Gaze direction effects on perceptions of upper limb kinesthetic coordinatesystem axes

The effects of varying gaze direction on perceptions of the upper limb kine sthetic coordinate system axes and of the median plane location were studie d in nine subjects with no history of neuromuscular disorders. In two exper iments, six subjects aligned the unseen forearm to the trunk-fixed anterior -posterior (a/p) axis and earth-fixed vertical while gazing at different vi sual targets using either head or eye motion to vary gaze direction in diff erent conditions. Effects of support of the upper limb on perceptual errors were also tested in different conditions. Absolute constant errors and var iable errors associated with forearm alignment to the trunk-fixed a/p axis and earth-fixed vertical were similar for different gaze directions whether the head or eyes were moved to control gaze direction. Such errors were de creased by support of the upper limb when aligning to the vertical but not when aligning to the a/p axis. Regression analysis showed that single trial errors in individual subjects were poorly correlated with gaze direction, but showed a dependence on shoulder angles for alignment to both axes. Thus , changes in position of the head and eyes do not influence perceptions of upper limb kinesthetic coordinate system axes. However, dependence of the e rrors on arm configuration suggests that such perceptions are generated fro m sensations of shoulder and elbow joint angle information. In a third expe riment, perceptions of median plane location were tested by instructing fou r subjects to place the unseen right index fingertip directly in front of t he sternum either by motion of the straight arm at the shoulder or by elbow flexion/extension with shoulder angle varied. Gaze angles were varied to t he right and left by 0.5 radians to determine effects of gaze direction on such perceptions. These tasks were also carried out with subjects blindfold ed and head orientation varied to test for effects of head orientation on p erceptions of median plane location. Constant and variable errors for finge rtip placement relative to the sternum were not affected by variations in g aze direction or head orientation. Thus, the perceived position of the trun k-fixed median plane is not altered by varying gaze direction, The implicat ions of these results for mechanisms underlying kinesthetic perceptions and their potential roles in programming of upper limb movements to visual tar gets are discussed.