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Professor John Broadhurst investigates the magnetic physiology of the brain
by Jeremy Paschke
In 1870, when two German scientists -- Eduard Hitzig and Gustav Frisch -- touched rudimentary electrodes to the cerebral cortex of a patient undergoing surgery, they inadvertently caused anger and fear in the patient. The brain, Hitzig and Frisch discovered, is an electrically structured organ.
In later years, finer electrodes enabled biophysical researchers to draw connections between given regions of the brain and specific modes of behavior. For instance, stimulation of the hypothalamus induces rage, and it was found that the difference between pleasure and pain is separated by the distance of a single cell. Messages in the nervous system travel by electrical impulse along chains of neurons, the building blocks of nervous tissue.
Mapping the complex circuitry of the brain has been an enchanting yet elusive goal to many scientists. Now a new attempt is under way to reach that goal, and it involves studying electricity's closest of kin -- magnetism.
Manifest in every electrical current there is a magnetic field - electricity and magnetism being inseparable - which radiates in concentric circles out from the chain of neurons as they convey an electrical signal. For most of the twentieth century, detecting these magnetic fields was an impossible task since they are incredibly weak - a hundred million times weaker than the earth's magnetic field. Technologies springing forth from superconductivity, however, have made detection realizable and have opened the way for the revolution in magnetic brain research.
At the University of Minnesota, physics Professor John H. Broadhurst is playing an active role in analyzing the magnetic physiology of the brain. He investigates the magnetic patterns involved in conditioned response and researches how the brain processes a combination of sight and sound stimuli. Broadhurst hopes that his endeavors may contribute to a greater knowledge of how the brain works. Broadhurst hypothesized that two combined signals would create a pattern of magnetic fields in the brain that was distinctly different than an overlap of two independent signals. He based this hypothesis on the natural tendency to connect sight with sounds.
When humans view matter in motion, the brain is conditioned to expect some aural input. For instance, when abaseball player hits a ball, we expect to hear the crack of the bat. But consider the example of a shooting star.
"One gets a strange feeling when watching a shooting star," says Broadhurst. "This is because the shooting star is a spectacular sight with no accompanying sound." Contemplating this and other observations led Broadhurst to wonder how the brain processes a combination of sight and sound stimuli. Could the processing be a simple overlap of the two stimuli, or are the signals combined by the brain in some unique fashion?
In order to test his hypothesis, Broadhurst and his graduate student, Kevin Knuth, designed an experiment where a human volunteer is placed beneath a magnetometer, which measures the magnetic fields in the brain, to observe the magnetic fields produced within the brain after exciting both optical and auditory senses of a human subject. A flash of light and an auditory "click" is sent to the subject - flash / click /flash / click /flash / click - hundreds of times over, conditioning the brain to connect the flash with the click.
Then, the experiment operator suspends the click, and the subject sees only the flash.
Broadhurst observed that for the first five or six iterations, a magnetic field surged in the subject's brain within the first hundredth of a second after the flash / click event. However, once eight to ten of the flash / click patterns were repeated, the subject's brain learned to combine both sight and sound signals. After eight to ten repetitions the initial magnetic field surge disappeared. When the click was suspended and the pattem broken, the initial magnetic surge returned as the brain adjusted itself on what to expect, either flash / click, or flash / silence. Broadhurst studied this adjustment.
Repeated experiments proved that after a volunteer developed a conditioned response, the magnetic field generated in his or her brain by a silent flash was, as Broadhust predicted, vastly different than the magnetic field generated by both flash and click. This experiment was the first to provide solid evidence that the brain's examination of combined visual and auditory impulses is different than a simple overlap of two separate signals. Broadhurst concluded that the brain adapts itself in order to process the combined signals, and the brain is confused when the combination is broken.
