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Transcranial Magnetic
Stimulation (TMS)
TMS is one of the most exciting tools available to us here in the Human
Movement Lab. The knowledge that the nervous system can be manipulated
by magnetic fields has been around for some time, but only in the last 20
years has it become possible to use this knowledge to study brain functions.
Effectively, TMS utilises an electromagnet to cause a very temporary
disruption in the firing of neurons at the site of stimulation. TMS has a
very high level of spatial resolution, so it is possible to pinpoint very specific
areas of cortex for stimulation. TMS also has excellent temporal resolution,
such that disruption from a single pulse lasts for between 20 and 80ms,
depending on which neurons are being stimulated. This high resolution allows
investigators to study precisely which areas are needed for particular
cognitive tasks, and when exactly these areas are needed. By disrupting brain
activity at crucial times and at critical locations, it is possible to
discover which brain areas are necessary to perform certain cognitive
functions. TMS
has two modes of function. Single pulse TMS delivers a single stimulation to
the target area, often in conjucntion with the
participant performing a cognitive task (such as identifying rapidly
presented letters on a screen). Delivery of the pulse is time-locked to the
presentation of stimuli so that experimenters can determine when an area of
cortex is required to perform a cognitive task. Of course the disadvantage of
this method is that if you stimulate at the wrong time, you won’t disrupt the
process you’re interested in. Repetitive TMS (rTMS)
involves the repeated stimulation of an area using a train of pulses. This
technique lacks the fine temporal resolution of single pulse TMS, but is an
excellent way of establishing that an area is crucial for a particular task.
In my own research I have used rTMS to investigate
how stimulation of the Frontal Eye Fields (FEF) affects visual attention,
because we know exactly where FEF are, but not how or when they are involved
in attention. .
TMS has become a vital part of the arsenal of tools available to cognitive
neuroscientists because it allows us to answer questions about causality that
cannot be addressed by other techniques, such as functional brain imaging.
Functional brain imaging such as fMRI can show us
which areas of the brain are active during a task. However, all this tells us
is that that area of cortex is active. fMRI
cannot tell us whether that area is necessary for the task we are
investigating. By contrast, stimulating the same area with TMS and observing
the effects of the stimulation on behaviour can tell us if that area is required for the task. It
is important to note that the although the effects of TMS are temporary
(often lasting for less than 80ms) and that once the TMS effects have worn
off neuronal activity returns to normal, there is a small risk of seizure
associated with repetitively stimulating the brain for long periods. As a
consequence of this TMS is never used on participants who have at risk from
epilepsy, fainting or are very old or very young. For full safety guidelines
see Wassermann (1998) For
a more comprehensive review of how TMS is used in cognitive neuroscience try
reading one of Vince Walsh’s excellent review articles:
Walsh, V., & Cowey,
A. (1998). Magnetic stimulation studies of visual cognition. Trends in
Cognitive Sciences, 2(3), 103-110. Walsh, V., & Cowey,
A. (2000). Transcranial magnetic stimulation and cognitive neuroscience. Nature
Reviews Neuroscience, 1(1), 73-79.
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