Visuo-motor Transformation Process in the Central Nervous system

Transformation of the Coordinate System

Abstract

When an animal moves its eyes and head to fixate a visual target which has appeared in the periphery of the visual field, its central nervous system has to synthesize the motor command to execute well-coordinated activation of a large number of extraocular and neck muscles based on the visual information as to the target location perceived by the retina. Thus, the visuo-motor transformation process during such behavior involves an ill-posed problem and the process should have some constraints for the brain to solve the problem uniquely. However, neural implementation of the constraint is not well understood. The superior colliculus (SC) is a center for the control of visually-guided orienting movements of the eyes and head, whose output encodes a vector of gaze movement. With regard to the control mechanism of the head movement, the output command from the SC is transmitted to neck motoneurons via two major subgroups of interneurons in the brainstem reticular formation; one group in the medial pontine reticular formation and the other group in Forel's field H in the reticular formation of the mesodiencephalic junction. The former group of interneurons control the horizontal component, and the latter control the vertical component of the head movement. Each of these interneuronal populations represents a “functional synergy” of the neck muscles. Such parceling of head movements into two major subcomponents may serve as a neural constraint to reduce the number of degrees of freedom of the complex and redundant skeletomotor system such as the neck and may help the brain solve the ill-posed problem imposed on the visuo-motor transformation process during gaze movements.