Neural pathways underlying human 3D motion perception

Julie HarrisMartin Giesel, Benjamin Portelli, Alex Wade, Milena Kirstner, Ryan Maloney, Marina Bloj, Alison Bruce (funded by BBSRC)

We use our eyes and brain to move confidently within our surroundings without bumping into things, identifying objects as dangerous or attractive and parsing subtle changes in facial expression. Vision is a hugely complex process that uses much of the brain’s resources and involves a constant trade-off between energetic efficiency, speed and accuracy. How is this achieved? Clues to answer this question come from fundamental biology. Anatomically, it is striking that there are multiple pathways in the visual system and neurons in different visual areas and pathways appear differentially sensitive to certain types of visual information, such as colour or motion. It is clear that some visual brain areas and pathways have evolved at different times and for different functions. Dedicating different pathways to different functions can be a way of reducing the complexity of the processing problem – allowing the brain to compute independent properties in parallel. Here, we are interested in a specific set of pathways that seem to show strong independence of this type: those involved in the perception of motion in three dimensional space. Whilst motion is known to be critical for the ‘where’ functions of the dorsal pathway, very little attention has been placed on how binocular information for motion is processed, nor what pathways carry out that processing. In this project we explore how binocular visual information about motion-in-depth (MID) is processed and carried by several different visual pathways. Two computational processes have been proposed for using binocular information for MID, and there is evidence for each of them being useful for human vision. Are these signals processed along different fundamental pathways in the brain? Why is it interesting to ask this question? (1) Because neither pathway is fully understood: the sites and natures of the computations involved in processing MID in two ways have not been identified. Even more intriguingly, while one pathway, has been much studied, the other is barely explored, very poorly understood and potentially ancient, in an evolutionary sense. (2) The two pathways might perform different functions, and we propose a series of studies to specifically explore what those functions might be.

Our project has very broad scope, we explore the nature of the putative pathways at the anatomical level, using functional magnetic resonance imaging (fMRI) to localize function, and source-imaged electroencephalography (EEG), which can be used to understand the temporal dynamics of visual processing. We are using psychophysical behavioural studies to study what computational processes take place during MID perception, using both a normal population to explore normal function, and a clinical group of subjects (strabismic amblyopes) that we know have compromised MID processing using one specific pathway. Additionally using Transcranial Magnetic Stimulation (TMS) to degrade information in a particular visual area we are testing the causal relevance of MID-responsive regions identified with fMRI and EEG. Finally, we also employ eye-tracking methods to understand what specific sources of MID information are useful for. Using all these techniques will allow us to get a full picture of the processes underlying MID. To achieve this we require the expertise of three institutions (St. Andrews, York, Bradford). Our work is primarily core visual neuroscience. However, it has implications for human health. One of our techniques will exploit the fact that a person with a squint (strabismic amblyopes) is unable to use a core source of MID information, namely binocular disparity. There are hints that this group may be able to use other sources of MID. Our group are the first to explore this issue comprehensively.