Motion induced blindness
Even if you fix your gaze at one of these yellow dots, it will eventually disappear! A newly uncovered visual illusion poses problems for theories of conscious vision. Striking visual disappearance in normal-sighted observers under natural conditions Of all that your eyes capture, how much do you really see? Many are aquainted with visual illusions, […]
Even if you fix your gaze at one of these yellow dots, it will eventually disappear! A newly uncovered visual illusion poses problems for theories of conscious vision.
Striking visual disappearance in normal-sighted observers under natural conditions
Of all that your eyes capture, how much do you really see? Many are aquainted with visual illusions, such as the Necker cube, where the viewer is put in a state where two or more percepts of the same physical presentation are both coherent and mutually exclusive. Other figures might produce a so-called “filling in” effect, where parts of a visual scene are (gradually) left out. This might be shown with a demonstration of the blind spot, a part of the visual scene that corresponds to the region where the optic nerve leaves the retina, and there are no retinal receptors. Another example is the famous Kanizsa triangle.
Now, in a paper in Nature, Dr. Yoram Bonneh, of the Smith- Kettlewell Eye Dots disappear approximately 40% of the viewing time Research Institute in San Francisco, and his colleagues have studied a remarkable new visual illusion, consisting of a swirling pattern of blue dots superimposed on three stationary yellow dots (click on the image to go to the demonstration page). When viewing the figure, and paying attention to the blue dots, the yellow dots sometimes disappear. (The animation is reprinted with permission)
Bonneh et al. pursues the so-called motion induced blindness (or MIB) phenomenon, and they have tried out many variations of the same effect. First of all, they studied the frequency of diappearance of the dots. It turned out that the dots disappear approximately 40% of the viewing time, with durations as long as up to 10 seconds. Although variations such as a smaller amount of dots (down to 10 dots), and decreased luminance between the yellow and blue dots have an effect on the percentage of disappearance, it nevertheless does not eliminate the effect altogether. Even actively attending one specific dot did not hinder the effect.
Furthermore, the dots disappear in alternation – in the same manner as “traditional” visual rivalry. That is, the dots tend not to disappear at the same time, but rather one at the time, while the other dots are seen. The effect is also very similar to clinical cases of attention deficits, in which partial invisibility might occur despite the primary visual corticies being intact. But it would seem a semantic stretch to call this effect “filling in”, since it is not necessary for the blue dots to intersect the yellow disc for the effect to occur.
Bonneh et al. also document cases in which dots are brought back from the void. Large eye movements tend to disturb the effect, and the dot reappears. At the same time, large and abrupt transients tend to bring back the disappeared patterns. Thus, in normal cases, the effect appears to be rather robust, and it seems that rather gross changes are required for eliminating the MIB effect.
Some scientists have already pursued the findings from the MIB effect. Professor Jack Pettigrew, from the Vision, Touch and Hearing Research Centre Large eye movements tend to disturb the effect at the University of Queensland, has used Transcranial Magnetic Stimulation (TMS) – a focused magnetic field that tends to inhibit neuronal activity in one part of the cortex. Pettigrew used TMS just when the dot disappeared, to inhibit activity in either the left or right hemisphere. He made a striking finding: when “zapping” the left hemisphere the missing dots came back immediately. Right hemisphere TMS, on the other hand, tends to prolong the disappearance. Pettigrew interprets this finding to a kind of denial effect, where the left hemisphere adopts a belief that there is a single entity visible – a rotating 3D surface – and denies that there is anything else. A fully operational right hemisphere could be thought of as “dragging” the left hemisphere back to visible reality.
What does this tell us about consciousness? We do not know yet. But we know that the disappearing yellow spot is still in the physical signal when it disappears, so that it must activate the retina and send activity some way into the visual system. If Pettigrew is right, the yellow spot neuronal signal may go well into cortex. And yet it fades from consciousness. Bottom line is; any theory of the relationship between consciousness and the brain must somehow give an explanation of this phenomenon.
Copyright © T.Z. Ramsøy
Further Reading
Keywords:Motion induced blindness, visual illusions, visual rivalry
References
Y. S. Bonneh, A. Cooperman & D. Sagi (2001). Nature 411, 798-801
