Translation of sensory signals into commands for control of saccadic eye movements: role of primate superior colliculus

Afferent signals that guide orienting movements converge in the deeper layers of the SC in a wide variety of animals. The sensory cells are arranged topographically according to their receptive-field locations and, thereby, form maps of sensory space. Maps of visual, somatosensory, and/or auditory space have been obtained in the iguana, mouse, hamster, barn owl, chinchilla, cat, and monkey. The deeper layers of the SC also contain neurons involved in the generation of movements of the eyes, head, vibrissae, and pinnae. Thus the SC, a site containing multiple sensory maps and perhaps multiple motor maps, has been selected by many investigators as a structure for investigating the problem of sensorimotor integration. In the mammalian nervous system, emphasized in this review, much remains to be learned about the structure, organization, and function of the SC. While anatomical studies continue to add to the knowledge of the sources of afferent projections, their pattern of laminar termination, and the source and destination of efferent projections, relatively little is known about the intrinsic organization of the colliculus, especially the deeper layers. Recently, electrophysiological studies have moved from an emphasis on the sensory and motor properties of collicular neurons to an examination of the maps of auditory and somatosensory space and the correspondence of these maps. In the future, major efforts aimed at identifying the functional properties of cells that project to the SC from diverse brain regions as well as the functional properties that project to the various structures receiving input from the colliculus are needed. A combination of anatomical and electrophysiological methods is required to describe the signal transforms that occur between the SC and motor areas (such as the paramedian pontine reticular formation) closer to the final common pathway. Conceptual and empirical work is needed to develop and test models of how the dynamic visual and auditory maps found in the primate SC are generated. In general, new and/or improved models of the role of the SC in sensorimotor integration are needed as guides for future research. A point of view emphasized here is that it may be fruitful to examine the function of the SC from a motor perspective. The nature of the motor command imposes constraints on the configuration of signals that can initiate movements and thereby determines the required transformation of sensory signals.