W1 - TMS Flashcards
What is TMS
- “Non-invasive” technique to create virtual cortical “lesions”, which are:
- Temporary
- Reversible
- Localised
- Alllowing us to understand function of specific brain regions
Why can’t we always use patients in studying causality?
- Insufficient patients with circumscribed lesions to study all cognitive functions
- Lesions in single, specialized areas are rare
- Recovery and brain plasticity might compensate for lesions over time
How is TMS applied. Go through the process
TMS is applied externally via. a coil on scalp
- Produces a rapidly changing magnetic field
- Induce electrical _currents i_n the brain
- Depolarize neurons in a small, circumscribed area of cortex
- Neurons fire randomly, increasing neural noise, mask neurons which are firing correctly
Fritsch & Hitzig (1870)
D’Arsonval (1896)
Magnusson & Stevens (1911)
What did the researchers find?
Fritsch & Hitzig (1870)
Electrically stimulate the cortex of animals
D’Arsonval (1896)
Discovered that the magnetic stimulation of the visual cortex can elicit “phosphenes”
Magnusson & Stevens (1911)
Developed the first “head coil” covering the entire head
What is rTMS. What does it require?
Modifiation of TMS where it creates fast sequence of pulses
- Very fast loading times
- 100-200 μs
- Short duration
- <1 ms
What is the most common coil in TMS. How does it work and what is the advantage?
Figure 8 Coil
- Generates magnetic fields in opposite direction
- Generates offset current loops that circulate in opposite directions
- Strongest effect in centre with radius 3-4 mm
- More precise stimulation and more focal of cortex compared to round coils
What is the first way TMS can be used to Bio Psych research
Injection of “neural noise” approach:
Using single-pulse TMS to disrupt cognitive processing
What does the ‘nerual roise’ approach establish. How do we do use it?
- Single TMS pulse to a specific region of the cortex disrupts a cognitive function
- Establishes causality, unlike neuroimaging (correlations)
- Interfere with process of interest at exactly the
time window during which the regions is required by inducing “neural noise”- Interferes with normal funcitoning (Regions still work)
“Neural Noise” Amassian et al. (1989): Overview and Aims
Overview
- 3 alphabetical letters presented under difficult viewing conditions
- Magnetic stimulation was applied over visual cortex (~2 cm above the inion)
Aim
- Effects on letter perception when varying the interval between visual stimuli and time point of TMS stimulation
“Neural Noise” Amassian et al. (1989): Results
- Critical period (40 – 120 ms) stimulation
- Affected detection performance
- Shifting the stimulation site from left to right
- Impaired perception of letters in the contra-lateral visual field
- Shifting TMS stimulation from top to bottom at midline (letters displayed vertically)
- Stimulation above the reference line suppressed letters at the bottom of the display
- Stimulating below the centre not possible as bone was in the way
“Neural Noise”. Amassian et al. (1993). Aims
Aim:
- Find out whether ‘visual mask’ can itself be ‘masked’ using single-pulse stimulation, thereby ‘unmasking’ the stimulus
- Since TMS disrupts stimuli, it could potentially disrupt processing of mask, thereby preventing that the stimulus is suppressed
“Neural Noise”. Amassian et al. (1993). Results. And what dotheir 2 studies tell us
Critical Period:
- 60-140ms (20ms more than first one)
- TMS Informs us about time-course of processing
At 100ms
- Without TMS, detection rate of target 0.37
- With TMS, detection rate of target increased to 0.9
What is the second way TMS can be used to Bio Psych research
Using repetitive TMS to interrupt or enhance cognitive processing
Why is rTMS used? What are the guidelines
- Inhibit cognitive functions for a longer period of time
- rTMS before or during task
- Measure whether (and for how long) a specific cognitive task is impaired
- There are strict safety guidelines for rTMS
What is the third way TMS can be used to Bio Psych research
The “probing excitability” approach using single-pulse TMS
What does the “Probing Excitability” approach test in particular
For the Motor System
What is the idea behind “Probing Excitability”. What is the measure of interest and what does it not do?
Idea
- If M1 is required for a cognitive task, then it should already be activated when single-pulse TMS is delivered.
Measure of Interest
- How strongly M1 reacts to the pulse
- It does NOT aim to disrupt cognitive function and examine effect on performance
- Measure MEPs for each stimulation and compare average MEPs between experimental conditions
“Probing Excitability”: How does it measure M1 reaction
Recording Motor Evoked Potential (MEP) using Electromyogram (EMG), which is electrical activity of muscles
“Probing Excitability”. Eisenegger et al. (2007): Aim
Aim: Is M1 involved in MR
“Probing Excitability”. Eisenegger et al. (2007): Result
- Stimulation of M1 during mental rotation elicited stronger MEPs as compared to baseline, reading aloud, reading silently
- M1 is more excitable during mental rotation and might be already activated, and hence, involved in the cognitive process
“Probing Excitability”. Bode et al. (2007) Aim
Aim: Is M1 dependent on strategy?
Has been suggested that some objects can easily be imagined as rotated by hand (e.g., tools) while others can’t (e.g., building)
“Probing Excitability”. Bode et al. (2007) Result and Limitations/Future Directions
- MEPs were equally high for mental rotation of all different stimuli, so probably strategy does not play a role
- M1 could be more excitable because adjacent and interconnected regions (e.g., SMA) were activated
What is the fourth way TMS can be used to Bio Psych research
Probing information transfer using “Paired-pulse approach”
What is the logic behind “Paired-Pulse”
- Two pulses in brief succession
- Sub-threshold
- Supra-threshold
- Examine how strongly the first pulse influences the effect of the second pulse
- First pulse preactivates, what will happen to the second pulse?
“Paired Pulse” Fitzgerald et al. (2003). Overview
- Schizophrenic suggested to have difficulty inhibiting motor cortex.
- Reduced c_ortical silence period:_ Impaired suppression of tonic (slow) motor activity that follows descending excitatory activity
- Cortical Silence Period: Cooling down period to get rid of motor activity
Study
- Sub-threshold pulse
- Produced excitatory activity in left M1
- Supra-threshold pulse
- Measured excitability via MEP
“Paired Pulse” Fitzgerald et al. (2003). Results
- Compared to controls, patients with and without medication showed stronger responses to the second pulse.
- i.e. cannot inhibit first sub-threshold pulse
- Suggesting general deficits in motor inhibition.
What is TMS mainly used for in clinical application. Why?
-
Depression
- Linked to PFC imbalance between hemispheres
- TMS can stimulate PFC (one hemisphere) to balance it out
- Last minute resort
Though it also works for other mental disorders
