Disclaimer: Please note I am not an expert in the field of neuropsychology. Some of the information presented here may be factually inaccurate. Consult the Reference section for more in-depth coverage on this subject.
One of the most interesting examples of perception rivalry is called binocular rivalry . Binocular rivalry usually occurs when a person is exposed to completely different images simultaneously, one for each eye. The resulting effect is that only one of the images is perceived at a time, or perhaps patches of both images seen in some way, interwoven along contrasting features depicted in the images. Simultaneous perception of two complete images occur rarely, even though the optical stimulus on each retina does not change throughout the experience. The resulting visual experience would be akin of two images cross-fading between each other, in a periodic fashion.
Motion-Induced Blindness, or MIB, is an illusion triggered by moving objects in a scene. For example, a cluster of background objects moving behind smaller static objects would highlight the illusion. The objects in the background will cause the observer to become blind to the foreground objects for a few seconds before they reappear again .
Both binocular rivalry and MIB exhibit perceptual oscillations — the observer experiences visual changes in cycles. Interestingly, Pettigrew et al. postulated that such perceptual oscillations may actually share the same underlying timing mechanism, both for binocular rivalry and for MIB. Furthermore, the cause of perceptual rivalry might be due to the brain's switching action between the two hemispheres [3, 4].
The brain's switching action can be described as hemispheres constantly alternating between "active" and "suppressed" states (to some extent). The observer would be aware of the sensory information processed by the active hemisphere, but not aware of the stimulus processed by the suppressed (other) half. This could explain why binocular rivalry occurs — if each eye has some specificity to its own brain hemisphere, the perceived visual stimulus would mostly correspond to the active hemisphere/eye pair. When a hemisphere switch occurs, images from the other eye will start to dominate and so on.
Lateralisation in the brain can also influence one's visual perception, depending on which of the hemispheres is currently active. The left hemisphere tends to be more analytical and logical, whereas the right hemisphere is simultaneous and intuitive. For example, the left side of the brain would allow the observer to focus more on individual objects, while the right side would allow the observer see the "bigger picture" more effectively. MIB tests demonstrate this effect: The observer can focus on the static foreground objects better while the left hemisphere is active. These foreground objects would become partially, or completely imperceptible to the observer, once the right hemisphere becomes active. The right side is better at observing the main pattern in the background.
The average rate of switching between brain hemispheres is slightly different from person to person. One's ability to focus on a specific problem, or to deal with complex multitasking situations may actually depend on the rate of hemisphere switching in the brain. Someone who is more analytical tend to be slow switchers. Whereas someone with artistic traits, such as playing music, tend to be fast switchers. People suffering bipolar disorder tend have much slower perceptual oscillations, compared to individuals without the disorder [3, 7, 8].
Observing Perceptual Oscillations
Dots is a simple OpenGL based program, written specifically to allow individuals to observe their own perceptual oscillation. The application is available to download in the Appendix section. The figure below illustrates how the test looks like. The test consists of a cluster of blue dots moving in the background. In the foreground, the program renders five stationary yellow dots. The observer is required to sit at a comfortable distance from the display, while the program is running, and focus on the central yellow dot. The observer should see some of the yellow dots periodically disappear and then reappear after a few seconds. The disappearance of the yellow dots is caused by the Motion-Induced Blindness discussed earlier, and the cyclic nature of the illusion is explained by the hemispherical switching in the brain. The program Dots was inspired by the MIB tests devised by Bonneh et al., which originally used three yellow static dots .
According to the MIB tests conducted by Pettigrew's team, participants involved in the test experienced an average of 4 to 6 perceptual oscillation cycles for every 30 seconds. Participants that scored less than 4 cycles usually pursued interests involving problem solving tasks and mathematics. Those who scored above 6 had a background in performance arts, typically music related . Apparently, performance artists benefit from fast switching, because quicker oscillations allow better co-ordination between the two brain hemispheres. On the other hand, possessing a brain with a slower switching rate would be an advantage for someone who tackles analytical problems which demand prolonged mental concentration.
Anyone wishing to undertake the MIB test should follow the above procedures for running the Dots program. The test should be timed for 30 seconds. While the test is running, count the number of times the yellow dots reappear after they disappear. Compare your score with the average figure of 4 to 6 cycles (per 30 seconds). It would be interesting to see how people with autism, or someone with unusual mental abilities would fare in this test.
P. Walker, Binocular Rivalry: Central or Peripheral Selective Processes?, Psychological Bulletin 85, pp. 376-389, 1978
Yoram S. Bonneh, Alexander Cooperman and Dov Sagi, Motion-Induced Blindness in Normal Observers, Nature 411, pp. 798-801, 2001
Steven M. Miller, Guang B. Liu, Trung T. Ngo, Greg Hooper, Stephan Riek, Richard G. Carson, John D. Pettigrew, Interhemispheric Switching Mediates Perceptual Rivalry, Current Biology 10, pp. 383-392, 2000
Olivia L. Carter & John D. Pettigrew, A Common Oscillator for Perceptual Rivalries?, Perception 32(3), pp. 295-305, 2003
Yoram Bonneh, Alexander Cooperman and Dov Sagi, Motion Induced Blindness, www.weizmann.ac.il/home/masagi/MIB/mib.html, 2000
Jonica Newby, Paul Schneller and Maria Cellabos-Wallis, Brain Switch, Catalyst, www.abc.net.au, 2004
S. M. Miller, B. D. Gynther, K. R. Heslop, G. B. Liu, P. B. Mitchell, T. T. Ngo, J. D. Pettigrew and L. B. Geffen, Slow Binocular Rivalry in Bipolar Disorder, Psychological Medicine 33, pp. 683-692, 2003
John D. Pettigrew and Steven M. Miller, A "Sticky" Interhemispheric Switch in Bipolar Disorder?, Proc Biol Sci. 1998 November 22; 265(1411), pp. 2141-2148, 1998
Source Code and Binaries
- Complete Source Code - GitHub