layers) reliably signaled low-velocity motion only. 6. Results support the hypothesis that response latency is related to differences in RF organization between layers.

The “fluttering heart” and spatio-temporal characteristics of color processing—II: Lateral interactions across the chromatic border

Two experiments are reported investigating the hypothesis that lateral interactions between the color processing mechanisms across the chromatic border are involved in the “fluttering heart” phenomenon. The fact that the effect can be destroyed by separating the colored target from the colored back-ground by a black line is compatible with this notion. The extent of the interaction (10′ and 35′, respectively) corresponds well with similar data in the literature. Estimating the extent of their interaction from a variation of the target width leads to the same result.

The “fluttering heart” and spatio-temporal characteristics of color processing—I: Reversibility and the influence of luminance

The classic phenomenon of the “fluttering heart” was investigated in two experiments. It was demonstrated that the effect consists of a genuine delay and not a phase difference. Its reversibility presents serious problems for two earlier explanations based on the dependence of perceptual latencies on wave-length and the differential latencies of the rod and cone apparatus, respectively. It was further shown that a luminance difference between target and background is neither a sufficient nor a necessary condition, but that a color difference is necessary. It was suggested that interactions between the color processing mechanisms across the chromatic border are involved.