Colour Vision

POSTER at AVA/BMVA 2012 in Cambridge: Image discomfort: its biological basis and utility - a review

by Sarah M Haigh

Arnold J Wilkins and Sarah M Haigh
A review of published and in prep work

Observers show consistency in the images they judge to be uncomfortable. The discomfort can be predicted from the... more Observers show consistency in the images they judge to be uncomfortable. The discomfort can be predicted from the statistics of the image, and this applies to images ranging from photographs of everyday scenes to geometric arrays, coloured or in motion. Images are generally rated as uncomfortable to view if (1) the luminance contrast energy has a power spectrum that departs from 1/f, or (2) the CIE UCS chromaticities are widely separated, or (3) the contours flicker at about 20Hz. For the same variety of images, those that are uncomfortable result in a large cortical haemodynamic response, measured either with fMRI or near infrared spectroscopy. Individual differences in the size of the haemodynamic response appear to reflect a cortical hyperexcitability, common in migraine, and providing clues as to the mechanisms of discomfort. The biological utility of the discomfort can be understood not only as homeostatic, but also as an evolutionary adaptation because images of venomous animals possess the properties of uncomfortable images and are indeed uncomfortable to view. There are far-reaching implications for the design of our visual environment because much of our environment has characteristics that are uncomfortable.

POSTER at ECVP 2011: Accommodation to chromatic gratings.

by Sarah M Haigh

Haigh, S. M., Allen, P. M., & Wilkins, A. J. (2011). Accommodation to chromatic gratings. Perception, 40 (ECVP Abstract Supplement), 80.

Wilkins, Tang, Irabor and Coutts (2008) measured discomfort from isoluminant square-wave gratings and showed that the... more Wilkins, Tang, Irabor and Coutts (2008) measured discomfort from isoluminant square-wave gratings and showed that the discomfort increased with the separation within the CIE UCS diagram of the chromaticities of the component bars, regardless of the hue. The gratings with larger separation elicited a cortical haemodynamic response of greater magnitude. The discomfort and larger haemodynamic response may arise because accommodative mechanisms relax when the chromaticity difference is large – Allen at al. (2010) found a greater lag of accommodation to achromatic gratings for those who found them uncomfortable. We therefore used an open field autorefractor to measure the accommodative response to the gratings. No correlation was found between the separation of chromaticities and the accommodative response, suggesting that the discomfort is not due to a failure to accommodate to the stimuli. However, participants who experienced pattern-related visual stress, showed a greater accommodative lag to the gratings overall than those who were symptom-free. This suggests that although accommodative mechanisms are unlikely to cause the discomfort, those who find gratings uncomfortable generally relax their accommodation when looking at an uncomfortable target.

References
Allen, P., Hussain, A., Usherwood, C., Wilkins, A. (2010). Pattern-related visual stress, chromaticity and accommodation. Investigative Ophthalmology and Visual Science, 2010(51), 6843-6849.
Wilkins, A. J., Tang, P., Irabor, J., Baningham, L., Coutts, L. . (2008). Cortical haemodynamic response to coloured gratings. Perception, 37(ECVP Abstract Supplement), 144.

POSTER at AVA 2011: Colour separation and aversion

by Sarah M Haigh

Haigh S M, Tang P, Barningham L, Coutts L, Allen P M, Wilkins A J, 2012, "Colour separation and aversion" i-Perception 3(4) 232

Aversion to achromatic patterns is well documented but relatively little is known about discomfort from chromatic... more Aversion to achromatic patterns is well documented but relatively little is known about discomfort from chromatic patterns. Large colour differences are uncommon in the natural environment and deviation from natural statistics makes images uncomfortable (Fernandez & Wilkins, 2008, Perception, 37(7), 1098-113; Juricevic, Land, Wilkins & Webster, 2010, Perception, 39(7), 884-899). We report twelve studies documenting a linear increase in aversion to chromatic square-wave gratings as a function of the separation in UCS chromaticity between the component bars, independent of their luminance contrast. Two possible explanations for the aversion were investigated: (1) accommodative response, or (2) cortical metabolic demand. We found no correlation between chromaticity separation and accommodative lag or variance in lag, measured using an open-field autorefractor. However, near infrared spectroscopy of the occipital cortex revealed a larger oxyhaemoglobin response to patterns with large chromaticity separation. The aversion may be cortical in origin and does not appear to be due to accommodation.

