Electricity on the BRAIN
The electroencephalogram is a voltmeter for the brain
Electrical activity in the human brain was measured for the first time by an electroencephalogram (EEG) more than 60 years ago. EEG remains a valuable tool. The Montreal Neurological Institute was among the pioneers in its development, opening an EEG laboratory in 1939 under the direction of Dr. Herbert Jasper, a seminal figure. The last 15 years has seen the introduction of newer brain-scanning machines magnetic resonance imaging (MRI) and positron emission tomography (PET) but the EEG in many cases is still indispensable.
Physicians and research scientists use EEG in both clinical and experimental
settings to look at normal and abnormal brain functions and rhythms. Dr.
Jasper and Dr. Wilder Penfield, founder of the MNI, pioneered the use of
EEG in the analysis and treatment of epilepsy, for example.
The basic idea behind EEG machines has not changed since Dr. Jaspers day, although the technology has undergone considerable refinement. Todays models are portable and many record data digitally on computer screens rather than on rolling sheets of paper, as was done in the past.
EEG measures the electrical activity of brain cells during physiological processes in both normal and pathological states, during awake periods, and during sleep. To obtain data, scientists connect electrodes from the EEG machine to the surface of the scalp. The structure and chemical composition of neurons leads to the generation of an electrical potential, which is a relative difference in electrical charge across the nerve cell membrane.
In the early days, there was no consensus about where to place electrodes
on the scalp, nor was there a standard number of electrodes. In the 1950s,
Dr. Jasper introduced a system
in which electrodes were numbered and placed in specific spots, the odd-numbered electrodes on the left side of the head and the even-numbered ones on the right side. His system became the international standard. Using this system, physicians anywhere in the world were able to compare results.
Since then, other electrode placements have been introduced. Standard clinical tests today use between 16 and 32 electrodes. The head is divided into either five or ten equal segments. Electrodes are placed on the skull, the ears and the bridge of the nose. In research experiments, the number of electrodes can be much greater as many as 100.
The voltage generated by brain cells and picked up by EEG is extremely small between 20 and 100 microvolts after amplification on the order of ten thousand times. The signal is so small that electrical interference, called artifacts, from outside sources for example, motors, overhead lights, even an eye blink is often as strong as the signal that the EEG is trying to detect. When reading EEG charts, physicians need skill and experience to distinguish artifacts from brain activity and to decode the brains electrical rhythms into diagnostic information.
The skull is a poor conductor of electricity that interferes with the transmission of electrical charges to the scalp. Although the brain and the scalp are separated by only a few millimetres, the distance is an enormous chasm in EEG terms.
Two other factors that play a role in determining EEG signal strength
are cell alignment and cell firing synchronicity.
Cells that are not aligned in the same way act to cancel out EEG signals. Some parts of the brain such as the hippocampus have
well-organized neurons that offer relatively strong EEG signals. The amygdalas cells, by contrast, are aligned poorly in relation to each other and generate weak EEG signals. EEG also works more effectively when many neurons are firing synchronously,
as occurs during epileptic seizures. For EEG to recognize the electrical discharge in a particular part of the brain, many cells need to fire at the same time.
Telemetry is a method of measuring EEGs at a distance, and is often used to monitor EEG activity continuously in patients who are hospitalized but not hooked up to the EEG machine. Telemetry at the Montreal Neuro is carried out for the most part at the five-bed facility of the Epilepsy Monitoring Unit. Here epilepsy patients are monitored by EEG and videocameras 24 hours a day. Sophisticated computer software developed by MNI researcher Dr. Jean Gotman is used at the Unit as well as in the majority of epilepsy monitoring units throughout the world. Epilepsy patients who are taking medication to control their seizures are asked to reduce the dosage to allow seizures to recur. Sometimes patients must remain in the unit for a week before a seizure happens. Analysis of the data helps to identify the type of seizure, to determine where in the brain the seizure is arising, and to discover why it occurs.
EEG is also used at the Montreal Neuro to study sleep patterns. Some sleep disorders pose serious health problems. Narcolepsy, for example, causes a person to fall into a deep sleep without warning. Dr. Barbara Jones, a researcher in the Complex Neural Systems research group, studies neurons that control animals wakefulness and sleep. She is particularly interested in the function of certain cells in the brain stem and the basal forebrain. Human sleep studies involve use of a poly-somnogram, a device that combines EEG data with information about the patients respiration as well as heart and muscle activity and eye movements. The combined data are examined by pulmonary specialists, neurologists and cognitive neuroscientists. It is a multidisciplinary team approach to neuroscience.
Dr. Luis Felipe Quesney has studied EEG since 1972 and now heads the EEG department. He believes that EEG has a long future. One development that excites him is the combination of EEG with brain-imaging technology to create three-dimensional images of lesions. EEG data provide a guide to the area in the brain where seizures are occurring and MRI pinpoints where the seizure-causing lesions are.
Dr. Quesney also sees greater use for EEG in further understanding cognitive functions. In our research, weve been trying to understand some functions of the brain using EEG, he says. Today we can say that a given function is linked to a specific region of the brain but we cant determine which circuit is in-volved or which neurotransmitters. If we could determine this, then we would know exactly what to treat in the case of neurological disease. Perhaps one day we will be able to.