Variations of vision come in many flavors. Some are dramatic and lend themselves to fairly accurate descriptions and timing, e g, transitory retinal strokes (which usually evolve over a few minutes) and classical migraine auras (of the order of 20 minutes). At the other end of the spectrum, variations may be much more subtle, more difficult to describe, and repetitive on short time-scales (minutes, seconds, fractions of seconds). A physiological variant is the Troxler phenomenon, the fading-from-view that normally occurs with a steady fixation, with immediate return of vision upon blinking or shifting fixation. For a striking demonstration of the Troxler effect, try the Lilac Chaser illusion. A familiar example of abnormal variation of vision is the obscurations frequently associated with papilledema from raised intracranial pressure. There are many other variants of short-term, repetitive variations of vision which might fit under a heading of unstable vision. This is the topic of this presentation.
Unstable vision is a frequent complaint but many patients find it difficult to provide detailed descriptions and very few are prepared to illustrate their observations graphically. Indeed, graphic libraries of unstable visual perceptions seem to be lacking. Part of the difficulty of coming to grips with symptoms like these is the lack of a colloquial terminology and the very instability of the percept, i e, its variation over time. Fortunately, variation over time is easily arranged in computer graphics, so allowing the generation of a symptomimetic reference library.
By modifying the fixed reference image shown to the right, the display aims to present several variants of unstable vision on an adjustable time scale. The idea is to try to find an approximate match to the individual patient's perceptions and to obtain a rough quantitative measure. Select a type of image modification under the Feature heading and click on the display to start variation. Click again to stop. The Mode heading allows selection of regular or irregular variations. Adjust the rate of change by means of the slider. Older browsers cannot display sliders but will instead present a text input field. To vary the rate of change, enter a value in the 50 to 95 range and tap the Enter key.
Unstable Vision Simulator © Lars Frisén 2007
The display deliberately has been made very simple. Readers familiar with modern drawing software will recognize the potential for much finer graduations and also for mixing various types of unstable percepts. However, such refinements would require a much more complex interface, with a much steeper learning curve, and also would cause slower renditions.
New 2016: a refined version of the above display has been included with a host of other symptomimetic displays in a new dynamic visual library. The library has the format of an iPhone/iPad app named Visual Disorder Atlas.