A model of colour appearance based on efficient coding of natural images
An object’s colour, brightness and pattern are all influenced by its surroundings, and a number of visual phenomena and “illusions” have been discovered that highlight these often dramatic effects. Explanations for these phenomena range from low-level neural mechanisms to high-level processes that incorporate contextual information or prior knowledge. Importantly, few of these phenomena can currently be accounted for when measuring an object’s perceived colour. Here we ask to what extent colour appearance is predicted by a model based on the principle of coding efficiency. The model assumes that the image is encoded by noisy spatio-chromatic filters at one octave separations, which are either circularly symmetrical or oriented. Each spatial band’s lower threshold is set by the contrast sensitivity function, and the dynamic range of the band is a fixed multiple of this threshold, above which the response saturates. Filter outputs are then reweighted to give equal power in each channel for natural images. We demonstrate that the model fits human behavioural performance in psychophysics experiments, and also primate retinal ganglion responses. Next we systematically test the model’s ability to qualitatively predict over 35 brightness and colour phenomena, with almost complete success. This implies that contrary to high-level processing explanations, much of colour appearance is potentially attributable to simple mechanisms evolved for efficient coding of natural images, and is a basis for modelling the vision of humans and other animals.
Colour processing in the mouse brain for vision and beyond
Colour vision plays important roles in regulating animal behaviour, yet understanding of how such information is processed in the brain is still incomplete. Here I discuss our work addressing this issue in mice where, despite aspects of retinal organisation that might suggest limited capacity for colour vision, we find evidence of extensive cone-dependent spectral opponency across subcortical visual centres. In particular, our data both reveals important contributions of such colour signals to non-image-forming functions (regulation of the circadian system) but also indicate surprisingly sophisticated support for more conventional aspects of colour vision.
The neuroscience of color and what makes primates special
Among mammals, excellent color vision has evolved only in certain non-human primates. And yet, color is often assumed to be just a low-level stimulus feature with a modest role in encoding and recognizing objects. The rationale for this dogma is compelling: object recognition is excellent in grayscale images (consider black-and-white movies, where faces, places, objects, and story are readily apparent). In my talk I will discuss experiments in which we used color as a tool to uncover an organizational plan in inferior temporal cortex (parallel, multistage processing for places, faces, colors, and objects) and a visual-stimulus functional representation in prefrontal cortex (PFC). The discovery of an extensive network of color-biased domains within IT and PFC, regions implicated in high-level object vision and executive functions, compels a re-evaluation of the role of color in behavior. I will discuss behavioral studies prompted by the neurobiology that uncover a universal principle for color categorization across languages, the first systematic study of the color statistics of objects and a chromatic mechanism by which the brain may compute animacy, and a surprising paradoxical impact of memory on face color. Taken together, my talk will put forward the argument that color is not primarily for object recognition, but rather for the assessment of the likely behavioral relevance, or meaning, of the stuff we see.
The Blurry Beginnings: What nature’s strangest eyes tell us about the evolution of vision
Our study reveals the most elaborate opsin expression patterns ever described in any animal eye. In mantis shrimp, a pugnacious crustacean renowned for its visual sophistication, we found unexpected retinal expression patterns highlighting the potential for cryptic photoreceptor functional diversity, including single photoreceptors that coexpress opsins from different spectral clades and a single opsin with a putative nonvisual function important in color vision. This study demonstrates the evolutionary potential for increasing visual system functional diversity through opsin gene duplication and diversification, as well as changes in patterns of gene coexpression among photoreceptors and retinula cells. These results have significant implications for the function of other visual systems, particularly in arthropods where large numbers of retinally expressed opsins have been documented.
Young IBRO NextInNeuro Webinar - The retinal basis of colour vision: from fish to humans
Colour vision is based on circuit-level comparison of the signals from spectral distinct types of photoreceptors. In our own eyes, the presence of three types of cones enable trichromatic colour vision. However, many phylogenetically ‘older’ vertebrates have four or more cone types, and in almost all their cases the circuits that enable tetra- or possibly even pentachromatic colour vision are not known. This includes the majority of birds, reptiles, amphibians, and bony fish. In the lab we study neuronal circuits for colour vision in non-mammalian vertebrates, with a focus on zebrafish, a tetrachromatic surface dwelling species of teleost. I will discuss how in the case of zebrafish, retinal colour computations are implemented in a fundamentally different, and probably much more efficient way compared to how they are thought to work in humans. I will then highlight how these fish circuits might be linked with those in mammals, possibly providing a new way of thinking about how circuits for colour vision are organized in vertebrates.