A New View of Vision: Two Scientists Present a Startling Theory about How the Brain Processes Visual Signals
Vaughan, Christopher, Science News
A NEW View OF VISION
The brain may operate on the same principles as an FM radio. That is the simple way of describing the conclusions of two federal scientists after eight years of research on the brain's visual system. What they believe they have discovered is not that the brain operates by radio waves, but rather something that neurophysiologists find almost as surprising: The visual system seems to transmit information as a compound signal.
Frequency-modulated (FM) radio and many telecommunications networks send and receive information in a very compact manner, by adding together more than one signal and sending them as a compound wave. When the compound wave reaches its target, it is split back into separate signals. The ability to send compound signals, a technique called multiplexing (SN:2/23/85, p.119), is what allows FM radio stations to broadcast music in stereo.
Now, after thousands of tests of how monkeys react to visual patterns, Barry Richmond of the National Institute of Mental Health (NIMH) and Lance Optican of the National Eye Institute have come up with the multiplex filter hypothesis -- a complex, mathematical theory that challenges scientific orthodoxy by proposing that visual nerves transmit information via multiplexed, encoded signals. If their hypothesis proves correct, it could modify Nobel prize-winning work of 20 years' standing and become the cornerstone of a whole new way of thinking about the brain. In addition, it might change the way computer and robotics researchers approach artificial-intelligence problems and model brain processes.
When a picture is flashed in front of the eye, signals are sent from the eye to the brain in short, machine-gun bursts of nerve cell firings. This collection of signals, called the "spike train," passes from nerve cell to nerve cell in the brain. Standard neurophysiological theory holds that the number of spikes in the spike train determines the message the neuron sends: A very intense, staccato burst of nerve firing might be the strong signal generated by a bright visual pattern, for instance.
Richmond was measuring this neural response in monkeys briefly shown a rectangular bar of light when he noticed something interesting. When the bar position changed, the number of spikes sent along one of the visual neurons didn't change, but the pattern of firing -- the spacing of spikes in the spike train -- did change in a very reproducible way. Richmond could take a measurement with the bar at one position and get one pattern, record other patterns at other positions, then return to the first position and get the first spike-train pattern again. "I knew that anything that regular had to be meaningful," Richmond says.
Others in the lab told him to forget it, that only the intensity of the firing and not the pattern is important, and that people had wasted years looking at those patterns. Besides, they said, there were ways to get rid of the pattern differences in the data so that only firing-intensity data would remain. "Fortunately, I couldn't forget it," says Richmond.
Richmond asked biomedical engineer Optican if there might be some systematic approach the two of them could take in analyzing the spike-train patterns. Richmond hoped eventually to learn whether there was a systematic way of finding what neurons were doing on a basic level, and whether that information could be synthesized into a model of what perception is. Optican, who was involved in systematic, mathematical studies of eye movement, "couldn't believe" studies of visual perception were not done the same way. "I said, 'You mean they don't take a systematic approach to vision?'" he recalls.
Optican discovered that the actions of neurons are very well understood in the retina of the eye, but the meanings of individual nerve signals from the eye become less and less well defined as scientists follow those signals deeper and deeper into the brain. …