Human Hand Moves Proactively to the External Stimulus: An Evolutional Strategy for Minimizing Transient Error

We investigated particularly the proactive nature of the visual-motor system by steady and transient experiments of a hand-tracking task, and confirmed that the hand motion precedes on the average the target motion in steady runs within a finite frequency range of the sinusoidal target motion. The question why and how much the hand motion should precede was answered by frequency-jump experiments. The results implied that the positive phase shift of the hand motion represents the proactive nature of the visual-motor control system which is adaptationally developed for each person to minimize the transient error of the hand motion when the target motion changes unexpectedly.

Figure 1

Results of steady experiments. (a) An example of the sampled data (target frequency 0.8 Hz). The solid line shows the target motion and the dotted line shows the hand motion. (b) An example of histogram for the number of sampled events with various phase shifts observed between the hand motion and the target motion. Frequency dependence of the mean-value (c), and standard deviation (d) of the phase-shift histograms (b). The two graphs were calculated by averaging over the data of all the subjects. The error bars represent standard deviations.

Figure 2

Result of transient experiments. The dark circles show transient error in FA cycle as a function of the instantaneous phase shift in LB cycle (${\psi }^{LB}$). The initial target frequency is 0.8 Hz. (a) Data taken from all the subjects. (b) Data for only one subject as an example. The dotted line illustrates the best fitted parabola curve [$r^2=0.83$ for (a), $240({\psi }^{LB}-0.068{)}^2+13.4$, $r^2=0.82$ for (b)].

Figure 3

Correlation between the steady phase shift at 0.8 Hz and the instantaneous phase shift in LB cycle corresponding to the minimum transient error at initial frequency 0.8 Hz for eight subjects.

Figure 4

Simulation results of steady and transient experiments in the present model [Eq. (1)]. The open diamond and the dotted line show the frequency dependence of the steady phase shift. The closed circle and the solid line show the estimated phase shift corresponding to the minimum transient error as a function of the initial frequency. The inset illustrates the transient error for the initial $\mathrm{\text{target frequency}}=0.5\mathrm{H}\mathrm{z}$ as a function of ${\psi }^{LB}$. The values (1.5, 0.1, 0.1, 0.05, 0.05) were used for the parameters ($\gamma$, ${\tau }_1$, ${\tau }_2$, $\delta$, $\xi$), respectively.