Cognitive spatial-motor processes. 5. Specification of the direction of visually guided isometric forces in...

The effects of an external constant force bias on the information transmitted (Ti) by the direction of isometric force exerted in 2-dimensional (2-D) space by human subjects were studied using an isometric manipulandum and random dot stereograms generated in a color display (Massey et al. 1988, Massey et al. 1990). Subjects exerted force on the manipulandum such that a visual force-feedback cursor would move in the direction of a visually defined stimulus in the stereo display. The time course of force development and the gain of directional information during increasing force intensity were also studied. We found the following. (a) When no bias force was applied, the force exerted by the subject increased from near zero to greater than 200 gram-force at the end of a trial and was close to the visually defined direction. When a constant bias force of 110 gram-force was applied in various directions in blocks of trials, the force exerted by the subject increased in time, as above; however, its direction also changed in time so that the instantaneous vector sum of the bias force and the force exerted by the subject pointed close to the visually defined direction. The Ti and the reaction time (RT) did not differ significantly in the two experimental conditions. These results suggest that the directional control of isometric forces is very efficient, especially in relation to visuomotor coordination. (b) The Ti was calculated at various levels of force intensity, as the latter increased from approximately 50 gram-force to 200 gram-force.(ABSTRACT TRUNCATED AT 250 WORDS)