Session 7 - Non-Invasive Brain Stimulation TMS and tDCS Flashcards
(23 cards)
1
Q
What is TMS (transcranial magnetic stimulation)?
A
- non-invasive brain stimulation (NIBS)
- transcranial = ‘through the skull’
- allows to modulate brain activity
- first introduced by Anthony Baker in 1985
- based on the principle of electromagnetic induction
2
Q
Why is TMS important for research?
A
- in neuroimaging (MRI, EEG): correlation appraoch
–> behaviour correlates with neural activity - in TMS: demonstration of causality
–> neural activity correlates with behaviour
3
Q
Electromagnetic induction
A
- TMS induces electrial currents in the brain via Faraday’s law of electromagnetic induction (1831)
–> pulse send through a wire coild generates a magnetic field
–> fluctating magnetic field generates secondary current in a nearby conductor - Example: bike dynamo
4
Q
TMS set up
A
- coil = generates magnetic field
- stimulator = delivers electric currents
5
Q
Stimulating the brain
A
- electric current quickly pulses through a coil
- rapid fluctuations of this current produce a magentic field (up to 2.5T) prependicular to the plane of the coil
- fluctuating magnetic field passes thriugh the skull and induces electric currents in the brain
–> depolarisation of neurons
6
Q
TMS frequency
A
- precision: 3-5mm
two frequency tyoes in repetitive TMS (rTMS)
- low frequency: inhibitory ≤ 1Hz = “virtual lesion”
–> knock out/shut down specific brain regions for seconds/minutes –> not a real lesion because function will come back
- high frequency: excitatory ≥ 5Hz
- depends on the brain state
7
Q
TMS safety
A
- TMS is a relatively safe and painless procedure: creates ‘tapping’ sensation and noise (clicking)
- common side effects:
–> activation of facial muscles causes twitches (uncomfortable)
–> headache (sitting position, noise)
–> neck pain (sittting position)
–> ear ringing (clicking sounds)
–> mood change (in PFC –> wanted in depression cases) - very rare risk = seizure, only in high frequency protocols (safety guidelines/questionnaire to rule risk patients out)
8
Q
TMS coils
A
- different coil shapes –> create different electrical fields
–> figure of eight coil: precise - circular, figure-of-eight coil
9
Q
TMS devices
A
- different devices
- most used: simple one with only coils and stimulator
- there is also devices with chairs and static coils (can only be adjusted in certain ways) –> often used in depression patients
- deep TMS device, reach deeper structures –> but also stimulate upper layers too –> could also be reached by activating connected regions (FC)
10
Q
Motor cortex stimulation
A
- always first step in TMS procedure
–> meassure resting motor threshold first to determine which threshold is safe to use (subjects often have EMG electrodes in hand to measure movement)
11
Q
What is the Motor Threshold (MT)?
A
= indicator of cortical excitability –> stimulation intensity
- output neccessary to produce a motor response in at least 50% of attempts (5/10)
- motor evoked potentials (MEPs) of at least 50µV amplitude
- usually meassured during resting condition but active muscle activation is possible too
- important for safety measures –> stimulated at 110/120% of MT
- easy to observe: visually or with EMG –> objective
- breaks needed between stimulations to prevent overlap of stimulations/acitvity (state dependency, also usually not said when first pulse comes)
- needs to be repeated for each session
- phosphene stimulation: participants close eyes, stimulation of visual cortex –> they see ‘stars’
- first dorsal iterroseus muscle (hand, index finger-thumb) –> stimulation to standardise
–> hotspot = stringest finger movement (the 5/10 stimulations are usually measured there)
12
Q
Neuronavigation
A
- can be done with/without –> visualisation of TMS coil relation to brain in real time
- anatomic landmarks to co-register head and coil –> glasses/headband, infrared
- visualisation of TMS coil in relation to the brain real time
13
Q
Key steps for determining the resting MT
A
- Ensuring safety –> TMS Safety Questionnaire + ear plugs
- Placing electrodes on the targeted hand muscle (FDI)
- Resting condition (head should be relaxed)
- Placing the TMS coil (handle pointing backward and laterally at 45°)
- Finding the ‘hot spot’ (check for visual movement of the finger)
- Finding MT (adjusting intensity –> 5/10 method)
14
Q
TMS as a research tool: online vs offline TMS
A
- online = TMS applied during the task
–> TMS + Task + TMS + Task …
–> intensity, state dependency, etc important - offline = TMS applied before the task
–> TMS + Task + Task + Task … - online more precise base line: with or without simulation, more precise timing of impulse
–> direct effect of stimulation (effect still there, same strength
–> could be distracting during task (loud, facial twitches, tingling sensation) - placebo coils (expensive): need to be switched during the task, does not feel the same
–> could simply tilt coil 90° –> but might induce current, won’t feel non-naive
–> or look at a very different part of the brain (vertex) –> but real or confounding effects?
15
Q
TMS for Depression
A
- dlPFC –> therapy and TMS
- high frequency rTMS (10Hz) applied over the left dlPFC
- daily treatments for 4-6 weeks
- increase abnormal acitivity in that area and improves symptoms of depression
- clinical trials in 2007: significant decrease over 6 weeks (no long-term study)
- approved by FDA in 2008 for medical-resistant depression
16
Q
How to get to the dlPFC from the motor cortex?
A
- 5cm rule: motor cortex stimulation then move 5cm towards front –> reach dlPFC target
- anticorrelation in subgenual cingulate and dlPFC –> more effective targets, can be used to carefully target deeper structures
- only hit the dlPFC ~40% of the time
17
Q
Fitzgerald dlPFC target
A
- 18% responders in M1 + 5cm
- 42% with brain simulation (fMRI) ??
18
Q
Subgenual cingulate: can we predict individual patient responses to TMS?
A
- abnormally increased activity in depression in subgenual cingulate –> target in deep brain stimulation
- 25 depressed patients
- treated at BIDMC using the standard protocol
- normative connectome rsfMRI data from N = 1000
–> more effective individual TMS targets are significantly more anticorrelated to the Subgenual Cingulate than less effective targets
19
Q
tDCS: What and what is needed?
A
= transcranial direct current stimulation
- 9V battery delivering a constant current
- 2 surface (sponge) electrodes
- 1-2mA direct current applied between the 2 electrodes
20
Q
tDCS: mechanism
A
- subthreshold alteration of resting membrane potential:
- depolarisation under the anode –> excitatory effects
- hyperpolarisation under the cathode –> inhibitory effects
- tDCS does not directly elicit action potentials –> changes the likelihood that an incoming action potential will result in postsynaptic firing (or not)
21
Q
tDCS: safety
A
- most commonly reported side effects are mild
–> itching (39.3%)
–> tingling (22.2%)
–> headache (14.8%)
–> discomfort (10.4%)
–> burning sensation (8.7%)
22
Q
TMS versus tDCS
A
TMS
- electromagentic induction of currents in the brain
- high frequency (excitatory) or low frequency (inhibitory) stimulation
tDCS
- application of weak direct currents via scalp electrodes
- anodal (excitatory) or cathodal (inhibitory) stimulation
23
Q
Take home
A
- TMS is a non-invasive brain stimulation (NIBS) technique
- induces electrical currents in the brain via electromagnetic induction
- stimulation effects depend on:
–> coil shape (circular vs Figure of 8)
–> frequency (high vs low - virtual lesion)
–> intensity –> Moto Threshold MT
–> neuronavigation - TMS as a research tool:
–> online vs offline
–> control conditions - TMA as a therapeutic tool
–> eg. dlPFC stimulation in depression - tDCS is another NIBS technique
–> direct current flows from anode to cathode
