Hard to Break by Russell A. Poldrack

Author:Russell A. Poldrack
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2021-02-10T00:00:00+00:00

BOX 5.3. Brain stimulation

In order to know whether a brain region is directly involved in some mental process, it’s necessary to know if disrupting that area leads to a change in behavior. Sometimes we can study this by examining people with brain lesions, as we have already seen. However, it can often be difficult to find enough patients with lesions in any particular location to allow us to study them in a robust way. For this reason, researchers have also developed ways to stimulate the brain noninvasively, in effect creating temporary “virtual lesions.” The most common technique to create virtual lesions is transcranial magnetic stimulation (TMS). This technique takes advantage of a basic law of physics, which states that a changing magnetic field will cause an electrical current in any material that conducts electricity; this is the same law that gives us hydroelectric power generators. TMS is performed by placing a coil against the scalp and then running a brief but strong electrical pulse through the coil, creating a very short-lived magnetic field that causes an electrical current to be induced in the neurons below the coil. The effect of this stimulation on the neurons depends on the timing of the magnetic pulses. A single pulse that is applied during performance of a task will disrupt the activity of neurons at a specific point in time, allowing researchers to determine when that area plays its role in the task. Repeatedly stimulating an area over time can result in longer-lasting changes that either reduce or increase the excitability of that part of cortex, depending on how rapidly it is stimulated. One of the challenges of interpreting results from TMS studies is that the stimulation does not just affect the area that is directly stimulated, but also flows throughout the brain, affecting other areas that are connected to the stimulated region.

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