SAN DIEGO—In the textbook explanation for how information is encoded in the brain, neurons fire a rapid burst of electrical signals in response to inputs from the senses or other stimulation. The brain responds to a light turning on in a dark room with the short bursts of nerve impulses, called spikes. Each close grouping of spikes can be compared to a digital bit, the binary off-or-on code used by computers.
Neuroscientists have long known, though, about other forms of electrical activity present in the brain. In particular, rhythmic voltage fluctuations in and around neurons—oscillations that occur at the same 60-cycle-per-second frequency as AC current in the U.S.—have caught the field’s attention. These gamma waves encode information by changing a signal’s amplitude, frequency or phase (relative position of one wave to another)—and the rhythmic voltage surges influence the timing of spikes.
Heated debate has arisen in recent years as to whether these analog signals, akin to the ones used to broadcast AM or FM radio, may play a role in sorting, filtering and organizing the information flows required for cognitive processes. They may be instrumental in perceiving sensory inputs, focusing attention, making and recalling memories and coupling various cognitive processes into one coherent scene.
It is thought that populations of neurons that oscillate at gamma frequencies may unite the neural activity in the same way the violin section of an orchestra is coupled together in time and rhythm with the percussion section to create symphonic music. When gamma waves oscillate in resonance, “you get very rich repertoires of behaviors,” says Wolf Singer, a neuroscientist at the Ernst Strüngmann Institute in Frankfurt, Germany, who researches gamma waves. Just as your car’s dashboard will vibrate in sync with the motor vibrating at a resonant frequency, so too can separate populations of neurons couple in resonance.
Phillip Gander, a neuroscientist at the University of Iowa, presented results at the meeting suggesting how gamma waves could contribute to working memory, the brain’s mental scratch pad for immediately recalling information. Gander’s study demonstrated that when a person remembers a tone, gamma waves kick up in the idling brain during the interval between when a test tone is sounded and when the subject is asked to recall it. The gamma frequencies surging through the auditory and frontal cortex may assist in temporarily remembering a sound or another sensory input, holding the thought in mind in the same way a tuning fork set vibrating sustains a pitch long after it is struck.
The growing interest in gamma waves is not shared by all neuroscientists. It generated controversy that drew an overflow crowd to a session at the giant Society for Neuroscience (SfN) annual meeting in San Diego in early November. Debate there centered on whether gamma waves are fundamental to the brain’s operations or instead just an irrelevant byproduct, like the hum of an electronic amplifier.
A substantial body of evidence for the role of gamma waves in mental processing is generally lacking. Critics point out that oscillations arise everywhere one looks in nature, from surf crashing rhythmically onto shore to the ear-piercing squeal of microphone feedback in a public address system. The mere existence of electrical circuit oscillations, they contend, does not mean that they are integral to neural functioning. “Right now these are a lot of beautiful theories without a lot of experimental testing,” says Jessica Cardin, a systems neuroscientist at Yale University’s School of Medicine, a skeptic in the SfN debate. She notes that the evidence amassed so far is not based on rigorous tests looking for a cause-and-effect relationship between gamma waves and specific neural processes.
One area where a consensus has emerged relates to the role of gamma waves in neurological and psychological disorders. “I am a psychiatrist and one of the reasons I am interested in this is because gamma rhythms are clearly disturbed in a number of psychiatric conditions, most commonly schizophrenia and autism,” says Vikaas Sohal, a systems neuroscientist at the University of California, San Francisco. Sohal, also acknowledges that it has yet to be ascertained whether the oscillations are a cause of cognitive dysfunction.
Some researchers have already begun to test whether altering gamma waves may help treat mental disorders. Studies by Singer and others have shown that people can learn to control the power of their gamma waves in specific regions of their cerebral cortex by using neurofeedback as a possible therapeutic technique. Neuroscientist Elizabeth Buffalo, a professor of physiology and biophysics at the University of Washington, Seattle, who moderated the debate at the SfN meeting, noted: “There are a couple of small studies on autistic kids with biofeedback that have promising results.”
Also, various forms of rhythmic electrical brain stimulation to alter gamma waves—and other frequencies of neural oscillations—are starting to be tested for a wide range of disorders, including chronic depression, autism, schizophrenia and others. Cardin cautions, however, that the therapeutic approach being tested—using techniques such as deep brain stimulation and transcranial magnetic stimulation—may simply disrupt a brain circuit by excessively stimulating or inhibiting electrical activity, thereby shutting down aberrant signaling. In the extreme, electroshock therapy is effective in treating depression, but the treatment may not correct abnormal oscillations that underlie the condition.
This same criticism about blunt tools undermines experimental efforts to test whether oscillations are fundamental to brain function or just fumes from the brain’s cognitive engine. Existing techniques alter brain activity too severely to provide a convincing test. What’s more, manipulating brain waves also alters the timing of spikes, making it difficult to determine which activity—the waves or spikes—may be involved in either normal or aberrant neural processing. What is needed is yet-to-be-invented technology to manipulate the phase of oscillations and the timing of spikes independently in specific neurons, the scientists agree. Only then will it be possible to tell which one is responsible for a given cognitive process.