Neurons got Rhythm

Stefanie Liebe had been training her rhesus monkeys for almost a year, and now the animals were finally ready to start her experiment. The very first results caught her attention. “There was no need for statistical analysis; the synchronous oscillation was already very clear in the raw data,” recalls the neurobiologist. “I thought that was extremely cool.”

When researchers investigate processes involving multiple brain areas, they often find – as Stefanie Liebe did in her study from 2012 – synchronous oscillations, i.e., synchronized activities of entire neuron assemblies firing in identical frequency bands. Many scientists now suspect that such recurring electrical patterns are very important for complex cognitive performance. By analyzing them, they hope to better understand how the brain works.

For her doctoral thesis at the Max Planck Institute for Biological Cybernetics in Tübingen, Stefanie Liebe wanted to find out how different areas of the brain work together in short-term memory. To do this, she taught monkeys to look at images on a monitor. The animals learned that they had to press a lever as soon as an image appeared. The monitor then went black for a moment before another image appeared. The monkeys then had to decide whether this image was the same as the first one. If, for example, a butterfly was shown first, the monkeys were trained to release the lever only when another butterfly image appeared.

The researcher was now interested in what happened in the monkeys' brains when only a black screen was visible between the two images for about 30 to 50 milliseconds. This was the phase in which the animals remembered what they had seen. Using tiny electrodes, she measured the electrical voltage of individual cells at rest and how it changed after receiving impulses via the cells' synapses, known as the postsynaptic potential.

She focused on two areas of the cerebral cortex: an area in the prefrontal cortex, which is essential for short-term working memory, and a section in the visual cortex at the back of the brain, which processes visual information. These two areas are co-active when the monkey remembers a butterfly image – so they must communicate with each other in some way.

And indeed, the measured voltage in the visual cortex began to oscillate in a specific pattern. The area of short-term memory oscillated in exactly the same pattern. The two areas of the brain had thus found a common rhythm across the distance; they were oscillating in unison, so to speak. Such synchronous oscillation only occurred when the monkeys were actually able to remember. If they were unable to do so, no synchronicity could be seen in the oscillations.

Ever since Hans Berger performed the first electroencephalograms (EEG) in 1924, doctors have known that electrical impulses in the brain constantly occur in rhythmic patterns that differ significantly between sleep and wakefulness, but also during certain perceptual processes in the brain. An EEG illustrates this in a regular sequence of zigzag lines. It measures the electrical activity in the brain using electrodes placed on the head. The device outputs the voltage changes in the form of many simultaneous wave lines. Because the electrodes are quite far away from the actual active areas of the brain, each signal can only be a very rough approximation of what is actually happening under the scalp and skull. Nevertheless, the EEG shows similar deflections in areas of the cerebral cortex that are far apart.

In the meantime, measurements taken directly in the brain have made it possible to assign numerous oscillations to neural networks. In recent decades, researchers have developed theories to explain the phenomenon of common frequency changes.

Stefanie Liebe suspects that the synchronous oscillation she measured improves the exchange of information between brain areas. “You can imagine it as if the synchronized oscillation in both areas causes a revolving door to start turning,” says the researcher. “This allows signals to be sent back and forth more easily.” A neural network oscillates at the same rate as another so that it is particularly sensitive to its information, believes Stefanie Liebe.

Some researchers go one step further. They believe that synchronous oscillations are not only important for effective information flow, but that they enable more complex cognitive performance in the first place. Neurobiologist Wolf Singer, now at the Ernst Strüngmann Institute (ESI) for Neuroscience in Frankfurt, measured synchronous oscillations in cats back in the 1980s. He immediately suspected that this could solve a fundamental puzzle that has been occupying brain researchers: the so-called binding problem.

It has long been known that sensory impressions are fragmented in the brain. When a person sees a red ball flying past, for example, several small neural networks in the visual cortex become active. One area signals “something red”, another “something round”, and a third “movement from right to left”. However, people are not aware of these partial impressions; they only perceive the whole, i.e., the red ball flying past. But how does the brain connect this fragmented information to form an overall picture? Researchers summarize this question with the term “binding problem”.

