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Summary Task 9 - Transcranial Magnetic Stimulation
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Summary of Task 9 of Methods of Cognitive Neuroscience
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TASK 9: TRANSCRANIAL MAGNETICSTIMULATION
FUNCTIONAL PRINCIPLES OF TMS & TDCS
TMS
TMS effects typically small – alter RT but don’t elicit overt behaviour
HOW DOES TMS WORK?
1. Stimulator (capacitor) produces a very strong current,
which gets discharged through a thick cable &
runs into electromagnetic coil
2. A change in electric current in the wire of the coil
generates a magnetic field perpendicular to the
current
The greater rate of change in electric current, the greater the magnetic field
Magnet field builds up quickly & decays quickly, because current goes back to
capacitor
3. Magnetic field can induce a secondary electric current perpendicular to itself to flow in
another wire placed nearby
4. Secondary current is induced in the neurons below the stimulation site depolarisation
Induced electric current caused by making them fire in same way they would
when responding to stimuli in environment
5. Magnetic field acts as bridge between electric current in stimulating coil & current
induced in the brain
If stimulated neurons have critical cognitive function – stimulating them will disrupt
that function
Virtual / reversible lesion – effects are quickly reversed
Effect of TMS can be thought of as inducing “noise” into neural processes
Induces activity that is random with respect to goal-state of area stimulated
Induces disorder rather than order disrupting task performance
Neurons are activated from both internal source (task demands) & external source
(TMS) with the latter disrupting the former
If region not involved in task no interference
STIMULATION CHARACTERISTICS & PARAMETERS
Stimulation parameters – physical properties of the applied magnetic stimulation: (1) pulse
intensity, (2) pulse amplitude, (3) pulse frequency, (4) duration, (5) rise time, (6)
magnetic field distribution, (7) pulse wave form, (8) peak magnetic energy
, Changes can affect different stimulation characteristics in very different ways
Stimulation characteristics – induced physiological effect of TMS: (1) strength & distribution
of induced electrical field, (2) depth of penetration, (3) accuracy of stimulation
Determined by physical & physiological factors (e.g., coil geometry, size, scalp
shape, scalp-cortex distance, anatomical properties, conductivity of stimulated
tissue
Stimulation strength vs. distance
Strong charge signal goes more in depth BUT gets logarithmically weaker the
deeper it goes
Limited to a few cm of stimulation – direct stimulation only works for cortex
We can reach underlying structures indirectly by findings part of the cortex
connected to them
TDCS
Transcranial direct current stimulation (tDCS)
Uses very weak electrical current more distorted signal than TMS
Direct current involves flow of electric charge from positive side (anode) to negative
site (cathode)
Stimulating pad placed over region of interest & control pad place in a site of no
interest
After period of stimulation – cognitive task is performed & can be compared with
sham stimulation
OR anodal & cathodal stimulation can be directly contrasted
Cathodal tDCS stimulation tends to disrupt performance – affects glutamate system
Anodal tDCS stimulation tends to enhance performance – affects GABA system
o Repeated sessions used for cognitive enhancement & neurorehabilitation
Immediate effects vs. aftereffects – have to be considered separately
Immediate effects – believed to occur on resting membrane potential rather than
modulation at the synapse
Aftereffects – likely to occur due to changes in synaptic plasticity influencing
learning & perhaps affecting different neurotransmitter systems
Safety guidelines: upper limits on size of current & surface area of stimulating electrodes
If current concentrated on small electrode – can cause skin irritations
Very little discomfort otherwise – participants can’t tell whether machine is
switched on / used as sham makes it possible to compare active trials to sham
conditions
TMS COILS, PROTOCOLS & ASSOCIATED RISKS
COILS
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