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Measuring Brain Activity

From about 1940 to 1970, there were two main technologies for studying brain activity: (1) the EEG (electroenceph­alograph or brain wave machine); and (2) single-cell recording, in which activity of individual neurons was monitored with tiny wire electrodes or glass pipettes (small tubes).

Both the EEG and neuron record­ings are still used in neuroscience research, but both are greatly improved in their current versions. Single cell recording has been upgraded to multiple-cell recording, using implanted chips or arrays of electrodes that record from large populations of cells. The EEG has been made more sensitive and informative by computer data analysis.

What does the EEG measure?

The EEG or brain wave machine has been around for over 90 years. Early physiologists found they could record tiny electrical potentials from the surface of the brain. In 1924 Hans Berger was able to record electrical rhythms from the surface of the skull without surgery. This was the first EEG.

The electroencephalograph (EEG) records tiny electrical voltages from the brain. Because the electrode must record through the skull and protective layers (dura) outside the brain, the weak potential represents averaged activity of millions of neurons.

In the typical EEG setup, most of the electrodes are on the scalp, but one is attached to the earlobe with a clip, serving as an electrically neutral spot to compare with the scalp. The scalp may be covered with a dozen or more electrodes, each with a gooey electrolyte paste between the electrode disk and the scalp to maintain good electrical contact.

When a person perceives a stimulus or performs an action, or thinks about something, the EEG shows characteristic fluctuations in the EEG. A changed induced by a stimulus is called an evoked potential.

Well-known evoked potentials have names starting with a P if the initial voltage is positive, N if negative. For example, the N20 is a negative potential arriving at the machine 20 ms (milliseconds or thousandths of a second) after a nerve is stimulated.

What is the N20 evoked potential?

N20 signals the arrival of a signal from a sensory nerve to the cortex. It is called an evoked potential (EP) because it is an electrical potential triggered or "evoked" by stimulation. Not coincidentally, some of the earliest unconscious reactions to emotional stimuli occur after 20 ms, the first possible opportunity for the brain to react to an external stimulus.

N100 is an evoked potential appearing in the frontal lobe area in response to an unexpected stimulus. It probably represents the initial, pre-attentive response of neurons trying to sort out what it is they are receiving. It weakens when a stimulus is predictable or repetitive.

N170 appears in response to faces. It is strongest on the middle, side part of the brain (occipital temporal area) over the fusiform gyrus, which is known to process facial stimuli.

An example of evoked potential research appears in Chapter 7 (Cognition) in the section on thinking. Subjects in an experiment were asked to read sentences like, "He spread the butter on the bread."

For some subjects, the sentence ended with an unlikely word, such as "socks." ("He spread the butter on the socks.") This group showed a change in their evoked potentials that reflected their surprise or the need to re-process the meaning of the sentence, given the unexpected ending. The EP was the N400, which researchers believe reflects interpretation of a word's meaning, particularly when unexpected.

Evoked potential research started in the 1950s and 1960s. Like all brain research involving technology, it benefited greatly from increased power and decreased prices of computers as time went on. Today, relatively inexpen­sive computers can extract a lot of information from a noisy EEG signal.

The CT Scan

The first of the brain scanning techniques emerged in the 1970s, as a variation of X-ray technology. It was called the CAT scan, and it involved rotating an X-ray machine around the axis of a person's body. This produced a series of images like slices through the brain.

By combining the information, neuro­scientists or physicians can build up a 3-dimensional X-ray of a person's brain. This shows tumors, enlarged ventricles (the fluid-filled cavities of the brain), and other physical deformities in the brain. It does not, however, show which areas are actively processing information.

CAT stands for "computer axial tomography." A tomograph is an X-ray showing a layer of tissue at some specific depth. An axial tomograph is one made by rotating the subject around an axis, which means twirling the subject or twirling the machine. During a CAT scan, the patient lies still and the scanner rotates around his or her body.

A student who received several CAT scans said they made her feel a bit claustrophobic. Her entire body was inserted into the large machine, which was then rotated around her, taking X-ray images from every angle.

As scanning techniques grew more sophisticated, the machines became less imposing. Now a slender doughnut-shaped structure is used for scanning, and receiving a CT scan (as they are more commonly called now) is not so much like being inserted into a huge machine.

What is a CT scan? What does it show?

The PET Scan

After the CT scan, the next major advance in brain imaging was the PET scan. PET stands for positron emission tomography. Unlike CT scans, which produce a three-dimensional still picture, PET scans could be made in real time and show motion.

