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.
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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