Hand trajectory invariance in reaching movements involving the trunk

Movements of different body segments may be combined in different ways to a chieve the same motor goal. How this is accomplished by the nervous system was investigated by having subjects make fast pointing movements with the a rm in combination with a forward bending of the trunk that was unexpectedly blocked in some trials. Subjects moved their hand above the surface of a t able without vision from an initial position near the midline of the chest to remembered targets placed within the reach of the arm in either the ipsi - or contralateral workspace. In experiment 1, subjects were instructed to make fast arm movements to the target without corrections whether or not th e trunk was arrested. Only minor changes were found in the hand trajectory and velocity profile in response to the trunk arrest, and these changes wer e seen only late in the movement. In contrast, the patterns of the interjoi nt coordination substantially changed in response to the trunk arrest, sugg esting the presence of compensatory arm-trunk coordination minimizing the d eflections from the hand trajectory regardless of whether the trunk is recr uited or mechanically blocked. Changes in the arm interjoint coordination i n response to the trunk arrest could be detected kinematically at a minimal latency of 50 ms. This finding suggests a rapid reflex compensatory mechan ism driven by vestibular and/or proprioceptive afferent signals. In experim ent 2, subjects were required, as soon as they perceived the trunk arrest, to change the hand motion to the same direction as that of the trunk. Under this instruction, subjects were able to initiate corrections only after th e hand approached or reached the final position. Thus, centrally mediated c ompensatory corrections triggered in response to the trunk arrest were Like ly to occur too: late to maintain the observed invariant hand trajectory in experiment 1. In experiment 3, subjects produced similar pointing movement s, but to a target that moved together with the trunk. In these body-orient ed pointing movements, the hand trajectories from trials in which the trunk was moving or arrested were substantially different. The same trajectories represented in a relative frame of reference moving with the trunk were vi rtually identical. We conclude that hand trajectory invariance can be produ ced in an external spatial (experiment 1) or an internal trunk-centered (ex periment 3) frame of reference. The invariance in the external frame of ref erence is accomplished by active compensatory changes in the arm joint angl es nullifying the influence of the trunk motion on the hand trajectory. We suggest that to make a transition to the internal frame of reference, contr ol systems suppress this compensation. One of the hypotheses opened to furt her experimental testing is that the integration of additional (trunk) degr ees of freedom into movement is based on afferent (proprioceptive, vestibul ar) signals stemming from the trunk motion and transmitted to the arm muscl es.