Virtual Vision Display Compared to Laser Pointer

The results of this study suggest that the laser pointer produced an effect as good as or better than the VV Sport display. Given that the laser pointer is approximately ten times cheaper (roughly $100 vs. roughly $1000), lighter, and more convenient to carry around, that might seem to be the end of the story.

Things are not so simple, however, for four reasons. One factor is that we were clearly not using an ideal version of the VV Sport display, as discussed in Chapter 6. In particular, a longer field-of-view is needed, to provide peripheral cues. A wider field-of-view, to provide cues for moving smoothly in directions other than straight forward, might also be useful.

A second factor is that we almost certainly were not providing optimal cues in the VV Sport display. We have not systematically investigated what the stimulus parameters are. This is discussed more fully in Chapter 7. Furthermore, it should be possible to project much more realistic cues with the VV Sport display than are possible with the laser pointer. To the extent that more realistic cues are more effective, this provides a qualitative advantage to the VV Sport display.

A third factor is that the laser spot is not visible outside on a bright day, possibly making the VV Sport display a better outdoor solution.

The fourth and perhaps most interesting factor is T.R.'s perception that chasing the laser pointer spot works in a fundamentally different way cognitively than tangible cues, cues in the VV Sport display, or stepping over the laser spot, and that chasing the laser spot is fundamentally less robust and more prone to breakdown in the presence of distractions.

In Appendix C, research is presented which tentatively suggests that akinesia results from a breakdown of the brain's ability to smoothly step between the individual operations of a motor program. Visual cuing bypasses this problem by using external cues to step between operations in the motor program.

The subjective difference between these two modalities is that the internal (non-cuing) modality is unconscious, whereas the external modality requires conscious attention. The internal modality is better in that it does not require conscious attention, but for akinetic patients it is much more prone to breaking down in the presence of distractions, and also tends to degrade over time.

For T.R., unmedicated walking begins with an external cue. If he succeeds in sustaining ambulation, it becomes unconscious and internal; otherwise he needs external cues for each successive step.

T.R. claims that when he is chasing the laser spot he is operating in the unconscious modality. He is not thinking of the laser spot as an external cue; instead the spot has something to do with enabling him to maintain an adequate stride length, which seems to be essential for him to maintain unconscious ambulation. Therefore it is not surprising that using the laser pointer spot as a target to chase may break down in the presence of distractions, unlike the VV Sport display. This suggests that a VV Sport display may be more suitable than the laser pointer for patients with relatively severe cases of akinesia.

As T.R.'s disease has progressed, he has lost the ability to ambulate by chasing the laser pointer spot, although tangible cues still work just as well as before and he can still use the laser pointer spot as a fixed cue to step over. This would tend to back T.R.'s claim that chasing the laser pointer spot is cognitively different from fixed cues, as provided either by real objects, the laser pointer fixated on a location, or space-stabilized images in the VV Sport display.

The trials with T.R. suggest that for more seriously akinetic patients, the current subtended vertical field-of-view of the VV Sport display is not sufficient. In addition, T.R. has suggested (Appendix A) that a modified VV Sport display covering the bottom half of the field-of-view for one eye might be more useful. This allows one to present a range of cues covering a wide area, potentially allowing smooth motion in any direction, instead of merely straight forward.

Aside from engineering difficulties, a possible problem with such a large display is that by blocking more of the visual field, the display might interfere with the ability to perceive objects stereoscopically. This price may or may not be worth paying. T.R. agrees that a display which stretched from the straight-ahead viewing angle down to the bottom of the visor in a narrow strip would be more useful than the current arrangement, although it would not have the potential to allow smooth turning in any direction.