Sensing the sky: how top-down mechanisms improve colour constancy (#454)
Colour constancy, the ability to perceive colours as being equivalent independent of changes in illumination, is a property assumed essential for colour vision. Colour constancy has been evidenced in various animals including: honeybees,1 hawkmoths,2 goldfish,3 primates 4 and humans,5 and the potential mechanism of chromatic adaptation has been included in commonly used colour vision models 6 7 8 9 10 by applying an elegant mathematical formulation first proposed by von Kries more than a century ago.11 Indeed, chromatic adaptation shows evidence of being an important contributing mechanism of colour constancy in bees.1 12 Nevertheless, recent behavioural data from bees also suggests that these animals can directly perceive changes in the ambient illumination 13 and use such information in complex environments. 14 Furthermore, evidence of the capacity of bees to use contextual information about illumination colour to make behavioural choices 15 suggests a possibility that ‘top-down’ mechanisms may also take part in solving difficult colour discrimination tasks. We addressed this question by modelling a visual system where a top-down mechanism, expressed as a priori information about the spectral characteristic of the ambient illumination, is used for estimating the coefficients required by the von Kries constancy model. Simulations predict that an a priori, biologically relevant guiding estimate of the ambient illumination provided to the visual system significantly improves colour constancy of flower targets when considering most conditions of daylight (Figure). Interestingly, our model also predicts significant failures in the chromatic adaptation process at low correlated colour temperatures as those observed in bees when performing colour discrimination tasks under artificial, tungsten light. 1 12 We discuss plausible physiological mechanisms facilitating top-down processing, and how such solutions could lead to bio-inspired developments to improve colour constancy algorithms required to be implemented in digital solutions for imaging in natural conditions. 16
Figure:Predicted colour shift for different flowers without (a) and assuming top-down information (b). Colour space makes no assumptions about opponent mechanisms.17
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- Garcia J.E., Greentree A.D., Shrestha M., Dorin A., Dyer A.G. 2014 Flowers through the lens: Quantitative measurement with visible and ultraviolet digital photography. PLOS ONE 9(5), e96646.
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