In other words, researchers could observe a PET scan change while a person thought about various things. The following figure shows locations of a brain responding to seeing, hearing, speaking, or thinking of a word.

4 pet scans
A PET scan shows areas activated by seeing a word, hearing a word, speaking a word, or thinking a word.

What is a PET scan? How does it use glucose?

PET scans use a radioactive substance that binds to glucose, being released when the glucose is consumed by brain activity. Glucose is the brain's main source of energy. As brain cells consume chemically labeled glucose, positrons are emitted. The PET scan reveals which areas of the brain are burning the most sugar.

The results of PET scans can show small areas of activity. For example, research­ers viewing some of the first PET scans were excited to see different areas of a rat cortex light up according to which of five separate whiskers was touched.

Another study showed "the first physiological evidence of voices in a schizophrenic." Schizo­phrenics evidently generate language in the normal language-production areas of the brain that they interpret as somebody else's voice (Silbersweig et al., 1995).

What are examples of specific activity detected by PET scans?

PET scans were cutting-edge technology during the first half of the 1980s, but they require a subject to ingest glucose labeled with a radioactive tracer. PET scans can only be done two or three times a year on the same person before radioactivity becomes a hazard.

Fortunately, several more modern brain scanning techniques do not require exposure to radiation or ingestion of labeled substances. These techniques–such as functional MRI–are also capable of producing images more detailed than PET scans.

MRI and functional MRI (fMRI)

Magnetic Resonance Imaging (MRI) was first demonstrated in 1973, about the same time the first modern PET scanners were presented to medical researchers. Its usefulness to psychologists really blossomed in the 1990s with the development of a variation called functional MRI.

What is MRI? What is the big advantage of MRI? What was the big problem with it initially?

In MRI, a powerful magnetic field is placed around the brain. It temporarily holds the nuclei of the brain's atoms in one direction. When released, the atoms wobble back to their original positions and emit a weak radio frequency signal that can be picked up by a sensitive receiving device.

The big advantage of MRI is its noninvasive character. Unlike the PET scan, the MRI requires no substance to be put into the body.

The big problem with MRI, when first invented, was that the image took several minutes to form, so patients had to hold still. Sometimes it was difficult to get a good image from children or severely ill people who could not avoid moving.

The imaging time problem was eliminated by a discovery by Sir Peter Mansfield at the University of Nottingham in 1980 of a technique 10,000 times faster than the original MRI. This led to a process called fast MRI (or echo-planar imaging, EPI). It was followed by an even faster form of MRI called functional MRI, or fMRI.

What are advantages of functional MRI? Why did one researcher call it a "wonder technique"?

By the early 1990s, fMRI was causing much excitement among researchers. One called it "the wonder technique we've all been waiting for" (Blakeslee, 1993).

The researchers were excited because fMRI cost about a fifth as much as PET scans, produced sharper pictures, and was less dangerous for the person scanned. Unlike PET, which shows broad areas of activity, functional MRI was capable of showing activity in areas a few millimeters across, as small as a BB.

With all its advantages, fMRI has been a boon to cognitive neuroscience researchers. Unless otherwise stated, it is the technique used when brain scanning is mentioned in chapters ahead.

A variety of other brain scanning techniques exist. MEG (magneto­encephalography) directly detects weak magnetic fields in small patches of nerve cells.

Other techniques like R-TOI (Real-Time Optical Imaging) provide additional ways to measure brain activity. R-TOI has the advantage of being an optical technique, which means familiar technologies can be used. It has the advantage of "rapid frame rates, high sensitivity, low cost, portability and lack of radiation."

What are some other techniques and their advantages?

Diffusion tensor imaging (DTI) is the method of choice for visualizing white matter in the brain. White matter gets its color from the fatty layer around axons, the communication lines from nerve cells. DTI picks this up, so it can be used to build up complex images of connection patterns within the brain.

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References:

Blakeslee, S. (1993, June 1) Scanner pinpoints site of thought as brain sees or speaks. New York Times, p.B6.

Kosaka, N., McCann, T.E., Mitsunaga, M., Choyke, P.L., & Kobayashi, H. (2010) Real-time optical imaging using quantum dot and related nanocrystals. Nanomedicine, 5, 765-776. doi:10.2217/nnm.10.49

Silbersweig, D.A., Stern, E., Frith, C., Cahill, C., Holmes, A., Grootoonk, S., Seaward, J., McKenna, P., Chua, S.E., Schnorr, L., Jones, T., & Frackowiak, R.S.J. (1995) A functional neuroanatomy of hallucinations in schizophrenia. Nature, 378, 176-179.


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