Lecture 7- TMS Flashcards
first person in the UK to report magnetically induced phosphenes
Thompson
How does TMS work?
Pass an intense,rapidly varying, electrical current through a set of coils
Causes a strong but transient and rapidly changing magnetic field over a region of the scalp (2 Telsa)
This magnetic field induces a rapidly changing electrical field in the underlying brain tissue
Results in an extraneous flow of current that interacts with local neural processing
Rises (very brief pulse; 0.1ms) and dips within 1ms
Two types of coil
Round coil
Figure of 8 coil
Round coil?
More general disruption
Figure of 8 coil?
Combination in the middle makes it more focal
What is Barker’s model?
If magnetic field is parallel to axon, no induced current
If perpendicular, depolarisation can occur = disruption
Disruptive mode?
Strong stimulation
Neural noise/brain lesion
e.g. Disrupting language areas while reciting
What do we need to think about when designing TMS experiments?
People blink/move eyebrows/move shoulders
The noise TMS produces (need sham control condition)
Productive mode?
Weaker stimulation
Can facilitate activation
Produces phosphenes/hand movements
Usually over primary areas
TMS is what type of technique?
Interference technique
TMS over primary motor area? (M1)
TMS depolarizes corticospinal tract neurons
Evokes contralateral hand muscle movements
as measured by MEP
Is TMS constrained to stimulation site?
No…
Spreads into connected and functionally coupled areas (inc. subcortical areas)
Allen et al
(2007)
Short rTMS trains (1-4s) at various frequencies in cat visual cortex
In Allen et al’s (2007) study, there was an enhancement of spontaneous neural activity of up to ____ lasting up to ___ after TMS
200%
1 minute
In Allen et al’s (2007) study, activity in the visual cortex evoked by sinusoidal gratings was suppressed up to __ after TMS and gradually recovered after ___
60%
10 minutes or longer
In Allen et al’s (2007) study, they found that effects of TMS depend on ____ and ____
frequency and duration
Based on Allen et al’s (2007) study, TMS disrupts the ___ relationship among ___ responses
Evidence?
phase
neural
Neural spikes were decoupled from ongoing oscillations for 30s across all frequency bands
Based on Allen et al’s study, what type of coupling is there between TMS-evoked neural responses and changes in cerebral haemodynamics?
Evidence?
Tight!
TMS led to initial increase and longer decrease in tissue oxygenation
Paus et al (1997)
Which method was CBF measured?
There is a ___ relationship between changes in CBF and ____ at the stimulation sight
PET
Positive
Number of trains
Temporal resolution of TMS in behavioural studies
At least 10ms (Ashbridge et al, 1997)
Temporal resolution?
Milliseconds to Seconds (depending on number of pulses)
Also depends on recovery rate of neural area
Ilmoniemi et al (1997)
TMS applied to left sensorimotor area
As measured by EEG:
Response spread to adjacent ipsi motor area within 5-10 ms
Response spread to homologous area in opposite hemisphere within 20 ms
There is a difference between physiological TMS and ___ TMS effect on behaviour
Functional
Spatial resolution?
Not as good as fMRI but better than MEG and EEG
Who ‘invented’ TMS?
Barker (1984)
Stimulation depth?
Cannot stimulate medial or sub-cortical areas
How can you elicit moving phosphenes?
Stimulate V5
Repetitive TMS use and general findings
Behavioural tests can be administered during and after the rTMS
rTMS can either enhance or impair performance on these tasks
Single pulse use
Deliver single pulse to specific brain area at a specific time during a task
Provides greater temporal resolution in assessing role of area in that task
Three types of rTMS
High frequency (typically online) 5 pulses at 10Hz
Low frequency (typically offline) (before)
1 Hz rTMS for 10/15 minutes
Lasts for 5-10 minutes
Theta burst (very high burst) 50 Hz Effects last a lot longer
Critical time for TMS delivery coincides with time at which ____ can be recorded which is earlier than ___
single unit responses
ERPS
Two types of control in TMS experiments
Control task
Control site
fMRI guided TMS neuronavigation
Best but most expensive method
Have PS complete the same task in fMRI and TMS
Can stimulate (with TMS) the most activated area (in fMRI)
MRI guided neuronavigation
Anatomically identify the area of brain which you want to stimulate based on MRI
TMS neuronavigation based on group coordinates
localise brain region via coordinates from other fMRI paper/study
- doesn’t take into account individual anatomy
TMS based on the 10-20 EEG system (anatomical landmark approach)
Conduct the same measurement as one would do for EEG
- not very precise
Functional TMS localizer
For each PS, the site at which TMS provides maximal disruption in a different task is determined
What did Sack et al find?
(2008)
Compared fMRI, MRI, group coordinates and 10-20 system neuronavigation
Determined how many PS were needed to gain sig. result
fMRI: effect size = 1.13, only need 5 PS for sig. effect
MRI: 0.82, only need 10 PS
Talairach/group coordinate: 0.67, need 13 PS
Landmark/EEG (P4): 0.3 - needed 45 PS
Silvanto et al
(2007) Patient GY
Blindsight patient but could detect and localise stimuli in the blind field
Shows two pathways:
1) Retina –> PVC
2) Brainstem –> Higher visual areas
TMS applied over V5
GY experienced TMS induced phosphenes in his blind field following bilateral stimulation
THEREFORE
V5 is necessary for awareness
Gobel et al (2001) (double digits)
When TMS is applied over left angular gyrus, much ___ ___ for numbers larger than ___
larger RT
65
Gobel et al (2001)
There was no sig effect of TMS on double digit numbers when TMS was applied to ____
Left supra-marginal gyrus
Gobel et al (2001) (single digits)
When TMS is applied over left supra-marginal gyrus, there is a significant ____ in ___ for numbers larger than ___
increase
RT
5
Gobel et al (2001) (single digits)
There was no sig effect of TMS on single digit numbers when TMS was applied to ____
Left Angular gyrus
The size of the magnetic field generated with a TMS coil can be up to…
2.5 Tesla
Which of these frequencies is NOT typically used when rTMS is applied to human subjects:
5
10
50
100
100
What did the TMS experiment with the patient GY show?
GY experienced TMS induced qualia in his blind field, only with bilateral stimulation
In Goebel et al. (2001) ____________ number comparison was significantly impaired when rTMS was applied over ___________________.
double digit/left angular gyrus
Grotheer et al (2016)
TMS over right Number Form Area: gave them 2 pulses at 0ms and 100ms after stimulus presentation, found a decrease in accuracy for both letters and numbers.
TMS over ILO (control): no significant effect on numbers or letters
O’shea (2007)
Wanted to know if changes in brain activation after stroke are due to altered structure or reorganisation of function
Two tasks: select (harder) or execute
Select: more activation in premotor cortex compared to execute
1hz of TMS over LD PMC: increase in BOLD in right PMC (compensatory) (only for selection task)
1Hz of TMS over sensorimotor cortex: no increase in BOLD in right PMC
THEREFORE
Changes seen in stroke patients in right dorsal PMC are due to reorganisation of function
What did d’Arsonval do?
Presented the first report of magnetically induced phosphenes in human subjects by stimulation of the retina, not the cortex
Painless stimulation of the nervous system
by electromagnetic induction
Effects of a single TMS pulse may last up to ___ (but not all ___ ____)
70ms
physiologically active
