Individual Differences in Color Vision Assessed with Chromatic Textures

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Veterans with mild traumatic brain injury (mTBI) often report visual problems despite normal or near-normal performance on clinical tests of afferent vision. Strikingly, a similar pattern is observed in the auditory domain, which suggests two potentially overlapping explanations for the (multi-)sensory deficits in mTBI: (a) damage to the ascending sensory pathways produces subtle deficits that may not be detectable on standard clinical tests; and (b) changes at more central levels of sensory processing or in cognitive, affective, or associative brain networks lead to deficits in processing complex sensory environments. Therefore, an ideal measure of sensory capacity in mTBI should account for changes at these different levels of processing. Returning to the domain of vision, recent research suggests that color vision is highly sensitive to acquired disorders or trauma affecting the early visual pathway. At the same time, color plays a significant role in higher visual functions such as object segregation and visual attention. These properties make color vision an ideal candidate for assessment of visual function in mTBI. However, current tests of color vision focus on detection of (primarily congenital) losses originating at the retinal color receptors. The proposed study begins to develop a new test of color vision designed to capture changes at low (e.g., retinal) and high (e.g., cortical) levels of visual processing. The test relies on discrimination of chromatic textures whose individual elements are assigned an equiluminant shade along a continuum from pure green to pure red. The distribution of colors follows a statistical pattern such that discrimination among two textures with opposing patterns relies on the visual system’s ability to extract those patterns. Different types of patterns are used to probe different aspects of color processing. Pattern extraction is assumed to rely on a limited number of mechanisms sensitive to different shades of red and/or green and a ‘mixing module’ that allows the mechanisms to be combined in different ways to encode different patterns. A mathematical model of texture discrimination performance is used to enumerate, for a group of observers: (a) the number and structure of the low-level mechanisms; (b) each observer’s sensitivity to those mechanisms; and (c) each observer’s unique mixing proportions. Performance across different chromatic texture patterns is taken to reflect a given observer’s unique profile of red-green vision. This proposal aims to validate this paradigm in groups of younger (25-34 yrs.) and older (55-64 yrs.) Veterans with normal color vision, given that low-level color sensitivity is known to decrease by ~10% per decade of life with considerable individual differences in high-level color processing within and between age groups. The first specific aim is to determine whether chromatic texture discrimination captures aspects of color processing beyond those captured by well-established chromatic sensitivity measurements. Red-green chromatic sensitivity thresholds from the Color Assessment and Diagnosis (CAD) test will be entered with chromatic texture discrimination thresholds in a factor analytic model, where the hypothesis is that chromatic texture discrimination thresholds will load on a different factor than CAD thresholds. A novel ‘suprathreshold’ measure of chromatic sensitivity based on color-defined coherent motion will be included to capture variation in chromatic sensitivity at higher levels of processing. The second specific aim will apply the aforementioned mathematical model of chromatic texture discrimination to characterize individual differences in bottom-up (mechanism sensitivity) and top-down (mechanism mixing) color processing. Achievement of these aims will allow deployment of color vision tests – texture-based or otherwise – to characterize the visual deficits in mTBI by providing a baseline for normal variation in color vision at multiple ages and levels of processing. Notably, the red-green texture discrimination paradigm can be easily extended to yellow-blue and grayscale vision. The long-term goal for this line of work is to combine texture-based and other tests of complex visual function with related tests of complex auditory function to determine the extent to which multisensory deficits at different levels of processing contribute to the functional deficits observed in mTBI.