To do: Fixate your gaze on the top green square in the upper left for approximately 30-60 seconds, or until the red spot disappears — then move your gaze to the lower green square and the red spot will reappear!
To notice: When the you fixate your gaze well enough, the neurons in your visual system will adapt to the red spot and it will disappear. To do this, it helps if the red spot is already somewhat difficult to see — which we have tried to achieve by making it approximately the same brightness as the background, or “equiluminant”. If the spot seems either too dark or too light, press the “Darker” and/or “Brighter” buttons until it is equal in brightness to the background. We have noticed that the spot sometimes disappears more readily while its brightness is actively being adjusted. If you still have difficulty getting the illusion to work, flicker-mode may help you to equate the brightnesses of the red spot with the background…press “Flicker”.
Flicker Mode: Fixate your gaze on the green square (now in the middle top). As the red spot flickers against the gray spot, adjust the brightness of the red spot (using the “Darker” and “Brighter” buttons) until the red spot is the same brightness as the gray spot. A trick that visual scientists use to do this task is to set the red spot to the exact brightness that minimizes the sensation of flicker — scientists call this technique “minimal flicker photometry”. When the red spot is equal in brightness to the gray spot, press the “Fixate” button and try the Visual Adaptation test again (see “To do:” above)…now the red will be of equal brightness to the gray background and so it should be easier to make the red spot disappear.
Visual Fading is an illusion in which an object can become invisible due to lack of eye-movements. It shows that small eye-movements are necessary during normal vision — even when we are fixating our gaze — in order to see.
Susana Martinez-Conde, Stephen L. Macknik & David H. Hubel (2004) The role of fixational eye movements in visual perception. Nature Reviews Neuroscience, 5, pp. 229-240.
Martinez-Conde S, Macknik SL, & Hubel DH. (2002) The function of bursts of spikes during visual fixation in the awake primate lateral geniculate nucleus and primary visual cortex. Proceedings of the National Academy of Sciences (USA), 99(21), pp.13920-13925.
Martinez-Conde, S, Macknik, SL & Hubel, DH (2000) Microsaccadic Eye Movements and Firing of Single Cells in the Striate Cortex of Macaque Monkeys. Nature Neuroscience Vol. 3(3), pp. 251-258.