The human eye position control system in a rehabilitation setting

Our work at Ireland’s National Rehabilitation Hospital involves designing communication systems for people suffering from profound physical disabilities. One such system uses the electro-oculogram, which is an (x,y) system of voltages picked up by pairs of electrodes placed, respectively, above and below and on either side of the eyes. The eyeball has a dc polarisation between cornea and back, arising from the photoreceptor rods and cones in the retina. As the eye rotates, the varying voltages projected onto the electrodes drive a cursor over a mimic keyboard on a computer screen. Symbols are selected with a switching action derived, for example, from a blink. Experience in using this mode of communication has given us limited facilities to study the eye position control system. We present here a resulting new feedback model for rotation in either the vertical or the horizontal plane, which involves the eyeball controlled by an agonist-antagonist muscle pair, modelled by a single equivalent bidirectional muscle with torque falling off linearly with angular velocity. We have incorporated muscle spindles and have tuned them by pole assignment associated with an optimum stability criterion. The dynamics also indicate an integral controller taking its input from a bang-bang element with dead zone. There is, in addition, a pure time delay element involved. Describing Function analysis and simulation demonstrate that in this application the time delay is outside the feedback loop, and is probably associated with set-point generation at a higher level in the brain’s hierarchy of control systems. A second input could be involved at the spindle level, active when tracking predictable target motions