Neural mechanisms of colour appearance across the lifespan

Healthy older adults struggle with many visual tasks that appear simple to younger adults. Visual problems increase during adult aging due to multiple factors, related both to reduced inhibition in the visual cortex and to overall increases in neural noise. Perhaps surprisingly, colour appearance is one process which remains relatively stable across life, with lower-level changes compensated for by central cortical mechanisms which remain little understood. But cortical mechanisms that compensate for changes in colour appearance are only effective for relatively large stimuli; appearance of small stimuli is still affected, since aging leads to enlarged perceptive field sizes (for a review, see Werner, Delahunt, Hardy, 2004). As spatial attributes of the image are a core determinant of colour appearance, their manipulation thus provides a promising route for the study of compensatory neural mechanisms that enable stability of colour appearance throughout the lifespan.

In this project, we aim to characterise these compensatory neural mechanisms for the first time, using a combination of psychophysical and electroencephalographic (EEG) methods.

The project combines the strengths of Dr Martinovic's colour psychophysics lab, which concerns itself with the neural underpinnings of spatiochromatic vision (e.g., Jennings and Martinovic, 2014), with Dr Jentzsch's expertise in the use of individual differences (e.g. aging) as a way of interrogating neural function (e.g., Strozyk and Jentzsch, 2012). We will achieve our aim by conducting a systematic investigation of colour appearance in healthy aging. We will investigate appearance of chromatic elements of different sizes, different spatial frequencies, with surrounds of different proximity and density, and of different luminance levels. We will aim to establish boundary conditions between desaturated and unaltered colour appearance by manipulating the above listed factors, which we will then assess using both transient and steady-state visual evoked potentials. We predict that the cortical responsiveness to low-level chromatic signals will be reduced in over-60 year olds, and that this will have a major impact on the appearance of relatively small patches as well as patches with surrounds falling within their perceptive fields. The central compensatory mechanism may eliminate such low-level influences that originate within perceptive fields for larger stimulus sizes, possibly through an increase in signal-to-noise ratio and reduction in neural noise.

The findings will be relevant both for understanding changes in the neural underpinnings of colour appearance with aging, and for understanding of how compensatory cortical mechanisms operate. They will also be relevant for industries which aim to improve the quality of life of the elderly through user-friendly design.

The project will offer excellent training prospects for the student in techniques of both vision research and human neuroscience, e.g. photometry and definitions of brightness and colour using various technical and physiological colour spaces, the use of Matlab to present stimuli and analyse data, designing, conducting and analysing psychophysical and EEG experiments, as well as the experience of working with and assessing older participants. The gained technical and experimental skills should enable the student for a range of jobs in academia or industry.