Magnetic fields are produced in the brain by electrical surges along neural pathways, so Broadhurst used magneto-encephalography (MEG) to measure the brain's magnetic field. Previous studies on the human brain used electroencephalography (EEG) which directly measures electrical activity in the brain, and which finds practical applications in diagnosing epilepsy and discovering tumors. EEG has limitations that MEG can supercede. First, MEG measures the magnetic field, which is a vector quantity, where EEG measures the electric potential, a scalar quantity. Researchers prefer vectors over a scalars because vectors have a magnitude and a direction, while a scalar only has a magnitude. A good example of a vector is the wind, where both the strength and direction are imperative. On the other hand, temperature and mass - both without direction - are scalar.
A second limitation of EEG is that in order to measure electric potential, physical contact must be made with the brain. To an accurate reading with EEG, study participants must undergo surgery to remove sections of their skull. Obviously, it is easier to find participants for MEG research than it is to find EEG participants. "Most people," jokes Broadhurst, "don't like having their skulls removed."
Even so, there are limitations to using MEG. Recording the magnetic field alone does not immediately yield the location of electrical currents in the brain. In fact, tracking magnetic activity is a very indirect route to watching how the brain's neurons are behaving. As Broadhurst's current graduate student, Alex Roitman, admits, "There are many different currents that produce the same magnetic field configurations."
Sifting through hundreds of possibilities for the most likely candidate is an arduous chore. Also, the human brain's magnetic field is extremely weak, on the order of 10-12 Tesla, which is a hundred million times weaker than the earth's magnetic field. Detecting such a weak field requires special equipment.
The University of Minnesota does not own the equipment necessary for Broadhurst's experiments, so he performs his research at the Scripps Institute in La Jolla, California. The MEG device at Scripps is called a Super-Conducting Quantum Interference Detector (SQUID).
The entire unit magnetometer is composed of many individual currents traveling along neural detectors, many SQUIDS. One SQUID consists of a coil of wire resulting from the brain's activity roughly one centimeter in diameter that is cooled by liquid helium to a superconducting temperature of 4 Kelvin. A collection of 500 to 1,000 coiled SQUIDs make up the whole magnetometer, which embrace the subject's head like an oversized football helmet. The SQUID magnetometer is bulky because it needs a cryostat to con tain the liquid helium.
The magnetometer reads the magnetic field produced by electrical currents traveling along neural pathways. The magnetic fields cause a change in the magnetic flux through the superconducting coil. The changing flux induces an electromotive force which is read as voltage. This is a fine application of Faraday's Law of Induction.
Again, the SQUID charts the magnetic fields produced by neural activity inside the brain. These traveling charges are the messengers of information. Whether a person is conscious or unconscious, neurons in their brain are constantly firing. However, the MEG method cannot read a charge traveling across a single neuron. The current flowing across just one neuron is too small and its magnetic field too weak to detect, so a hundred thousand neurons must all carry a charge in the same direction at the same instant for the SQUID to obtain a reading. When this occurs, Broadhurst is confident that something has excited the peripheral senses and set the brain into action.
Broadhurst and other specialists in biophysics have developed a theory on how external stimuli are analyzed by the brain. First, scientists know that all sounds, from the buzzing of a mosquito to the loud clanging of church bells, originate with vibrating matter causing waves of high and low air pressure. When these pressure waves enter the ear, they push tiny hair follicles back and forth, "like chaffs of wheat swaying in the breeze," explains Broadhurst. The oscillations of the hair follicles set electric charges in motion beneath the inner membrane of the ear, and send electrical messages to the brain.
Before the message from our ear is analyzed, the volume must be adjusted. "This step allows us to have conversations in a noisy room," says Broadhurst, "where we must pay particular attention to a specific source of sound."
Next, our brain spatially maps the sound. Broadhurst gives a clever evolutionary example to illustrate the importance of spatial mapping. For example, "if a sabertooth tiger is chasing you he says, "you have to know which way to run." Broadhurst believes the human brain's ability to spatially map sound is a natural defense. Natural selection preferred those brains that could quickly associate a sound with its location in space. Birds of prey display an exceptional ability to map sounds. Owls, for instance, map sounds especially well and are therefore effective nocturnal hunters.
