The deep layers of the superior colliculus

Abstract

The evidence is compelling that the deep layers of the SC are involved in the translation of sensory signals into motor commands for the control of saccadic eye movements. The deep layers receive inputs from brain areas involved in the analysis of stimuli (visual, auditory and somatosensory) used to guide orienting movements. Patterns of activity recorded from collicular neurons are appropriate for the initiation and direction of saccades, and neurons carrying these signals are organized topographically, forming a map of motor (saccadic) space. Efferent projections from the deep layers are to brainstem nuclei having direct or indirect connections with motoneuron pools innervating extraocular muscles. Finally, reversible inactivation of neurons in the deep colliculus severely impairs the ability of animals to generate accurate saccades. Major gaps in our knowledge of the anatomical and functional organization of the SC remain. The physiological signals conveyed to the SC over afferent pathways are, for the most part, unknown. The intrinsic organization of the SC is poorly understood; and the question of whether or not there is extensive communication between neurons in the superficial and deep divisions has not yet been resolved. Very little is known about the morphology, location or physiological response properties of the cells of origin of the major efferent pathways; and the neural circuits involved in decoding information about the direction, velocity and amplitude of saccades (contained in the spatial and temporal pattern of neural activity of collicular cells) are still unspecified. In general, the contribution of the sensory responses of deep collicular neurons to the initiation of orienting movements is unknown. There is no experimental evidence that the activity of deep collicular neurons responsive to sensory stimuli is either necessary or sufficient for the generation of the motor signals observed in the SC, and patterns of connections within the SC specifically involved in the translation of sensory signals into motor commands have not been identified. The alignment (in anesthetized or paralysed animals) of auditory, somatosensory and visual maps in the SC has influenced thinking about the process of sensorimotor integration. It is commonly assumed that the deep division of the SC contains topographical maps of sensory space--i.e., a point in the space surrounding the animal is represented by neurons residing at a particular location in the SC. It is assumed, further, that sensory signals from different modalities originating from the same point in space activate a common pool of collicular neurons located within this representation.(ABSTRACT TRUNCATED AT 400 WORDS)