Feedback Control of Hand-Movement and Fitts' Law

Abstract

Fitt's empirical result is stated and its information theoretic interpretation briefly discussed. An alternate derivation from a model assuming continuous velocity control of hand position is shown to fit the motion time data equally well. Detailed studies of hand motion trajectories in Fitts' reciprocal tapping task have confirmed the exponential target approach predicted by this model but also revealed systematic fluctuations that it cannot explain. A further alternate model is therefore presented, based on intermittent feedback with target approach by a sequence of discrete positional corrective motion “impulses”. The second model also predicts motion times in accord with Fitts' Law. Detailed studies of target aimed translational hand and rotary wrist motions have shown that the intermittent corrective impulses are of approximately Gaussian integral form, with minimum s.d. on the order of 30 msec. for wrist rotation, and recurrence rate about 10/s. Mechanical and physiological interactions capable of explaining this corrective impulse trajectory are discussed, and a model based on balance of forces between agonist and antagonist muscles is briefly developed. The interaction between external (visual) and internal (proprioceptive or kinaesthetic) feedback channels determining impulse amplitude is discussed in the light of results obtained using force disturbance and with amplified, attenuated or delayed visual feedback. It seems that an internal “secondary positional reference” must be postulated to explain results obtained when S is deprived of visual feedback.