TES Flashcards by Steliana Yanakieva

What is TMS?

n-pole moving towards and n-pole moving away from the coil. If the magnet is pushed towards the coil it will be negative when the magnet is pulled away from the coil.
It is a method in which a changing magnetic field is used to cause electric current to flow in a small region of the brain via electromagnetic induction.

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What are some types of TMS?

Single pulse
paired pulse
Repetative TMS (rTMS)
Theta burst (TBS)

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What is single pulse TMS?

Single pulse inducing stimulation
phosphenes - measure of excitability on the occipital cortex
Causes muscle evoked potential (MEP) - movement is observed in the target area usually motor cortex
Excitatory stimulation

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What is EMG?

Muscle twitch occurs but cannot be seen with the naked eye and needs to be recorded with electromyography (e.g. actual muscle threshold may be lower than that needed to see a twitch)

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How is the threshold detected?

The lowest intensity required to induce phosphenes, MEP in 5 out of 10 trials

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What is Paired pulse TMS?

Sequential pulses are applied to the same cortical area at variable intensities with variable interstimulus intervals
Has a condition pulse and a test pulse
Causes SICI, LICI,ICF

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What does SICI stand for?

short-interval intracortical inhibition
U-shaped distribution between the initial stimulus and amount of inhibition
2 phases of inhibition involving the GABAgeric a-pathway

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What does LICI stand for?

long-interval intracortical inhibition

GABA b-pathway modulated effect that reduces the excitability of SICI

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What does ICF stand for?

intracortical facilitation

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What is rTMS?

Focally changes patches of cortical activity
within-participant comparisons of normal systems
daily prefrontal stimulation shown to be effective in the treatment of depression in patients with treatment resistant depression (10-18Hz)
Can be high and low frequency
low intensity –>inhibition
high intensity —> fascilitation

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What is TBS?

thetha burst stimulation usually at 50Hz
Three bursts of pulses repeated every 200ms
disruption - virtual lesions
relatively new technique that may be useful in the treatment of depression
offline form of stimulation
can be continuous –> inhibitory
can be intermittent –> excitatory

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What is offline stimulation?

Comparisons of task performance after vs before longer period of stimulation

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What is online stimulation?

To affect the brain during a task

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what is MEP?

Motor evoked potential that can be triggered by single of repetitive pulse
Can occur in subthreshold stimulation

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What is resting motor threshold?

Muscles at rest (e.g. lack of EMG)

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Control sites used for TMS

Usually vertex, but better interferences can be made with an active site within the circuit
sham stimulation
coil position so pulse is not delivered

What is TES machine powered by?

9V battery

How strong is TES electric current?

1-2mA

What is the tES voltage?

Can be constant or can change over time

What is TES?

Electrically induced stimulation
The current is passed though the skull alternating endogenous activity
poor spatial resolution
Can be applied for extended periods of time

TES applications

studying physiological phenomena
brain-behaviour relationships
To facilitate recovery in neurological disorders
Rehabilitation of motor skills
learning

What does TES do and how many types?

There are 3 types of stimulation used to answer different research questions.
Each has different mechanism, montages, long and short term effects

What is Direct Current stimulation (tDCS)?

Weak and constant electrical current
Two or more scalp electrodes with opposite polarities
Anode-positive –> membrane depolarisation; increases excitability; typically imprges in behaviouroves performance
Cathode - negative –> membrane hyperpolarisation, inhibitory effect, typically poorer performance
Effects are nor linear to intensity
opposite polarities may cause opposite change

tDCS design

one electrode on the area of interest and the other one on a reference site (neutral/return) - bilateral, above the eye (frontal lobe); top of the arm
Distribution of the current is dependant on the electrode placement and individual anatomy

Are the effects of tDCS linear?

No effects are not linear and therefore greater stimulation duration and/or intensity does not mean improved processing Distribution of the current is not often localised and uniform the flow direction can depend on topology of the gyri and sulci

What can TES do?

can dial up or down neural activity might shift bias towards speed vs accuracy May modulate areas that are already engaged in the task greater or supressed excitability depends on the stimulation intensity and the direction of the current flow

What is Stochastic Response?

Current injects neural noise | maximum benefit when the signals are just below threshold for propagation

What is Random noise stimulation (tRNS)?

Randomly selected current with frequencies between 0.1 and 640 Hz current is oscillatory shuttles back and forth between electrodes therefore no need for a reference easier to predict effects and less sensations than tDCS Usually associated with improved performance Avoids polarity related issues Prevents homeostatic adaptation to direct current as the current constantly changes sub-threshold signal is amplified by adding noise

What is transcranial alternating current stimulation tACS?

Sinusoidal current in a fixed frequency range Stimulation can be done at different frequency bands Entrains oscillations rather than causing excitatory or inhibitory effects No polarity - all areas are stimulated in a similar way Different electrodes oscillate at counter phase

What is entrainment?

exogenous tACS oscillations modulate the power and of the phase of endogenous oscillations

How is TES used?

Current may be diffused but electrode placement must remain the same and be done consistently and accurately

How are electrodes placed?

10-20 EEG system Neuronavigation but requires MRI physiology based placement - e.g. thresholds

What are some control strategies?

Sham stimulation double - blinding control areas opposite polaries for tDCS - effect of the polarity used control task control frequency for tACS - specificity of the frequency

What are some issues associated with tDCS polarities?

anodal is not always excitatory or cathodal inhibitory Effects needs to be interpret with caution as the polarity may depend on the cell morphology and effects under the electrodes are not uniform

What are some issues associated with duration of the stimulation?

time effects - does the peak effects occur during the stimulation or after? Can the effect be missed?

Other points to consider...

Are effects purely due to stimulation of the site or interactions with other sites sham may be poor control and is often good to combine it with other type of control e.g. control site/polarity/task to allow for assessment of specificity