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

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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.