TES Flashcards
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.
What are some types of TMS?
Single pulse
paired pulse
Repetative TMS (rTMS)
Theta burst (TBS)
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
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)
How is the threshold detected?
The lowest intensity required to induce phosphenes, MEP in 5 out of 10 trials
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
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
What does LICI stand for?
long-interval intracortical inhibition
GABA b-pathway modulated effect that reduces the excitability of SICI
What does ICF stand for?
intracortical facilitation
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
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
What is offline stimulation?
Comparisons of task performance after vs before longer period of stimulation
What is online stimulation?
To affect the brain during a task
what is MEP?
Motor evoked potential that can be triggered by single of repetitive pulse
Can occur in subthreshold stimulation
What is resting motor threshold?
Muscles at rest (e.g. lack of EMG)
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
