Chapter 14 How visual inputs to the ponto-bulbar reticular formation are used in the synthesis of premotor signals during orienting

Abstract

The primate superior colliculus (SC) is known as a structure subserving the transformation of visual information into "commands" for orienting eye movements. Collicular burst neurons discharging with short lead times in relation to visually triggered or spontaneous saccades are supposed to be the output elements linking the SC to immediately premotor pattern generators. In this paper we summarize some data available for the cat's SC neurones, identified as tecto-reticulo-spinal projection cells (TRSN), and reticulospinal neurones (RSN), identified as receiving excitatory collicular input. Some TRSNs respond to visual stimuli in the absence of orienting movements and, hence, their signals cannot be regarded as motor "commands", in spite of their proven connections with premotor pools in the brain stem and with the spinal cord. Moreover, a small fraction of RSNs belonging to polysynaptic descending collicular pathways also displays visual responses dissociated from movement, in addition to discharges related to the performance of orienting eye-head synergies. The processes of visual to motor transformation, assumed by current models as being definitively accomplished in the SC, appear thus to be partially performed in the reticular network incorporating the overlapping collaterals of tectal projection cells and their target neurons in the reticular core. It is concluded that, at least as for visuomotor transformations underlying orienting movements in the cat, the deep division of the SC and the brain stem reticular formation represent an ensemble, rather than a sequence of hierarchically arranged levels of processing.