Early advantage of luminance for object representation and its cross-talk with chromatic pathways in human visual scene analysis

Detection and identification of objects is the most crucial goal of human visual perception. Several parallel channels in the visual system processes incoming information with that purpose, segmenting the images in both eyes through a series of rapid hierarchically organized stages. This rapid hierarchical processing stream involves a cascade of neural activity that encompasses a series of brain areas, from primary sensory regions that analyse separate visual features (low-level vision), through parts that organise the percept into figures and background (mid-level vision), to parts of the brain that store semantic knowledge on familiar objects (high-level vision). A coherent representation of our environment is thus formed in less than 300 milliseconds of processing time within a range of highly varied brain regions. In the study on visual perception, it is crucial to investigate in which way does the brain manage to coordinate the processing of information on simple visual features such as colour and luminance along each of the transformative stages (low, mid, high-level) that lead to the perception of objects. It is well-known from studies on animals that separate visual channels in mammalian brain process achromatic and chromatic information: magnocellular pathway processes luminance information, while parvo- and koniocellular pathways predominantly subserve colour processing. Both are crucial for everyday object vision but their contributions differ, with luminance considered to be more relevant for rapid processing of lines, edges, shape and motion and colour being more relevant for segmentation of visual scenes. But the extent to which various visual pathways function independently or interactively at different stages of visual processing remains unknown even after many years of study, due to the difficulties in 1) producing stimuli that selectively elicit processing along different pathways and 2) analysing the rapidly evolving neural processes that subserve normal human vision. We intend to conduct a study that will overcome these problems through an innovative experimental approach which joins electroencephalography's (EEG's) ability to divulge millisecond differences in rapid neural processes that underlie human visual perception with the tools of colour psychophysics which allow us to separate out different visual processing streams by defining our stimuli in three-dimensional colour space (a luminance dimension and two chromatic dimensions). The stimulus displays will be controlled through a visual stimulus generator that would enable systematic and concurrent control of inputs into chromatic and achromatic mechanisms. The timecourse of cortical activations and their underlying generators will be identified and the interactions between different pathways modelled by using stimuli that elicit excitations in single (magno-, parvo- or konio-) or multiple (two or all three combined) pathways. The findings of this study will provide an important insight into the ways in which the human brain utilises different types of information during parallel visual processing and will thus significantly contribute to current knowledge on the relations between parallel (magnocellular, parvocellular or koniocellular) and hierarchical (low, mid, high-level) processing. We will be able to describe the neural mechanisms that allow preferential inputs of luminance information into object representation processes and thus enable it to drive everyday vision. A further advantage will be provided by the description of koniocellular contributions to vision which have so far not been studied extensively since this pathway has only recently become the subject of systematic study in human participants.

Luminance information plays a unique role in everyday vision by providing inputs that drive the efficiency and speed of object representation. Chromatic information also contributes to high-level vision, both independently and through interacting with luminance information at higher stages of cortical processing - but the extent to which they function independently or interactively remains unknown. In our study, we will join electroencephalography's (EEG's) ability to divulge millisecond differences in rapid neural processes with the tools of colour psychophysics which allow us to bias processing towards the luminance or chromatic pathways. We will define our stimuli in three-dimensional colour space whose directions correspond to magno-, parvo- and koniocellular pathways. This will enable us to test the early neural markers of processing that is biased towards a single pathway, drives two pathways or drives all three pathways at different stages of the visual hierarchy (low-, mid- or high-level). Early event-related activity elicited by stimuli that are processed predominantly along a single pathway will be measured to obtain its characteristic activation. This will allow us to model the evolution of rapid cortical responses elicited by summation of this information from different geniculate pathways. Source localisation will allow us to to look for differences in the excitation of various extrastriate areas that relate to the degree of activation of the luminance pathway. In study 1 which examines low-level vision, participants will perform orientation discrimination between circular horizontal or vertical Gabors. In study 2 which examines mid-level vision, participants will discriminate displays with fully-random arrays of gabors from displays with a pseudo-random array containing a contour. In study 3 which examines high-level vision, participants will discriminate line-drawings of real-life objects from scrambled images of such objects (so-called non-objects).

The primary impact of the proposed project will be an increased understanding of processes that underlie visual perception. Perceptual experiences form the basis of human consciousness - for example, the percept of colour red is related to an internal awareness of the sensation of redness, which is a question that is of interest to cognitive scientists, philosophers as well as the public at large. Many popular science books treat this topic. But the study of basic analysers in vision (luminance or colour pathways, for example) has so far often been neglected by scientific writers as unapproachable to the general public due to its focus on the study of elementary neural processes related to simple stimuli (e.g. gratings, gabor patches, lines of different orientation). The proposed study will answer important questions about the role of elementary visual processes for everyday vision. It will demonstrate how certain types of information have preferential role in forming high-level representations of our environment. This is a matter that is of intrinsic interest to all members of our society. The research will also be highly interesting to both vision scientists and clinical neuroscientists, as deficits in the luminance pathway have been identified in a series of conditions such as dyslexia (K. Nandakumar and S.J. Leat, (2008), Clin. Exp. Optom., 91, 333-340), schizophrenia (P.D. Butler, et al., (2007), Brain, 130, 417-430), autism (K. Plaisted and G. Davis, (2005), Cah. Psychol. Cogn.-Curr. Psychol. Cogn., 23, 172-179) or Parkinson's disease (W.G. Sannita, et al., (2009), Vision Res., 49, 726-734). Research by Butler and colleagues shows that subcortical magnocellular deficits drive secondary cortical impairments in schizophrenia. By investigating the function of the visual pathways and their potential interactions in high-level vision, the proposed work will provide a starting point for clinically applied research which could potentially lead to symptom alleviation or prevention techniques. The work will be submitted for publication in high-impact journals. The resluts will also be communicated through media with an interest in scientific breakthroughs. Another major way of publicising the proposed research will occur through networking at scientific meetings, within the UK research community as well as abroad. Some of these meetings are dedicated to the application of scientific work and thus bring together the UK scientific community and the engineering sector: e.g. the Applied Vision Association or the UK colour group. This will provide an opportunity to network with experts who are ideally placed to implement possile insights gained from our work into improvements of human-machine interaction devices. For example, air traffic control instruments could benefit from knowledge about types of visual information that result in highest rates of detection or fastest reaction times. The reputation of the United Kingdom as one of the leading countries for fundamental research into visual perception (in particular, colour vision) and a center for cutting-edge discoveries in brain and cognitive sciences will be upheld by the advances made in the proposed research. The appearance of this work in top journals may attract more world-class researchers to work at institutions within the UK and further advance the scientific community here.