back when proprioception is present, but becomes the only possible cue in its absence.
The aim of this chapter is to analyze some of the constraints in the intersegmental organization of a simple goal-directed motor task. Prehension movements directed at visual objects have been selected as an example because they involve two clearly different (although superimposed) segmental components. One of these two components corresponds to transportation of the hand as a whole at vicinity of the target-object (reaching, or transportation component). It involves proximal joints and muscle groups, and reflects computation, by the visual system, of spatial location of the object with respect to the body. The second component corresponds to an adjustment of the hand posture and of the respective position of the fingers (grasping, or manipulation component). It involves distal joints and muscles and reflects visual computation of shape, size, and weight of the object ( Jeannerod, 1981).
In normal subjects prehension movements are highly accurate, in spite of being relatively fast. In addition to a precise computation of the target-related parameters of the movement, this accuracy requires coordination of the different segmental components. Our approach here is (1) to report an analysis of the intrinsic structure of prehension movements in normal subjects, in order to disclose the pattern of intersegmental coordination, and (2) to try to determine the respective contributions of anticipatory and feedback modes of control exerted at the various stages of such movements. Information from visual and somatosensory origin is considered here as critical for movement control. Although visual information related to prehension movements can be easily manipulated in normal subjects, this is not the case for somatosensory information. We take advantage of pathological conditions in one patient with a lesion in the parietal lobe. This lesion had created a disconnection between somatosensory input and motor areas at the cortical level, thus producing effects similar to those produced by peripheral deafferentation of somatosensory feedback. Such a situation is of a particular interest since, when visual feedback from the movements was also suppressed experimentally, it allowed observation of motor patterns which could only be the expression of the central mechanisms activated in anticipation of the movement.
Subjects in this experiment were seven young normal adults plus one patient (see section 2 for a clinical description). Subjects sat in front of a cubicle divided