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>> Transcranial
Magnetic Stimulation (TMS) Activity
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.
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:
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