Motion mechanisms and cortical areas in human vision : psychophysics and fMRI

Our visual world contains both luminance- (first-order) and contrast-defined (second-order) information. Distinct mechanisms underlying the perception of first-order and second-order motion have been proposed from electrophysiological, psychophysical and neurological studies. In this thesis psychophysical and human brain imaging (fMRI) experiments are described that support the notion of distinct mechanisms, but extend the previous studies by providing evidence for a functional dissociation and a relative cortical specialization for first- and second-order motion.Using psychophysical methods, a directional anisotropy was found for second-order but not first-order motion in peripheral vision. This anisotropy is interpreted as a functional dissociation implicating the second-order mechanism in optic flow processing.Identification of early visual cortical areas is a prerequisite to any functional assessment of these visual areas. To this aim a novel human brain mapping method has been developed which automatically segments early human retinotopic visual areas. Unlike previous methods this procedure does not depend on a cortical surface reconstruction and thereby greatly simplifies the analysis.In a combined psychophysical and fMRI study, distinct cortical regions, in occipital and parietal lobes, were preferentially activated by either first- or second-order motion. These results provide evidence for the idea that first-order motion is computed in V1 and second-order motion in later occipital visual areas. In addition the results suggest a functional dissociation of the two kinds of motion beyond the occipital lobe consistent with a role for the second-order mechanism in optic flow analysis