On the utility of neural perturbation experiments for identifying the neural components of an integrated system.

International audienceLiving animals are sensorimotor systems endowed of properties that allow them to explore and interact with their environment. The orienting reaction is one of these systems by which animals can establish equilibrium between their endogen properties and the constraints imposed by the milieu. Indeed, variations in the environment (like the sudden appearance of an event) or fluctuations in the inner state (e.g., the hypovolemia) often break this equilibrium and lead to coordinated movements which are aimed at restoring it. As such, the orienting reaction can be viewed as a transition, whose detailed analysis can provide a picture of the dynamic properties of a neurobiological system, at different time scales (e.g., from the systematic orientation to its habituation) and spatial scales (e.g., from the saccade and its underlying neural network to the bistable properties of recruited neurons). Most often, this reaction consists in orienting the sensory organs toward the source of external events. Many neuronal groups are involved in generating a reaction that fits with the spatiotemporal properties of targeted events. These groups are distributed in several brain regions whose interactions generate streams which are more or less independent or cooperative. Among those regions, the medio-posterior cerebellum (MPC) is known to play a major role in the adaptability of the orienting reaction.By a series of experiments which consisted in reversibly perturbing, in the feline and primate species, the output nucleus of MPC, the caudal fastigialnucleus (CFN), we have been able to identify different sub-systems involved in orienting the fovea toward a visual target, with or without the contribution of the head, and at different times from the target appearance to its foveal acquisition. Our studies suggest that the CFNs, through their connections toward the rostral Superior Colliculi, would adjust miniature saccades generated when foveating a target presented in the central visual field. Through their connections toward premotor centers in the reticular formation, the CFNs would adjust the balance between excitatory and inhibitory commands for generating the proper drive to quickly and accurately orient the eye and the head toward peripheral targets.Differences observed in the effects of functional perturbations between the feline and primate species reveal different strategies which depend upon the nature of the target and the neuro-morphological organization of the oculocephalic system. Future comparative studies should reveal the evolutionary aspects of a complex system which progressively integrates the properties of the environment and the inner physiology