About the Project
Colour is one of the most relevant visual feature dimensions, widely used to cue attention in both art and industry - and, of course, in vision research experiments on attention. And yet, in spite of decades of research, it is still not fully understood which colour representations drive attentional selection. This is likely to be due to the lack of commonality between colour vision research and attention research: attention researchers often have impressive neuroscientific techniques at their disposal, but do not have the spectroradiometric tools and background knowledge of colour spaces necessary for running parametric studies on attention to colour. Meanwhile, colour vision science is generally concerned with colour appearance models and even when it addresses questions related to attention, it often lacks the neurscientific tools necessary for the in-depth study of its mechanisms.
Neural representations underlying colour perception in humans are highly complex, involving two types of opponent processes. At the earliest stages, the signals from the three cones (Short, Medium and Long-wavelength) are opposed into two chromatic, cone-opponent mechanisms which arise in the retina and are fully established already at the level of the lateral geniculate nuclei. However, in striate visual cortex, these signals are remapped into colour-opponent representations with multiple, finer tunings to both unique hues such as red, green, blue and yellow, as well as to various intermediate hues such as orange, magenta or turquoise. The striate cortex is considered to be the locus of the salience map that drives bottom-up visual attention. However, recent psychophysical evidence suggests that crucial inputs into salience stem from early, pre-cortical cone-opponent representations rather than colour-opponent representations (e.g., Switkes, 2008).
We aim to resolve this long-standing issue on cone-opponent and colour-opponent contributions to salience by measuring neural correlates of salience map activation for carefully selected colours stemming from both cone and colour-opponent representations. These neural correlates of salience map activation have been recently established in a series of electroencephalography (EEG) studies (e.g., Töllner et al., 2012). Töllner and colleagues have used pop-out visual search tasks, in which a single element in a display differs from the other elements in one feature (here, colour). When a target is present, an event-related potential (ERP) component known as the Posterior Contralateral Negativity (PCN) is observed, with its amplitude reflecting target salience. We will conduct a series of experiments using pop-out targets from different colour spaces, presented in different contexts (e.g., coloured targets among achromatic or chromatic distractors) in order to fully map out the contributions of colour representations to neural salience computations. In these experiments, we will use a psychophysical salience metric for different colours established by Switkes (2008). By using a range of iso-salient colours of different contrasts, we will be able to model the contribution of cone-opponent signals to reaction times and to the EEG signals. As a further aim, we will map out the cone-opponent contributions to another widely used neural measure of attention, the steady-state visual evoked potential (SSVEP). The SSVEP is a continuous oscillatory response to flickering visual stimulation that originates from the visual cortex. In a previous study, we have established the contribution of cone-additive luminance signals to salience as measured with SSVEP (Andersen, Müller, Martinovic, 2012), but it is still not known to what degree the SSVEP reflects the salience of colours and their constituent cone-opponent signals.
Our research has the potential to build a bridge between colour vision science and cognitive neuroscience of attention, joining the expertises of Dr Martinovic and Dr Andersen to provide the prospective student with in-depth knowledge from both disciplines. It will offer excellent training prospects for the student in various techniques of both colour science and human neuroscience, e.g. photometry and definitions of colour using various technical and physiological colour spaces, the use of Matlab to present stimuli and analyse data, as well as the aforementioned psychophysical and EEG experimental techniques. The gained technical and experimental skills will enable the student for a range of jobs in academia or industry, with both colorimetry and EEG being highly relevant to industry, with colour specifications relevant for architecture and industrial design, and SSVEPs being a core technique for brain-computer interfaces designed to communicate with patients with consciousness disorders.
References
Andersen, SK., Müller, MM. & Martinovic, J. (2012). Bottom-up biases in feature-selective attention. Journal of Neuroscience, 32, 16953-16958.
Switkes, E. (2008). Contrast salience across three-dimensional chromoluminance space. Vision Research, 48, 1812-1819.
Töllner, T., Rangelov, D., & Müller, HJ. (2012). How the speed of motor-response decisions, but not focal-attentional selection, differs as a function of task set and target prevalence. Proceedings of the National Academy of Sciences, 109 (28), E1990-E1999.