Space coordinate systems in the brain

Abstract

The central nervous system (CNS) processes information mapped on a large number of different coordinate systems. In case of goal-directed movements such as orienting and forearm reaching, the spatial location of the target perceived via various sensory modalities are mapped onto a common flame of spatial coordinates. To execute movements toward the target, information about its location, mapped on such spatial coordinates is transformed into the spatial and temporal pattern of activation of many skeletal muscles, mapped on the body coordinate system. Such coordinate transformation includes ill-posed problem, since the dimension of the body coordinates is usually much larger than that of the spatial coordinates. To overcome such problems, the CNS should involve some neural constraint. In case of orienting head movements, the spatial information mapped on the superior colliculus (SC) is processed by the interneuronal system in the brainstem reticular formation and drives neck motoneurons. The brainstem interneuronal systems are largely divided into two major groups, one in the medial pontine reticular formation and the other in the mesodiencephalic junction. The former neurons control the horizontal component of head movements and the muscle synergies for it, while the latter control the vertical component and the muscle synergy for it. Such "intermediate coordinate system" may serve as the neural constraint resolving the illposed problem. The above described neural constraint for the coordinate transformation is one of the major issues related to coordinate processing in the brain. However, mapping of information in various sensory modalities onto the common flame of spatial coordinates and calibration of these different coordinates is another important issue. The SC is a key structure of such common frame of spatial coordinates. Several studies showed that in the case of the SC, visual input guides the spatial map of other modalities such as the auditory system to the common frame of the spatial map.