Often, the binding problem is not limited to one sense, such as sight. Wolf Singer likes to use the example of a barking dog that is being petted. Many areas of the visual system are active in this process: the size, color, and movement of the animal are analyzed. In addition, information about the texture of the fur is transmitted from the stroking hand to the brain. This information is processed in the areas of the brain responsible for the sense of touch. Because the dog is barking, the auditory cortex is also involved in processing the auditory impression.

The person stroking the dog must also evaluate the dog's behavior, as the animal could become aggressive. This is why the limbic system, which is responsible for processing emotions, is also active. This creates an overall impression: this is a collie with long, reddish-brown, very soft fur, which barks loudly but is actually harmless. All of the information is linked together and assigned to the dog.

As a solution to the binding problem, it was initially discussed whether there might be an area in the brain that is exclusively responsible for conscious perception. There could be, for example, a nerve cell that only fires when the collie with the soft coat barks. Despite intensive research, however, no such area of the brain – that would represent consciousness, so to speak – has been found to date. There is another catch to this theory. “If there were a separate cell for every conceivable situation, it would result in a combinatorial explosion,” says Wolf Singer. Although there are a very large number of nerve cells, there are also a very large number of possible sensory impressions, and there would have to be at least one cell for every combination of these impressions. This is not feasible, even with billions of brain cells.
 

But how else could the binding problem be solved? When Wolf Singer measured synchronous oscillations in cats' brains almost 30 years ago, he came up with an idea. What if it is not where something takes place in the brain that is important, but when? The information “soft fur,” “loud barking,” and “reddish-brown dog” is perceived simultaneously. A pattern of brain areas specific to the situation is active and temporally related. So perhaps it is this simultaneity that is responsible for the overall impression of the barking collie. “The information is perceived uniformly. Where? Nowhere! The activity remains distributed throughout the brain,” says Wolf Singer. “Today, most brain researchers assume that the system deviates in time as an encoding space.”

For this to work, the brain must be extremely sensitive to temporal relationships. It therefore needs a very precise clock. And scientists suspect that the brain creates this clock itself through rhythmic oscillations. “You can imagine the synchronous oscillations as the pendulum strokes of a wall clock,” Wolf Singer explains. “They provide the beat so that the brain can define temporal relationships.”

Our conscious perception may therefore not arise in a specific area of the brain, but is encoded in time with the help of oscillations. Perhaps this also applies to all other conscious thought processes. As you read these lines, you feel addressed at this moment. And according to the theory, this thought cannot be localized to one place in your brain. It arises solely from the fact that a certain pattern of brain areas is active at the same time. It is difficult to imagine, but many research findings support the theory of the temporal correlation of consciousness.

According to Wolf Singer, schizophrenic people provide a possible example of what happens when we are no longer able to do this. His research group used magnetoencephalographs (MEGs) to compare brain activity. In healthy people, the scientists observed a sharp increase in high-frequency oscillations when they showed their test subjects specific images. In schizophrenic patients, the increase was significantly lower, and there was hardly any synchronicity between different areas of the brain. Schizophrenic brains are therefore not as good at establishing temporal connections. A key symptom of this disease is that patients have fragmented thoughts; they perceive things separately that actually belong together. This distorted perception could be related to an incorrect rhythm in the brain.
 

Further reading

• Liebe S. et al: Theta coupling between V4 and prefrontal cortex predicts visual short-​term memory performance. Nature Neuroscience. 2012; 15(3):456 — 462 (zum Abstract).
• Singer, W.: Binding by synchrony. Scholarpedia. 2(12):1657 (zum Text).
• Uhlhaas PJ, Singer W: Abnormal neural oscillations and synchrony in schizophrenia. Nature Reviews Neuroscience. 2010; 11(2):100 — 113 (zum Abstract).

Published on April 12, 2012
Last updated on October 9, 2025