Extent and duration of practice effects on performance with the Farnsworth-Munsell 100-Hue test

by David Foster

The extent and persistence of practice effects on serial performance in the Farnsworth-Munsell 100-Hue test (100-Hue... more The extent and persistence of practice effects on serial performance in the Farnsworth-Munsell 100-Hue test (100-Hue test) were evaluated in an experiment in which six subjects performed the 100-Hue test up to 17 times over six weeks, and then once more after 7 months. A practice effect occurred which was highly statistically significant for the group as a whole (P < 0.001) and for three individual subjects (P < 0.05). A practice effect was still evident 7 months after the last test performance. Error scores were reduced almost to zero after 5-10 retests so that it was unclear whether the effects of practice had disappeared or whether failure to improve further was a 'floor effect'. As a control against a floor effect, a second experiment was performed in which subjects' performance was impaired by placing neutral density fflters in front of their eyes (thus artiffcially raising their 100-Hue error score). Under these conditions, error scores continued to fall and were halved after 17 tests (P < 0.03). It is concluded that practice has a large effect on 100-Hue test performance which continues over many retests arid for many months after testing.

Isolation of opponent-colour mechanisms at increment threshold

by David Foster

An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment... more An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment threshold. The effectiveness of a uniform white conditioning field spatially coincident with a 1.05-deg uniform test field was assessed by measuring intensity thresholds for simple detection and for colour discrimination. Values were obtained both by a method of adjustment and by a two-interval forced-choice procedure. For sufficiently high luminances of the conditioning field (3000 td or greater) little or no difference was found between simple-detection and colour-discrimination thresholds over the critical test-flash spectral range 520-620 nm, implying that the paradigm produced almost complete isolation of the opponent-colour system at increment threshold. A control experiment in which thresholds were obtained for a conditioning field larger than the test field gave less satisfactory isolation; near 580 nm the luminance system was found to be at least 0.31 log unit more sensitive than the opponent-colour system. A comparison was also made of the spatially coincident field paradigm with a paradigm in which a modified test stimulus of low temporal and spatial frequency content was presented on a large conditioning field. Test spectral sensitivity curves for simple detection obtained by a method of adjustment showed little difference in effectiveness in opponent-colour isolation.

Four issues concerning colour constancy and relational colour constancy

by David Foster

Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of... more Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of colour constancy and relational colour constancy; (2) the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association of such ratios with von Kries’ coefficient rule; (3) the contribution of chromatic edges to colour constancy and relational colour constancy; and (4) the effects of instruction and observer training. It is suggested that cognitive factors affect colour constancy more than relational colour constancy, which may be an inherently more robust phenomenon.

Psychophysical estimates of the number of spectral-reflectance basis functions needed to reproduce natural scenes

by David Foster

Theoretical analyses of spectral reflectances of natural surfaces suggest that their perceived colors can be well... more Theoretical analyses of spectral reflectances of natural surfaces suggest that their perceived colors can be well reproduced by approximations comprising combinations of three or four spectral basis functions. The aim of the present work was to assess psychophysically the number of basis functions necessary to reproduce entire natural outdoor scenes. Hyperspectral images of 20 such scenes were each subjected to a principal component analysis and then reproduced with a variable number of basis functions. The quality of the color approximation under daylight illumination was quantified theoretically in CIELAB space and psychophysically by spatial and temporal two-alternative forced-choice measurements in which the original and the approximated images were compared on a calibrated color monitor. Although five basis functions produced on average unit error in CIELAB space, original images were visually indistinguishable from their approximations only if there were at least eight basis functions. The combination of the spectral diversity of the natural world and the observed levels of color discrimination suggest that estimates of the minimum number of basis functions necessary to reproduce natural scenes may need to be revised upward.

Individual differences provide psychophysical evidence for separate S-on and S-off channels

by Patrick Goodbourn

Bosten, J.M., Bargary, G., Goodbourn, P.T., Hogg, R.E., Lawrance-Owen, A.J., & Mollon, J.D. (2010, abstract only). Individual differences provide psychophysical evidence for separate S-on and S-off channels. Investigative Ophthalmology and Visual Science, 52, e-abstract 3907.

