TORONTO, August  30, 2001 --  Hugh R. Wilson, professor of biology and a neuroscientist at York University’s Centre for Vision Research, has created a method for measuring traveling waves in visual perception, using the visual phenomenon of "binocular rivalry" to trigger the wave in the brains of human subjects. The technique is based on concepts from Nonlinear Dynamics. His report, Dynamics of Traveling Waves in Visual Perception, with co-authors Randolph Blake and Sang-Hun Lee, is published today in the scientific journal Nature.

It is the first time that scientists have been able to measure the speed of traveling waves in visual perception. The achievement holds significant promise for furthering research into how vision changes as we age, as well as understanding epilepsy, visual auras generated by migraine headache, and other similar traveling wave patterns in the human brain.

Wilson’s findings shed light on the prevailing belief that what happens in the brain during binocular rivalry holds the key to understanding where consciousness is located. "Our research suggests the process is much more complex, and it no longer makes sense to talk about an exclusive brain area being responsible for consciousness," says Wilson.

The phenomenon of binocular rivalry has been known since 1593, and describes how the brain tries to process contradictory visual images. When each eye is made to view a radically different pattern, the brain can’t put together a coherent interpretation of the world and oscillates back and forth, turning nerve cells on and off as it tries to make sense of the two images. Subjects wearing red/green glasses to view the contradictory patterns in 3D, see the effect of the two images competing in the brain -- a slow, undulating shift back and forth between the patterns in their visual field about once every one or two seconds.

Scientists have traditionally used a vertical/horizontal grid shape as the visual trigger for this effect, and have not been able to determine the location and speed of the brain’s activity. They believed that competition between nerve cells responding to contradictory visual images occurred much higher in the brain.

"Our data show there are two levels of competition, one in the early visual brain and one at higher levels," says Wilson. "Our insight was to use a grid pattern in the shape of an annulus or donut as the visual stimulus. We developed a technique for triggering a wave at any point around the ring of the donut and we timed it to its destination."

Wilson says he was inspired by the work of former colleague Leon Glass at McGill University who used an annulus to measure traveling waves in cardiac tissue. "Traveling waves are ubiquitous in nature -- in sleep, in healthy cardiac tissue, in certain chemical reactions in a petri dish," Wilson points out.

His group has developed a computer model of nerve cells in the visual part of the brain to interpret the complex measurements, employing the theoretical framework of Nonlinear Dynamics – the nonlinear mathematical study of change in time.

Among Wilson’s findings:

• when mapped onto the early visual cortex, the waves traveled at a constant velocity of 2 ¼ cm/sec. This provides strong evidence that it is the early parts of the visual brain (primary visual cortex) where the phenomenon occurs, because only low levels of the visual cortext contain visual maps. However, these new results are compatible with the existence of a second form or rivalry -- object rivalry -- that takes place at higher levels of the brain responsible for form vision.
• it took an extra 1/6th of a second for the wave to cross from the brain’s left hemisphere to the right hemisphere, providing further evidence that the activity takes place in the primary visual cortex. The left and right halves of the visual world are represented in opposite brain hemispheres that are interconnected by longer nerve fibers.
• excitatory connections between visual brain cells (as opposed to inhibitory connections that shut the nerve cells off) speed up the wave motions, providing one of the most vivid experimental examples to date that these excitatory connections exist in the human brain, as they are already known to exist in the brains of monkeys and ferrets.

Wilson credits the theory of Nonlinear Dynamics for many of the breakthroughs in neuroscience. "Nonlinear dynamics reveals and elucidates a range of phenomena that are simply inconceivable in the more mundane world of linear systems theory. Memory and forgetting, decision making, motor control, action potentials, and perhaps even free will and determinism can no longer be intelligently conceptualized without a basic understanding of nonlinear systems," writes Wilson in the introduction to his book, Spikes, Decisions & Actions: Dynamical Foundations of Neuroscience (Oxford University Press, 1999).

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For more information, please contact:

Dr. Hugh R. Wilson
Centre for Vision Research
York University
416-736-2100, ext. 33140
hrwilson@yorku.ca

Susan Bigelow
Media Relations
York University
416-736-2100, ext. 22091
sbigelow@yorku.ca
YU/092/01