On the right you see a rotating array of blue crosses and 3 yellow dots. Now fixate on the centre (watch the flashing red/green spot). Note that the yellow spots disappear once in a while: singly, in pairs or all three simultaneously, right?
In reality, the 3 yellow spots are continuously present, honest! This is captively called “motion induced blindness” or MIB.
The popup button (3rd from bottom) is preset to “Dots” – try “Ring” for a change. I find it quite fascinating to watch the obliteration process, sometimes following the path, sometimes occuring in patches. Finally, try “Array”, there perhaps decreasing “size” a little; here the patchy appearance is strong.
You can change the speed (preset to 12 rotations per minute). Disappearance persists down to surprisingly slow speeds.
You can adjust the size (preset to 5 pixels). Disappearance persists up to surprisingly large sizes.
You can change the colour of the rotating crosses, the dots and the background. The dots disappear into whatever pattern and colour the background has. This underscores an important aspect: The dots do not simply disappear, but rather are replaced by the background, probably with the same mechanism as the one hiding our blind spot.
The ‘Reset’ button at the top restores the standard settings.
Steady fixation favours disappearance, blinks or gaze shifts induce reappearance. All in all reminiscent of the Troxler effect, but stronger and more resistant to residual eye movements.
In Feb 2008, John from Phoenix posed an intriguing question in the guestbook: If several people observe together, do the yellow dots disappear at the same time for everyone (synchronised)? I remember I briefly considered this years ago and rejected the hypothesis of synchronised perception as esoteric. I also informally tested this, and it did not occur. I still believe that the disappearence is an individual phenomenon, and thus not synchronised, but this should be formally tested with careful methodology. Should be an interesting experiment! Anyone?
There is no consensus as to the explanation in vision literature yet. I personally think that motion is not necessary, any (temporal) change in the image will suffice. [Note added 2008-03-07: see now Wallis & Arnold, 2008.] A more recent paper from that group (2009) sugggests a link of MIB to “motion blur / motion streak” suppression. If so, MIB would be illusion subserving a useful purpose in everyday vision. This also holds for a different explanatory approach by New & Scholl (2008) who conclude that “rather than being a failure of visual processing, MIB may be a functional product of the visual system’s attempt to separate distal stimuli from artifacts of damage to the visual system itself.”
A simple version of explanation: If you fixate steadily, all structures are imaged continuously on their retinal location. This leads to local adaptation in the retina (the Troxler effect, often incorrectly addressed as “fatigue”). By adding additional temporal modulation (here the rotation), effectively the background noise is increased. Thus the Troxler disappearence is more pronouned and/or happens faster.
Bonneh, Cooperman & Sagi (2001) Motion-induced blindness in normal observers. Nature 411:798–801
Wallis TSA & Arnold DH (2008) Motion-induced blindness is not tuned to retinal speed. JOV 8:11, 1–7
Wallis TSA & Arnold DH (2009) Motion-induced blindness and motion streak suppression. Current Biology 19:325–329
New JJ, Scholl BJ (2008) “Perceptual Scotomas” A functional account of motion-induced blindness. Psychological Science 19(7):653–659