Biomedical Engineering Reference
In-Depth Information
13.4 THE SACCADE CONTROLLER
One of the challenges in modeling physiological systems is the lack of data or informa-
tion about the input to the system. For instance, in the fast eye movement system the input
is the neurological signal from the CNS to the muscles connected to the eyeball. Information
about the input is not available in this system, since it involves thousands of neurons firing
at a very high rate. Recording the signal would involve invasive surgery and instrumenta-
tion that was not available in the 1960s. Often, however, it is possible to obtain information
about the input via indirect means, as described in this section.
In 1964, Robinson performed an experiment in an attempt to measure the input to the
eyeballs during a saccade. To record the input, one eye was held fixed using a suction
contact lens, while the other eye performed a saccade from target to target. Since the same
innervation signal is sent to both eyes during a saccade, Robinson inferred that the input,
recorded through the transducer attached to the fixed eyeball, was the same input driving
the other eyeball. He proposed that muscle tension driving the eyeballs during a saccade is
a pulse plus a step or, simply, a pulse-step input (Figure 13.9).
Today, microelectrode studies are carried out to record the electrical activity in oculo-
motor neurons in monkeys. Figure 13.10 shows a micropipette being used to record the
activity in the oculomotor nucleus, an important neuron population responsible for
driving a saccade. Additional experiments on oculomotor muscle have been carried out
to learn more about the saccade controller since Robinson's initial study. For instance in
1975, Collins and his coworkers reported using a miniature “C” gauge force transducer
to measure muscle tension in vivo at the muscle tendon during unrestrained human
eye movements. This type of study has allowed a better understanding of the tensions
exerted by each muscle, rather than the combined effect of both muscles, as shown in
Figure 13.11.
It is important to distinguish between the tension or force generated by a muscle, called
muscle tension, and the force generator within the muscle, called the
active-state tension gener-
ator
. The active-state tension generator creates a force within the muscle that is transformed
through the internal elements of the muscle into the muscle tension. Muscle tension is exter-
nal and measurable. Active-state tension is internal and not measurable. Active-state tension
Pulse
Step
0
t
1
Time
FIGURE 13.9
The muscle tension recorded during a saccade.
