Fifty years later: A neurodynamic explanation of Fitts' law

Abstract

An intrinsic property of human motor behaviour is a trade-off between speed and accuracy. This is classically described by Fitts' law, a model derived by assuming the human body has a limited capacity to transmit information in organizing motor behaviour. Here, we propose an alternative foundation, based on the neurodynamics of the motor circuit, wherein Fitts' law is an approximation to a more general relationship. In this formulation, widely observed inconsistencies with experimental data are a consequence of psychomotor delay. The methodology developed additionally provides a method to estimate the delay within the motor circuit from the speed-accuracy trade-off alone.

Figures

Figure 1

Network diagram of the VITE circuit with connections indicated as excitatory or inhibitory. TPC, target position command; PPC, present position command; DV, difference vector; G, gain signal.

Figure 2

Estimation of total sensory-motor delay (τ) and averaging rate (α) for the four experiments presented in Fitts' original 1954 paper (Fitts 1954). For each experiment, a contour plot shows the points in the parameter space (α,τ) for which the least-square difference between the speed-accuracy trade-off of delayed feedback and the experimental data is less than 5% (contours given at 1% intervals).

Figure 3

The speed-accuracy trade-off of the delayed circuit for parameters within the range estimated for each of Fitts' experiments. The solid line represents the speed-accuracy trade-off for the circuit. The broken line represents linear regression.