Within a twelfth of a second, the brain adjusts volume and maps the sound, then an area of the brain called the Silvius Fissure Region, which holds memory, analyzes the sound and attempts to connect it with sounds heard before.
Research with MEG reveals that most humans spend the first 150 milliseconds after hearing a sound scanning their memory to match it with a similar, known sound. If the sound is completely foreign, the brain chums away for a full half-second. Ideally, researchers would choose to always analyze human response to new sounds. However, new sounds are difficult to regenerate and study in a scientific manner because, as Broadhurst says, "new sounds are only heard once in your life." Familiar sounds, such as a semi-trailer's blaring horn or a harmonica's chord, precipitate an emotional or physical reaction because the sounds are connected to a host of other memories. Although there are given locations in the brain for memory, motor skills and other cognitive abilities, few scientists consider our brain as a strictly compartmentalized organ, where various functions only occur in their given location.
Instead, scientists think of the brain as "plastic." In other words, although there are preferred locations for memories and cognitive functions, on the whole, memories are stored in the order in which they are collected.
"Plasticity is a very good thing for humans ' " Broadhurst explains, "because it permits folding to occur." If the brain's outer layer, the cerebral cortex, can be folded, then the total surface area of the brain increases, and it can accommodate more nerve cells.
Moreover, a folded brain will hold the most important information enveloped within the "crease" of the fold, thus protecting that information from possible injury. A folded brain makes humans smarter thinkers. "Intelligence might be measured not by the size of one's brain, but by the degree of folding," says Broadhurst. Animals with folded, or highly corrugated brains, such as dolphins and humans, demonstrate far greater intelligence than animals without corrugated brains, such as birds and reptiles.
Scientists have a good idea what type of physiology is required for an intelligent brain, but those wishing to define every subtlety of human thought still face challenges. For anyone who fears that SQUID technology will reduce the complexities of conscious thought to a fundamentally identical mechanical blueprint for all human beings, Professor Broadhurst gives relieving counsel.
"Every brain is built different says Broadhurst, every person thinks differently, which is one reason why MEG research is so difficult. Although all human brains carry out similar processes and achieve similar goals, Broadhurst insists that we all think differently. Broadhurst compares the brain's neurons to a collection of several thousand logic gates in a program logic array. As a person acquires various cognitive skills riding a bicycle or leaming a list of names - neurons connect one to another. Each individual, as a result, builds a unique structure of logic gates in a unique neural framework.
SQUID technology is only 16 years old, but it offers great promise for improved resolution and increased data collection. What does this mean for a scientific understanding of human consciousness? Broadhurst says that current research on human consciousness is still at a very superficial level. The only neural activities MEG can detect are the reactions to external stimuli. Spontaneous or creative thoughts still lie well below detection threshold. "A true study of consciousness," says Broadhurst, would involve analyzing nonstimulated neural activities as well as stimulated ones."
However, these select reactions do not inhibit Broadhurst and Roitman from asking further questions with MEG. Their next investigation will probe the dependence of nervous response on a sound's frequency. In their study, a listener will hear a number of standard musical notes played at the same frequency from the same instrument, perhaps a cello. Then the experiment conductor will play a deviant note of the same frequency but from a different instrument, a French hom or an oboe. Roitman says the bulk their analysis lies in the "early stage of recognition, well before conscious connection is made." Roitman will use MEG to see if the volunteer subconsciously differentiates between the timbres of various instruments. If Broadhurst and Roitman prove that a sound's frequency is analyzed independent of its duration and timbre, they will make another stride in furthering the biophysical understanding of the human brain.
Scientists and non-scientists alike marvel at complexity of the human brain. The acclaimed biologist and Pulitzer Prize winning author, Edward 0. Wilson, argues that the rise of brain sciences will follow evolutionary theory, plate tectonics, and microbiology as the next great wave of scientific advancement. Wilson suggests that a superior understanding of consciousness will unite all sciences, along with the arts and human culture, under one platform of knowledge.
Meanwhile, here at the University of Minnesota, Professor John Broadhurst takes another step towards solving the last great mystery.
