Control of leg protraction in the stick insect: a targeted movement showing compensation for externally applied forces

Summary

In stick insects, the swing of each rear leg is aimed at the ipsilateral middle leg. The control of this targeted movement was investigated by applying external force to aid or oppose protraction of one rear leg as stick insects walked on a treadwheel.

In the first condition studied, the target middle leg was stationary during the protraction of the rear leg (Figs. 1a, 2). The opposing forces tested were 14 and 32 times greater than the peak force exerted during unobstructed protraction. Nevertheless, the rear leg continued to step to a constant position behind the middle leg (Fig. 3).

In the second condition, the target middle leg also walked on the wheel. As the force opposing protraction increased, the endpoint of rear leg protraction shifted caudally, the speed of protraction decreased, and the total protraction duration increased (Fig. 5; Table 1). The middle leg's position at the end of rear leg protraction shifted caudally but its posterior extreme position remained virtually unchanged. When the onset of the external force was abrupt, compensation often occurred within 20 ms (Fig. 6a).

External forces aiding protraction increased protraction speed only slightly (Table 2). When the force was suddenly removed, the leg continued moving forward but with reduced velocity (Fig. 6b).

It is concluded that position information is used only to determine the swing endpoint and that velocity is controlled during the movement. The results are compared with movements to a target by vertebrates and with models of motor control in general.