THE SENSORIMOTOR AND COGNITIVE INTEGRATION OF GRAVITY

In order to demonstrate that gravity is not only a load acting locally and continuously on the body limbs, but is also used by higher levels of the nervous system as a dynamic orienting reference for the elabor ation of the motor act, a review of several experiments conducted both in 1 g and 0 g are presented. During various locomotor tasks, the str ategy that consists of stabilizing the head with respect to gravity il lustrates one of the solutions used by the CNS to optimize the control of dynamic equilibrium. A question which remains to be solved when co nsidering experimental results obtained in weightlessness concerns, ho wever, the maintenance of motor schema that has evolved under normal g ravity. Results have suggested that the concept of conservative proces ses, that would adapt postural control to weightlessness by using prev iously learned innate strategies, must be reconsidered during goal-ori ented tasks. In fact, it is proposed that when conservative processes and existing solutions derived from a repertoire of terrestrial postur al strategies do not provide efficient output, the CNS has to create n ovel strategies through a slow learning process. As with the study of postural control, three-dimensional arm reaching movements also illust rate the central representation of gravity. Indeed, gravity can be reg arded as either initiating or braking arm movements and, consequently, may be represented in the motor command at the planning level. Finall y, from a prospective point of view, there is a need to determine new experimental paradigms in order to study the specific motor control of man in space. It is suggested that the formulation of experimental pa radigms should not consider man in space simply as a terrestrial biped . (C) 1998 Elsevier Science B.V. All rights reserved.