William Levick's major work was to relate structure and function in the visual pathway of mammals, giving functional accounts of the diverse types of ganglion cell in the retina. He then played a leading part in establishing with confidence a 1:1 correspondence between a particular morphological type and physiological class, with tentative identification of other correspondences. He made important contributions to the analysis of the mechanisms by which retinal ganglion cells respond only to specific trigger features of the image, and to an account of their responses to very small numbers of quantal absorptions.
William showed by direct experiment the high degree of specificity in the connections that these cells make with the next neurons in the brain. He was among the first neurophysiologists to use computer methods effectively and has been responsible for training many collaborators in this type of work. He was also at the forefront in probing the relation between neural activity and visual function.