Transcranial Magnetic Stimulation (TMS) Flashcards
What does TMS stand for?
Transcranial Magnetic Stimulation.
What is TMS?
TMS is a ‘non-invasive’ technique to create virtual cortical (relating to the outer layer of the cerebrum) ‘lesions’.
Why create ‘virtual lesions’ on the brain?
Because studies on subjects with real lesions have led to observable developments in cognitive sciences. Virtual lesions may allow us to specifically investigate how certain brain regions relate to particular functions/characteristics.
Why use the TMS?
TMS delivers temporary, reversible, small-scale and localised lesions and is an ethical manner in which the function of specific brain regions may be investigated.
Why not always use patients?
The specific lesions in particular brain area/s sought to study all cognitive functions may not be present in patients. Furthermore, lesions in single and specified regions are rare. Finally recovery and brain plasticity may compensate for lesions over time.
How is TMS applied?
The TMS is applied externally, using a coil that is placed on the scalp and produces a rapidly changing magnetic field to induce electrical currents in the brain, or a ‘pulse’. Currents depolarise neurons in a small and circumscribed region of cortex.
Does the TMS pulse occur only once?
Not always. The method of TMS can be delivered in single-pulses or repetitive pulses (repetitive TMS/rTMS). It can also be delivered in paired pulses (2 pulses in brief succession).
What are the main approaches through which TMS is used in research?
Via the injection of ‘neural noise’ (using single-pulse TMS to disrupt cognitive processes), ‘virtual lesions’ using rTMS, ‘probing excitability’ using single-pulse TMS and ‘probing information transfer’ using paired-pulse TMS.
Define the ‘neural noise’ approach.
The NN approach involves delivering a single TMS pulse to a brain region in order to disrupt cognitive processing. This is an attempt to either disrupt processing a stimulus, or to disrupt the inhibition of a stimulus (so that the stimulus is in turn no longer suppressed).
Define the ‘virtual lesion’ approach.
The VL approach involves delivering rTMS to a brain region in order to interrupt or enhance cognitive processing. This is an attempt to assess how a brain region inhibits certain cognitive functions by measuring if a specific cognitive task is impaired (and for how long - tasks usually become slower or function is lost totally).
Define the ‘probing excitability’ approach.
The PE approach involves delivering a single TMS pulse to a brain region in order to test how responsive (‘excitable’) a brain region is during a cognitive task. This is an attempt to observe if the brain region in question is required for a cognitive task. If it is, then the region should already be activated when the pulse is delivered.
When observing the Motor Cortex, what are MEP’s?
MEP’s are ‘Motor Evoked Potentials’. It is the electrical activity of muscle. Observed using an Electromyogram (EMG). Often observed when probing the responsiveness of the Primary Motor Cortex.
Define the ‘probing information transfer’ approach.
The PIT approach involves delivering a paired TMS pulse. This means that two pulses are delivered in brief succession (usually sub and supra-threshold respectively) to assess how strongly the first pulse influences the second.
