On Seeing With Defective Vision: 4

 Mouches volantes (floaters)

Localized condensations within the vitreous body may cause the perception of mouches volantes (literally, flying flies) or floaters. It is debatable if mouches should be included under the topic of defective vision but here is an animated display. It aims to capture the characteristic movements of floaters. Floaters generally trail slow (pursuit) eye movements quite accurately. The situation is different with saccades, where the floaters initially are a bit tardy, then accelerate wildly to overshoot with a wide margin, and finally drift back. The peculiar behavior is often most striking when first tracking a "sinking" floater and then restoring the original direction of gaze by means of an upward saccade. Notably, floaters transiently tend to elongate during saccades. The spectacle is often most dramatic when viewed against the blue sky. Click in the display area to start, click again to stop. The cross-hair represents the direction of gaze.

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 Filling-in and popping

Filling-in or perceptual completion is often held to explain why many visual field defects are quite unobtrusive. It is difficult to demonstrate filling-in to normal eyes, except in the blind spot region. The following display allows some explorations.

Rest an elbow on the table to provide a stable support for the chin. The display is set up for the right eye. Cover the left eye. Laterality can be changed under the Settings menu. Use a viewing distance of about 200 mm (8"). Fixate on the cross-hair. Move the mouse to drag the black bar inside the blind spot, which is centered some 15 degrees temporal to the fixation point and a few degrees below. Once the bar has been hidden, use small horizontal and vertical mouse movements to carefully center the bar inside the blind spot. Then, click the right mouse button to increase the length of the bar. Once the two bar ends extend outside the blind spot margins, the bar will be seen in its full length, due to perceptual completion. Click the left mouse button to decrease the bar length if excessive.

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There is a small selection of additional test targets under the menu. The cross target behaves much like the bar. The ring target is perhaps more interesting but also more difficult to adjust. It requires very careful centering before expansion is started. If the ring can be made to emerge symmetrically across the blind spot borders, the blind spot will suddenly and dramatically pop out as a solid black disk. Stable fixation is crucial for seeing the popping phenomenon.

Another option allows the addition of random background noise. It is remarkable how deftly the brain fills in the blind spot even with such a complex pattern.

Finally, there is a fluffy ball target. It has been included in an attempt to illuminate filling-in (or perhaps more precisely, fading-out) outside the blind spot region. Place the ball somewhere in the nasal visual field. Let the ball remain stationary while fixating on the cross-hair. The ball will disappear from view after some 10 or 20 seconds, as originally described by D. Troxler back in 1804. The reason for using a fluffy border is to prevent small eye movements from refreshing the original percept (confer [A]). The fluffy border is also associated with a striking after-image effect [B].

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