Science Forum

A number of years ago, a generous award was offered to anyone who could scientifically prove the existence of the soul. In spite of many frenzied attempts, the award was never collected.

One might suspect that the mind, or human consciousness, would be as difficult to scientifically prove as the existence of the soul. Both appear to be equally nebulous quantities. Yet some neurobiologists are crediting Wolf Singer of the Max Planck Institute for Brain Research in Frankfurt, West Germany, with just that startling feat. Singer, himself, is not so sure. A recent article in the journal Science, published by the American Association of the Advancement of Science, describes how Singer spotted a low-frequency electrical oscillation in the brain that may be related to how the human mind assimilates the stimuli that the senses provide it.

Last year, Singer’s research group published a dramatic finding in the journal Nature. While recording electrical signals from widely spaced neurons in the brains of cats, Singer noticed that they fired synchronous electrical impulses when responding to stimuli that stemmed from the same object. When presented with stimuli from different objects, the neurons fired but not in any kind of synchrony. The synchronous signals were oscillating waves with a frequency of 40 hertz or cycles per second. What has neurobiologists excited is that, up to Singer’s announcement, few believed that neurons in different parts of the brain could coordinate their efforts while responding to stimuli.

These low-frequency impulses may prove to be the answer to a problem that has plagued neuroscientists for years; how do distant neurons, unconnected in any apparent way, pool their responses to create a single visual image? The article uses the simple act of visualizing a pen to illustrate the complex process by which the brain assimilates the many separate bits of information relating to the pen. Some nerve cells, for example, respond to the orientation of the pen’s edges, others record its color, movement, size, etc. The nerve cells, or neurons, are widely separated in the brain yet somehow they manage to coordinate their information input into a single coherent image. The process by which this occurs, known as “visual binding,” has remained a mystery.

In 1981, Christoph von der Malsburg of the University of Southern California speculated that visual binding may occur when neurons briefly coordinate their activity when exposed to the same stimuli. At the same time he made the proposal, von der Malsburg doubted that it could ever be proven. How could you look for a synchronous pattern of activity among the billions of neural firings going on in the brain? It would be like looking for pairs of matching lightning bolts in a tremendous electrical storm.

“I despaired of finding signs of visual binding,” said von der Malsburg. “I assumed that the neurons involved in any particular visualization would be scattered all over the brain and there would be no way to pick them out.” In a memorable comment, von der Malsburg said, “I thought the mind would be invisible.”

In 1986, Singer had a postdoctoral associate in his lab working on electrical impulses emanating from the primary visual cortex, an area of the brain that responds to visual stimuli. The area consists of columns of nerve cells with each column responding to a specific orientation of the object. Some columns might be sensitive to vertical edges, others to horizontal edges while others respond to edges at an angle. The cells in a particular column fire off periodic bursts of electricity, called action potentials, when exposed to an orientation to which they are attuned. The “postdoc” was recording impulses from electrodes implanted in the orientation columns of kittens. She used filters to remove background noise in order to study the activity of individual neurons. With the filters removed a record of the field potential, or average electrical activity of all the neurons in the area, can be recorded. When this was done she noticed that for periods of less than half a second the field potential oscillated at 40 hertz. Singer, aware of von der Malsburg’s hypothesis, wondered if the almost unnoticeable oscillations were clues to the elusive visual binding.

Singer’s research team set out to see how, or if, the oscillations could be connected to visual stimuli. In what is considered a significant experiment, the group found that cells in columns separated by as much as seven millimeters fired in synchrony if presented with bars of light moving in the same direction. When the bars moved in opposite directions, the cells fired but not in synchrony. Such synchrony was once considered heresy by neurobiologists who believed that few, if any, long-distance connections existed between the columns.

Singer is not willing to go further than to say that the 40-hertz synchronous oscillations appear to a widespread neural response to visual stimuli that might represent visual binding. Others are willing to go much further for him. Christof Koch, a neurobiologist at the California Institute of Technology, says that the oscillations, “could be the neuronal expression of attention.” Koch says the brain may use similar oscillations to focus attention on objects not only seen but felt, smelled, tasted, or heard as well. Francis Crick of the Salk Institute has gone even further by theorizing that synchronous nerve-cell activity may not simply be related to sensory stimuli but also regulate such intangible activities as thoughts and ideas. Crick has moved light years ahead of Singer’s cautious claims by proposing a “neurobiological theory of consciousness” that suggests synchronous brain activity to be the fount from which the mind evolves. Koch points out that the time scale associated with the 40-hertz oscillations corresponds with that of attention flitting from one object to another.

Criticism thus far has been leveled more at Koch and Crick than at Singer’s data. This is probably due to the fact that he has made few far-reaching claims for it. The criticism is that Koch and Crick are formulating a theory of consciousness around a set of experiments performed on anesthetized cats. This is a valid point. How can a theory of consciousness be based on observations of animals that are not conscious? Singer agrees that if the oscillations are connected to visual binding, or even more complex phenomena, then they must also be confirmed in alert animals.

It might seem, in Shakespeare’s words, that “much ado is being made about nothing.” These are, after all, minor electrical oscillations that may simply be an artifact of some more complex reaction that is unknown at the moment. Nonetheless, neurobiologists are equally awaiting further developments. “Wolf Singer is onto something extremely important,” says von der Malsburg. “If this experiment materializes further, it could open a completely new era in our understanding of the mind.”

Clair Wood, a science instructor at Eastern Maine Technical College, is the NEWS science columnist.