Cognitive spatial-motor processes 5. Specification of the direction of visually guided isometric forces in two-dimensional space: time course of information transmitted and effect of constant force bias

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 > 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. There was a gain of information with force intensity for all experimental conditions studied (i.e. stereoscopic depth, absence of visual force feedback, and presence of force bias). This suggests that the specification of the direction of force improves as the force intensity increases, which could be due to a continuous comparison and correction of the force produced so that it is in the visually defined direction. The curves of Ti vs. force intensity, F, were negatively accelerating. The relation between Ti and F was a power function of the form: Ti = kFm where k and m are constants. Equation (1) is linear in a log-log scale: ln Ti = A + m ln F where A = lnk. The constant m was similar but the constant A lower in various experimental conditions in which stimulus conditions were manipulated (e.g. absence of visual force feedback). In contrast, the presence of constant force bias did not affect the values of m or A. These findings indicate that the rate of gain of information (that is, the slope m in the second equation above) is very similar in different cases but that the curve is displaced to lower levels of Ti by the degradation of visual definition of the instructed force direction. Finally, application of external force bias affects neither the rate nor the amount of information gained.