Purpose: There are two distinct populations of retinal ganglion cells that carry signals from the s-cones: those that... more Purpose: There are two distinct populations of retinal ganglion cells that carry signals from the s-cones: those that receive excitatory input (S+) and those that receive inhibitory input (S-). The two cell populations are morphologically, anatomically and functionally distinct. We used individual differences in human visual performance as a method of determining whether the two pathways contribute separately to perceptual abilities.

Methods: We used a psychophysical test based on the Cambridge Colour Test, but in which detection thresholds for S-cone spatial increments and S-cone spatial decrements were measured separately. Thresholds for increment and decrement detection were estimated each as the average of two ZEST staircases, run in four separate blocks. 1062 participants aged 16-40 took part, as part of the PERGENIC test battery. 105 participants were tested on two different occasions, separated by at least a week, and data from these participants form the basis of our test-retest reliabilities.

Results: Detection thresholds for S-cone increments and decrements each showed good test-retest reliability ( = 0.64, 0.67 respectively). The ratio of increment to decrement thresholds also showed significant test-retest reliability ( = 0.48). The correlation between thresholds for increments and thresholds for decrements was  = 0.65 (n = 1062).

Conclusions: Significant test-retest reliabilities show that stable individual differences in S-cone sensitivity can be measured psychophysically in a normal adult population aged 16-40. At least 40% of the variance in S-cone stimulus thresholds is shared between increments and decrements. However, a further portion of the variance (at least 20%) is stable across individuals, but unique to S-cone increments or decrements. Thus there is both a shared mechanism contributing to S-cone increment and decrement sensitivity, and mechanisms that are distinct for each type of stimulus.

Colour constancy from temporal cues: better matches with less variability under fast illuminant changes

by David Foster

To test whether temporal transient cues could improve colour- constancy estimates, surface-colour matches were made... more To test whether temporal transient cues could improve colour- constancy estimates, surface-colour matches were made across two Mondrian patterns illuminated by different daylights: the patterns were presented either in the same position in an alternating sequence or, as a control, simultaneously side-by- side. The degree of colour constancy was significantly higher with sequential stimulus presentation than with simultaneous presentation, in the best condition reaching 0.87 on a scale of 0 to 1 for matches averaged over 20 observers. The variance between observers was also markedly reduced with sequential stimulus presentation. The visual system appears to have mechanisms not requiring adaptation that can provide almost unbiased information about surface colour under changing illuminants.

Motion coherence across different chromatic axes

by Simon Cropper

Absence of linear subthreshold summation between red-green and luminance mechanisms over a wide range of spatio-temporal conditions

by Simon Cropper

The cone inputs to the unique-hue mechanisms

by Simon Cropper

Motion of chromatic stimuli: first-order or second-order?

by Simon Cropper

The detection of motion in chromatic stimuli: First-order and second-order spatial structure

by Simon Cropper

Detection of chromatic and luminance contrast modulation by the visual system

by Simon Cropper

Discriminating smooth from sampled motion: chromatic and luminance stimuli

by Simon Cropper

The detection of motion in chromatic stimuli: Pedestals and masks

by Simon Cropper

The perception of motion in chromatic stimuli

by Simon Cropper

Rapid colour-specific detection of motion in human vision

by Simon Cropper

Number of perceptually distinct surface colors in natural scenes

by David Foster

The ability to perceptually identify distinct surfaces in natural scenes by virtue of their color depends not only on... more The ability to perceptually identify distinct surfaces in natural scenes by virtue of their color depends not only on the relative frequency of surface colors but also on the probabilistic nature of observer judgments. Previous methods of estimating the number of discriminable surface colors, whether based on theoretical color gamuts or recorded from real scenes, have taken a deterministic approach. Thus, a three-dimensional representation of the gamut of colors is divided into elementary cells or points which are spaced at one discrimination-threshold unit intervals and which are then counted. In this study, information-theoretic methods were used to take into account both differing surface-color frequencies and observer response uncertainty. Spectral radiances were calculated from 50 hyperspectral images of natural scenes and were represented in a perceptually almost uniform color space. The average number of perceptually distinct surface colors was estimated as 7.3 x 10^3, much smaller than that based on counting methods. This number is also much smaller than the number of distinct points in a scene that are, in principle, available for reliable identification under illuminant changes, suggesting that color constancy, or the lack of it, does not generally determine the limit on the use of color for surface identification.