Biomedical Engineering Reference
In-Depth Information
distal joint. At first, only a local FORWARD behavior is stimulated (until
t
=
1375); then the LATERAL behavior is activated simultaneously. Because of the
equal strength of activation, the system now tries to walk forward as well as
laterally, which results in diagonal walking.
Approximately at time
t
= 1460, activation of the FORWARD behavior is
set to 0, which leaves only the LATERAL behavior to influence the oscillator
networks (see Figures 4 and 5A-C). Thus, the system walks laterally, which can
be observed from the data as the amplitude of the thoracic joint at 0 while the
basilar and especially distal joint perform large-amplitude oscillations. Subse-
quently, this process is reversed.
4.1. Local Reflexes
The approach described here for generation of rhythmic motion deals very
well with plain surfaces without obstacles. However, in the case of uneven
ground poor results are to be expected. Our approach to deal with uneven terrain
was to implement a set of reflexes in parallel with the motor end path (see Fig-
ure 5, right side), which override, for a short and predefined period of time, the
rhythmic activity of the oscillators. For example (see Figure 5, right side), if the
current values of the thoracic joint increase steeply and a significant angular
displacement error is detected at the same time, it is assumed that the "planed"
trajectory is blocked. This triggers a reflex, which moves the leg backward and
upward (via joint activity in the thoracic and basilar joints) and then forward at
maximum speed. This reflex is illustrated in Figure 5 (right side; start point at
t
= 46, stop at
t
= 58). The reaction time of the reflexes can be as fast as 1/100 s,
because they are directly in line with the motor control signals. The three pic-
tures also illustrate how fast the motor controller returns to the pattern given by
the oscillator, after the reflex is no longer active. It is important to notice that the
action of the reflex does in fact sit right on top of the ongoing rhythmic activity.
As can be seen in Figure 5 (right-hand traces) the oscillatory activity is always
present in the background (light gray lines); as soon as the reflex is terminated,
the locomotion returns to the oscillation.
5.
DISCUSSION AND
OUTLOOOK
Recently, the psychological point of view that grants the body a more sig-
nificant role in cognition has also gained attention in spatial cognition theory.
Proponents of this approach would claim that instead of a "mind that works on
abstract problems" we have to deal with and understand "a body that needs a
mind to make it function" (16).
