Cognitive spatial-motor processes 4. Specification of the direction of visually guided isometric forces in two-dimensional space: information transmitted and effects of visual force-feedback

The information transmitted (Ti) by the direction of two-dimensional (2-D) isometric forces at different stereoscopic depths was studied in 50 naive human subjects using an isometric manipulandum and random dot stereograms generated in a color display (Massey et al. 1988). Subjects viewed the display through appropriate color filters and perceived the image of a disk rotated about a horizontal axis on the frontal plane; the top of the disk was rotated around that axis by 15, 45, 60 and 80° away from the subject. Each of these disks involved a different amount of stereoscopic depth perception which was lowest for the 15° and highest for the 80° tilt. Subjects were instructed to exert force in the direction of a visual target presented on the disk in a reaction time task. The instantaneous force exerted by the subjects on the manipulandum was shown on the disk in the form of a feedback cursor. Information transmitted, reaction time (RT) and systematic directional deviations were calculated. We found the following, (a) Ti increased with input information but at a lower rate; at the highest level of input information studied (5.91 bits), Ti was 4.1 bits at the 15° tilt. This high value of Ti suggests that directional information for isometric force is processed very efficiently. How-ever, this Ti was consistently lower than that transmitted by the direction of movement (Georgopoulos and Massey, 1988). (b) Ti did not differ significantly among the 15-60° tilts but was 0.19 bits less for the 80° tilt. RT did not differ among the 15-80° tilts, (c) There was a small but systematic clustering of force directions along the diagonal directions (i.e., away from the major axes). This clustering decreased by changing the configuration of the arm relative to the manipulandum. (d) A significant decrease of Ti (by 0.29 bits, on the average) and an increase of RT (by 80 ms, on the average) were observed when the force-feedback cursor was turned off at the beginning of the RT. These findings suggest that the visual definition of the origin of the upcoming force vector is important for the generation of force in the appropriate direction.