Colour vision starts in the retina where light is absorbed in three different cone classes, sensitive to long-, medium-, and short-wavelength light. The cone signals then feed into three different post-receptoral channels, a luminance channel and two chromatic channels. Interestingly, these two chromatic channels do not correspond to perceptually salient colour mechanisms (red, green, yellow, blue, [1]), suggesting that the two sub-cortical chromatic channels are recombined in visual cortex into a different set of mechanisms.

unique hues

Colour constancy across the life span. Our behavioural experiments with a large sample of adult colour-normal observers [7] show that cortical hue mechanisms are almost invariant with age.  In contrast, chromatic discrimination performance declines with age. Understanding the age-related changes of the healthy visual system and how information is encoded in visual cortex is important for the early diagnosis of visual deficits brought about by neurodegenerative diseases such as Glaucoma [9].

Colour constancy across observers. Our results suggest that the human visual system is able to compensate for retinal  (peripheral) signal changes by adjusting the relative cone weightings of the cortical colour mechanisms. Such an adaptive weighting is useful to maintain colour constancy throughout the life span in the presence of known changes in the ocular media (yellowing of the lens) and retinal sensitivity losses. It may also be responsible for the small inter-observer variability [5,6,8] compared to the large differences in the observers’ retinal make-up. The mechanism underlying this hue compensation is still poorly understood, but it is likely that it utilises invariant sources in our visual environment. The idea that the human visual system can adjust the weightings of the chromatic mechanisms is also consistent with our recent results [10] showing that the relative scaling of the two chromatic mechanisms (L/L-M and the S/L+M) mechanisms seem to depend on the set of colours used in a particular experiment and on the task at hand. We speculate that observers may be able to adjust the weightings such that differences between colours are maximised.

Colour constancy across ambient illumination conditions. Human colour appearance mechanisms are not only remarkably constant across age and observers, they are also not much affected by different illumination conditions. Only unique green settings are affect by illumination changes (D65 to CWF)  [4], whereas red, yellow and blue are constant and not affected by a change in the prevailing illumination.

Colour coding in the brain. Our brain imaging results confirm that colour coding in the Lateral Geniculate Nucleus (LGN) is similar to post-receptoral coding in the retina [3]; in primary visual cortex (V1)  however, colour is encoded by unipolar (half-wave rectified) mechanisms, not by opponent mechanisms [2]. Our results are consistent with a hue map in visual cortex.




Dr. Laura Parkes, University of Manchester

Dr. Yannis Goulermas, Electrical Engineering, University of Liverpool

Dr. A. Choudhary, Eye & Vision Science,  UoL & Royal Liverpool Hospital

Dr. Kaida Xiao, School of Design, University of Leeds

Dr Dimos Karatzas, Barcelona, Spain

Dr. Jasna Martinovic, University of Aberdeen



[1] Wuerger, S. M., Atkinson, P., & Cropper, S. (2005). The cone inputs to the unique-hue mechanisms. Vision Research, 45(25–26).

[2] Parkes, L. M., Marsman, J.-B. C., Oxley, D. C., Goulermas, J. Y., & Wuerger, S. M. (2009). Multivoxel fMRI analysis of color tuning in human primary visual cortex. Journal of Vision, 9(1).

[3] Wuerger SM and Parkes L. (2011). Unique Hues: Perception and Brain Imaging. In: C. Biggam C, C. Hough, and D. Simmons (Eds). New Directions in Colour Studies;  Publisher: John Benjamin.

[4] Wuerger SM and Xiao K (2015). Colour Vision: Opponent Theory. In: Vision: Concepts.  Encyclopedia of Color Science and Technology, p.1-6.  Publisher: Springer. doi: 10.1007/978-3-642-27851-8_92-1

[5] Xiao, K., Wuerger, S., Fu, C., & Karatzas, D. (2011). Unique hue data for colour appearance models. Part I: Loci of unique hues and hue uniformity. Color Research & Application, 36 (5), 316-323.  DOI: 10.1002/col.20637

[6] Xiao, K., Fu, C., Mylonas, D., Karatzas, D., & Wuerger, S. (2013). Unique hue data for colour appearance models. Part II: Chromatic adaptation transform. Color Research & Application, 38(1), 22-29. doi: 10.1002/col.20637

[7] Wuerger, S.M. (2013).  Colour Constancy Across the Life Span: Evidence for Compensatory Mechanisms. PLoS ONE; 8 (5): e63921 DOI: 10.1371/journal.pone.0063921

[8] Kaida Xiao,  Michael Pointer, Guihua Cui, Tushar Chauhan and Sophie Wuerger (2015). Unique Hue Data for Colour Appearance Models. Part III:  Comparison with NCS unique hues. Colour Research & Application, doi: 10.1002/col.21898.

[9] Wuerger, S.M.,  Powell, J. Parkes L, Choudhary A. fMRI measurements in LGN and Primary Visual Cortex in Glaucoma.. Journal of Vision, 2015: VSS abstract .

[10] Martinovic, J., Wuerger, S. M., Hillyard, S. A., Müller, M. M., & Andersen, S. K. (2018). Neural mechanisms of divided feature-selective attention to colour. NeuroImage, 181, 670–682.

[11] Wuerger SM, Xiao, K, Ashraf, M, and E Self (2020). Color Vision: Unique hues and color-opponent processing. In: Vision: Concepts.  Encyclopedia of Color Science and Technology, p.1xx-xx  Publisher: Springer. doi: 10.1007 (under review).  To download: Data in XYZ, SPD of the display phosophors.


Acknowledgement of support

EPSRC EP/C000404/1

UK and Eire Glaucoma Society

Technology Strategy Board (now InnovateUK)