W1 - TMS
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What is TMS
“Non-invasive” technique to create virtual cortical “lesions”.
What are the basic properties of TMS and what does it allow us to understand
• Temporary • Reversible • Localized Allows us to understand function of specific brain regions
How is TMS applied. Go through the process
Externally - via a coil placed on the scalp • ) Rapidly change magnetic field to • ) Induce electrical currents in the brain • ) Current depolarize neurons in a small, circumscribed area of cortex • ) TMS-induced current causes neurons to fire randomly > neural noise increase > mask neurons which are firing correctly.
What is rTMS
Modifiation of TMS where it creates fast sequence of pulses
How does rTMS create a magnetic field strong enough for stimulation. What are the 2 requirements
Very fast loading times (~100-200 μs) and short duration (<1 ms)
What is the most common coil in TMS
Figure 8 Coil
How does the figure 8 coil generate currents. What is the result compared to round coils?
• Generates magnetic fields in the opposite direction • Generates offset current loops that circulate in opposite directions More precise stimulation and more focal of cortex compared to round coils
What is the advantage of have a more focal area stimulated
The researcher knows which part of the cortex is affected
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 is the logic behind the ‘neural noise’ approach
If single TMS pulse to specific region of cortex disrupts cognitive function > CAUSALITY (unlike neuroimaging)
How do we use the ‘neural noise’ approach
Interfere with the process of interest at exactly the time window during which the regions is required
‘Neural noise’: Do the regions associated with the process of interest stop working completely?
Region does not STOP working but the ‘neural noise’ interferes with it
“Neural Noise” Amassian et al. (1989): Overview of their study and Aim
Overview: • 3 alphabetical letters as stimuli presented under difficult viewing conditions • Magnetic stimulation was applied ~ 2 cm above the inion over visual cortex 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 of their study
1.) 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 due to bone]
“Neural Noise”. Amassian et al. (1993). Aims of their study
Aim: Find out whether ‘visual mask’ can itself be ‘masked’ using single-pulse stimulation, thereby ‘unmasking’ the stimulus (Since TMS disrupts stimuli, can it disrupt mask?)
“Neural Noise”. Amassian et al. (1993). Results of their study. And what does their 2 studies tell us
Without TMS, detection rate of target 0.37 at 100ms. With TMS, detection rate of target 0.9. Critical Period: 60-140ms Informs us about time-course of processing
What is the second way TMS can be used to Bio Psych research
“Virtual lesion” approach: Using repetitive TMS to interrupt or enhance cognitive processing
How does “Virtual lesion” via. rTMS work and why is it good
Inhibit cognitive functions for a longer period of time > Measure whether/how long a specific cognitive task is impaired
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
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, What does it NOT do: It does aim to disrupt cognitive function and examine effect on performance
“Probing Excitability”: How does it measure M1 reaction
Using Motor Evoked Potential (MEPs), measured 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 • M1 is more excitable during mental rotation and might be already activated, and hence, involved in the cognitive process