This research is concerned with the supranuclear control of the oculomotor system and, in particular, with the role of the superior colliculus of the midbrain in the generation and guidance of saccadic eye movements. Previous work indicates that the directions and amplitudes of orienting saccades are encoded in the pattern of discharge of neurons spread over a large region of the deeper collicular laminae. The central hypothesis guiding the research is that the collicular output signal is specified by the intersection of this large region of discharge with spatially graded projections to brainstem areas generating the vertical and horizontal components of saccades. In other words, the colliculus employs a distributed or ensemble code to determine the trajectory or vector of the saccade. Apart from its implications for oculomotor control, this possibility makes the colliculus an interesting model for the study of distributed codes in general. The work proposed here focuses specifically on the connectional gradients that are assumed to underlie the spatial code operating in the colliculus. The experiments are concerned first with establishing the true spatial configuration of the connectional gradients that link the superior colliculus to brainstem oculomotor circuits. These gradients have been assessed primarily by studying saccades evoked electrically from the colliculus, but such saccades are influenced by initial eye position as well as by the site of stimulation. The proposed experiments are designed to assess the possibility that this initial position dependence is mediated by an extra-collicular mechanism, to quantify the spatial gradients of initial position dependence, to extract a canonical motor map of the colliculus that is free of this influence, and to compare this canonical motor map with the retinotopic sensory map of the colliculus. Additional experiments will investigate the possibility that tectotectal cell (i.e. cells which project from one colliculus to the other) are part of the neural substrate of a functional gradient. The research will contribute to our understanding of how higher neural centers control the normal production of saccadic eye movements and will provide additional perspective on the usefulness of a non-primate animal model for the study of oculomotor control and its dysfunction.
