Alpha, beta, gamma: The language of brainwaves

I'VE just had a brainwave. Oh, and there's another. And another! In fact, you will have had thousands of them since you started reading this sentence. These waves of electricity flow around our brains every second of the day, allowing neurons to communicate while we walk, talk, think and feel.

Exactly where brainwaves are generated in the brain, and how they communicate information, is something of a mystery. As we begin to answer these questions, surprising functions of these ripples of neural activity are emerging. It turns out they underpin almost everything going on in our minds, including memory, attention and even our intelligence. Perhaps most importantly, haphazard brainwaves may underlie the delusions experienced by people with schizophrenia, and researchers are investigating this possibility in the hope that it will lead to treatments for this devastating condition.

So what exactly is a brainwave? Despite the way it is bandied about in everyday chit-chat, the term "brainwave" has a specific meaning in neuroscience, referring to rhythmic changes in the electrical activity of a group of neurons. Each neuron has a voltage, which can change when ions flow in or out of the cell. This is normally triggered by stimulation from another cell, and once a neuron's voltage has reached a certain point, it too will fire an electrical signal to other cells, repeating the process. When many neurons fire at the same time, we see these changes in the form of a wave, as groups of neurons are all excited, silent, then excited again, at the same time.

At any one time, a number of brainwaves are sweeping through the brain, each oscillating at a different frequency, classified in bands called alpha, theta, beta and gamma, and each associated with a different task (see diagram). This rhythmic activity turns out to be the perfect way to organise all the information hitting our senses. Every sensation we feel, from the itch of a sweater to the buzz of a cellphone, triggers a shower of neural signals. Brainwaves may provide clarity in this electrical storm by synchronising all the activity corresponding to a single stimulus - the words on this page, say - to a particular frequency, while neurons attending to another stimulus fire at a different frequency. This would allow brain cells to tune in to the frequency corresponding to their particular task while ignoring irrelevant signals, in much the same way we home in on different waves to pick up different radio stations.

"Brains have problems distinguishing signal and noise," says Karl Deisseroth, associate professor of bioengineering at Stanford University in Palo Alto, California. "We've found that in order for neuron A to talk with neuron B, it can better transfer information if it can synchronise its activity."

The importance of signal synchronisation becomes clear when you consider that the different aspects of a sensation - colour and shape in vision, for example - are processed in different parts of the brain before being sent to another region that binds them back together. "Imagine you are looking at an apple," says Deisseroth. "The apple's redness and roundness are picked up by different cells in the brain, but you don't see a red thing and a round thing - it's one item."

The rhythmic activity of brainwaves ensures that all the relevant signals relating to the sensation arrive at the binding region at exactly the same time. This allows the receiving neurons to process the signals together, recombining them into a single sensation. "If neurons are oscillating at the same frequency, signals from a stimulus would be treated together because the firing came in at the same time, and at the same point on the oscillation, so that the object is perceived as a whole rather than the separate details," explains Laura Colgin at the Norwegian University of Science and Technology in Trondheim.

Beyond their role in binding together all aspects of a sensation, however, the properties of brainwaves had remained murky. How, for example, do their specific characteristics, like the timing of each wave's rhythm, influence what we see, hear or remember? "We are only just beginning to understand these mechanisms," Deisseroth says.

Last year, Niko Busch, then at the Brain and Cognition Research Centre in Toulouse, France, found that the activity of a certain kind of brainwave determines whether something is seen or not. He used electroencephalography (EEG), which measures electrical activity through the scalp, to assess the neural activity of 12 volunteers exposed to rapid flashes of light. Surprisingly, the volunteers' brainwaves could be used to predict exactly which flashes they would see. If the flash coincided with the peak of a wave in the alpha or theta frequencies, they saw it, but if it occurred when the wave was at its trough, they didn't (Journal of Neuroscience, vol 29, p 7869). That fits, says Busch, since neurons are more likely to fire in response to visual input if they are already excited.