The Treatment of Akinesia using Virtual Images

by Jerrold D. Prothero

Virtual Vision

While the ability of visual cues to produce kinesia paradoxa in akinetic persons with Parkinson's disease is well-established in the medical community (see Chapter 2), it appears to have received little sustained attention, presumably because it appeared to provide limited therapeutic value. Obviously it would not be practical to paint two inch stripes everywhere a person with Parkinson's disease might wish to go.

The situation potentially changed in early 1993, with the development of a new, relatively low-cost, head-up display technology by Virtual Vision (VV).

In essence, the VV Sport [81] consists of a visor with a liquid crystal display (LCD) mounted above either the right or left eye. Mounted in front of the same eye, 24 below the line of sight, is a reflecting lens which projects the image from the LCD back into the eye. The LCD contains 96,600 color pixels; the lens occludes 15 by 22 of the visual field (see Figure 1.1). The rest of the visual field is left unobstructed, except for a tinted, see-thru screen. Since the Sport can be worn like sunglasses and weighs only 5 ounces, it can be thought of as providing a portable large-screen display.

 
Figure 1.1: Peripheral Viewing Display Angles

Ordinarily, a display mounted less than an inch from the eye would be useless, as focus in that range would be difficult and straining. However, the lens is used to optically modify the image, causing the image to appear at a greater distance in front of the user.

The principle at work here is called ``accommodation'', the adjustment of the curvature of the eye's lens depending on the distance to the object being viewed. In accommodation for near vision, the ciliary muscle contracts, causing increased rounding of the lens; the pupil contracts and the optic axes converge [78].

The brain uses the amount of accommodation necessary to bring the eye into focus as one of its depth cues. By collimating the light rays coming out of the Virtual Vision Sport lens, the eye is made to accommodate at an apparent distance much greater than that of the display. This ``fools'' the brain into thinking that the image is further away than in is. Furthermore, it avoids the strain of having to re-accommodate the eye when switching focus between the display and the outside world.

Accurate accommodation is important for objects within a few feet or so; beyond about six feet it becomes less important, because the changes in accommodation for objects farther than six feet away are small [81].

Virtual Vision provides a standard reflective lens which places the virtual image six or more feet in front of the user, depending on the manufacturing tolerances of the components separating the LCD from the lens. In addition, a precision reflective lens can be adjusted to place the display focus at any distance (see Figure 1.2).

 
Figure 1.2: Reflective Lens Options

A prototype version was introduced at the 1993 Winter Consumer Electronics Show in January, and it received considerable publicity. It was at this point that T.R., who has Parkinson's disease and who has given considerable thought to visual cues, conceived the idea of using the VV Sport display to provide portable visual cues which would appear to be on the ground. Such a display might allow him to function at a lower level of medication, and therefore with less side-effects (see Chapter 2).

T.R. contacted Virtual Vision with this idea. Virtual Vision referred him to the Human Interface Technology Laboratory (HITL). That was the beginning of the research described in this thesis.