Psychological Science, 2 ed.

Chapter 4: The Brain and Consciousness

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Transcranial Magnetic Stimulation (TMS) Activity

This activity includes: - TMS description - Activity Movie - Discussion Questions

Transcranial Magnetic Stimulation (TMS) is a technique for temporarily disrupting the function of specific cortical regions. The technique is noninvasive and is not known to have residual effects.

The brain is an organic device that works by the firing of cells called neurons. These neurons send electrical impulsesdown their length that trigger the release of chemicals known as neurotransmitters; neurons, therefore, may be thought of as electrochemical devices. TMS disrupts the electrical side of neuronal function by inducing a fluctuating magnetic field that generates an electrical current. When the field pulses, the electrical activity of the cortical region underneath the magnet is disrupted.

TMS stimulators are small hand-held devices that can be placed against the scalp directly over the cortical region of interest and then "fired" repeatedly. As the magnetic field pulses through the skull and cortex, the functioning of the region directly under the magnet is disrupted.

TMS has several applications in clinical neurology. It can influence movement, visual perception, speech, and even mood. Here you will see video clips that demonstrate the effects of TMS on speech production and explore the laterality of language representation.

Whilethe anatomy of the brain is fairly symmetrical, the two sides are specialized for different functions. For example, in most people, language is represented in the left (dominant) hemisphere in a region known as Broca's area. Damage to a certain area in the anterior of the left hemisphere often leads to language-production problems.

Using TMS, researchers have been able to temporarily disrupt the function of Broca's area in a healthy individual, as shown by the accompanying video clip. For example, if the magnetic field is first placed over Broca's area on the left hemisphere and then moved to the corresponding region of the non-dominant hemisphere, we see differing results.

You are about to hear the voice of the subject undergoing TMS. In this study, the subject will begin counting and then Broca's area will be stimulated by TMS. A clicking sound occurs when the TMS magnet is on. After stimulating Broca's area, the stimulator will then be moved to the corresponding area on the right, or non-dominant, hemisphere and the subject will again begin counting.

>> Watch the Video [ this movie requires the Flash 6 plug-in ]

Prior to the development of TMS, scientists and researchers were dependent upon disease or head trauma to "remove" the activity of specific cortical regions so that their function could be understood. TMS holds great promise for advancing brain research because the functional disruption is only temporary and without long-term side effects. At this time, TMS stimulators are about the size of an adult human palm and so can only stimulate large cortical regions and cannot be used with small mammals such as rats or mice. If TMS stimulators can be made smaller, the function of small cortical regions could be investigated, and animal models of disease could also be explored with TMS.

Discussion Questions:

1. Describe what occurred when the participant was exposed to TMS over Broca's area of the dominant hemisphere and then over the corresponding area of the non-dominant hemisphere. How did the two episodes differ?
   
   
   
2. Lateralization of language is but one dramatic example of specialization of function in the brain, a major theme of Chapter 4. Your text describes several major divisions of the cerebral cortex. What would you predict would be the effect of TMS for each of the major divisions of the cerebral cortex described in Chapter 4?
   
   
   
3. Recent research has suggested that stimulating a cortical region with low-frequency TMS (< 5 pulses/sec) depresses function of that region while higher frequency stimulation (> 25 pulses/sec) increases function. Why would it be useful in science or medicine to either excite or depress the function of a cortical region? Explain your answer and be sure to provide a specific example of the utility of both excitation and depression.
   

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