By Andrew O’Keeffe
Commonly, we think of communication occurring between individuals of a species. However, within the individual, different organs and cells must also communicate with each other. The nervous system can coordinate the functions of different organs throughout the body quicker than the blink of an eye.
A neuron (nerve cell) depicting the axon projecting out from the cell body. (Wikimedia Commons)
On first inspection, it seems as though communication among neurons within the brain ought to follow straightforward pathways. We know that wire-like projections (axons) extend from most neurons (nerve cells) to carry electrically-mediated impulses to other nerve cells. These electrical signals are then passed between nerve cells by the release of, neurotransmitters, chemical messengers of the nervous system. Conventionally, we assume that these electrical impulses (action potentials) are what transmits information from one location in the brain to another. However, something more subtle, and more interesting, may also be at work when different regions of the brain talk to each other.
In 1929 a German scientist, Hans Berger, placed electrodes on the scalps of human subjects and recorded the first ‘brain waves’, or what we now know as neural oscillations. These are fluctuations in electrical fields generated by the synchronized electrical activity of large populations of neurons. When large numbers of nerve cells are all acting synchronously, the signal they emit can be strong enough to be detectable using electrodes on the scalp or surface of the brain. Current thinking suggests that these changes in the synchronous electrical fields surrounding nerve cells could be assisting in communication between different regions of the brain. That is to say, the electrical fields generated by nerve cells in one part of the brain may be helping that area ‘talk’ to distant areas of the brain that are not directly connected by axons. If this idea is correct, it would constitute a super-fast system for the transmission of information within the brain. Faster even than the rapid connections between areas allowed for by axonal connections. But how can we tell if neural oscillations are helping to process information?
One method is to try to prevent these fields from occurring while preserving the firing patterns of neurons. This can show us if the lack of neural oscillations impairs the normal functioning of the brain. Indeed, we do have a technique that can disrupt electrical fields in the brain using strong and highly focused magnetic fields. This technique, called transcranial magnetic stimulation (TMS), can inactivate areas of the cortex (the surface of the brain) temporarily halting speech or paralyzing a limb. A short video illustrating the power of this can be found here.
However, by disrupting the electromagnetic field, TMS also appears to disrupt the firing patterns of nerve cells. While this demonstrates how important electromagnetic fields are for the normal functioning of the brain, it also highlights how difficult it is to separate electric fields from neural firing patterns in order to study them independently. To attempt to work around this problem in the study of neural oscillations, our group of surgeons and scientists at UCLA have sought a novel approach using Parkinson’s disease (PD) patients.
PD is a common neurodegenerative disorder caused by the loss of a small set of highly specialized neurons in the deep brain structures. The symptoms of PD include an inability to move, slowness of movement and a severe shaking of the hands. Many people can be effectively treated in the early stages of the disease by using medications that replace dopamine, a neurotransmitter depleted in PD. However, after several years on medications these therapies often become ineffective. At this stage a surgical option of implanting electrodes into deep brain structures, a procedure known as deep brain stimulation (DBS) can restore a good level of function in a select group of patients. This video illustrates how effective this technique can be, despite the fact that we know very little about how the technique works.
We propose that the symptoms of PD are due in part to abnormalities in the neural oscillations of the brain and that stimulating the brain using DBS acts to re-establish the regularity of the oscillations and improve symptoms. The DBS procedure provides a valuable opportunity to study the neural oscillations of the human brain at close quarters. During the implantation of DBS leads, we insert an additional strip of electrodes through the hole created in the skull so that recording contacts are lying over the wrinkled cortical surface of the brain (see Fig.1). The strip of electrodes is positioned to lie over both the motor cortex, responsible for coordinating muscle movements on the opposite side of the body, and premotor cortex that we think plays a role in sending movement instructions to the motor cortex. Meanwhile, our patients, who remain awake during the surgery, can be directed to carry out tasks that draw upon both of these brain areas and hence require the two areas to communicate with each other.
Using this approach, we have studied the patterns of neural oscillations during rest and movement in PD patients of different severities. The results have been both revealing and provocative. First of all it appears that during rest, the information in neural oscillations is travelling from the ‘higher order’ premotor cortex to the ‘lower order’ motor cortex. However, this situation reverses during movement. Our finding in this respect runs counter to our expectations. We predicted that during execution of a movement, information is sent from higher movement centers to lower movement centers. One possible explanation is that the signal we observe represents a message passing from the lower to the higher motor areas saying something like “I’m doing some movement here. Don’t interrupt me!”. Such a signal could be crucial to the stability of a movement plan and prevent it from being interfered with by competing movement plans.
We also discovered that some of the symptoms of PD could arise from an inability to desynchronize the neural oscillations between the two motor areas (motor cortex and premotor cortex). More severe PD sufferers show less ability to adjust the relationship between neural oscillations in the two areas, unable to put them in a state where they are oscillating independently of each other. This suggests the intriguing possibility that therapeutic DBS is acting to improve the symptoms of sufferers by improving the brain’s ability to change the relationship between electrical fields at the level of the cortex.
Not only do these results suggest a fascinating new mechanism behind the symptoms of PD but they raise some interesting philosophical considerations too. If neural oscillations are playing such a fundamental role in the regulation of movement, could they also be playing a similar role in the production of language and thought? At present we have no reason to believe that the parts of the brain governing these processes operate in fundamentally different ways to those that produce movement. Most importantly, however, this research gives us new insights into how different parts of the brain communicate with each other. This could help us to understand how problems with these brain signals contribute to diseases of the brain and nervous system leading to better treatments for these disorders. Modern science has firmly established the brain as the seat of thought and indeed, of consciousness itself. Knowing how the brain transmits and processes information is just one part of the puzzle in understanding how the matter of the brain generates the conscious experience of our inner worlds.
Andrew O’Keeffe is a 2012 International Fulbright Science & Technology fellow from the United Kingdom, he is currently taking time out of his neurosurgical residency in London to complete a PhD at the Brain Research Institute, UCLA. Andrew can be contacted via email if you would like more information.