biomedical sciences research institute - Research - University of Ulster

Dr Jan LauritzenLecturer in OptometryDr Lauritzen’s research focuses on human psychophysics of early visual processes in the retina, primary visual cortexand extrastriate areas, and computer modelling of those processes. In collaborations, Dr Lauritzen also has a widermultidisciplinary interest in computer modelling of physiological processes in general and the design of psychophysicalprotocols for sensory physiology and experimental psychology.Computer modelling:An array of model receptivefields used to represent simplecells in V1The early steps in vision are mediated by cells with relatively simple receptive fieldsthat have been well described. However, computer models using such receptive fieldsnevertheless provide poor predictions of psychophysical performance in tasks thoughtto be mediated exclusively or predominantly in the retina and primary visual cortex (V1),especially when the test image is complex, such as a natural image. Computer modellingof the connections between cells in V1 involved in contrast gain control mechanisms andresponses from outside the classical receptive field has been used to improve predictionof psychophysical results in contrast discrimination tasks involving natural images.A similar computer model has also been used to demonstrate that contrast gain controlcan adequately account for differences in the contrast in natural images produced bydifferent lighting conditions (diffuse and direct), thus providing a form of ‘contrastconstancy’.Example of an image pair of foliage taken in direct light (left) and diffuse illumination (right). A model of contrastnormalisation virtually eliminates the differences between the two images.Human psychophysicsContrast discrimination tasks, where human observers are detect a simple contrast stimulus before a maskingbackground, provide well-understood, predictable thresholds when the mask is also simple. However as the maskbecomes more complex, thresholds become rapidly less predictable, and for natural or pseudo-natural images resultscan be counter-intuitive. For example, masking experiments on natural images have shown that, when an image isfiltered to remove the contrast that should optimally mask a particular contrast channel, thresholds often increase(i.e. the task becomes harder). Intracortical interactions are thought responsible, and computer models are beingused to determine the nature of these interactions, in view of predicting psychophysical thresholds correctly fordifferent types of stimuli with a single model.117