7. TMS Flashcards
d’Arsonval (1896)
Beer (1902)
Thompson (1910)
Magnetically induced phosphenes (All were retinal stimulation).
- d’Arsonval presents 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)
- Thompson shows the same thing but on himself
When was the first successful stimulation of the human motor cortex (TMS)?
1985
How does TMS work?
Magnetic coil with current running in one direction creates an electric field which goes through the scalp and generates another magnetic field in the underlying tissue which runs in the opposite direction
Figure of 8 TMS coil induced the strongest electrical field…
where the two loops meet, the induction is very focal under that spot
What are the two modes of TMS?
Disruptive - induces neural noise
Productive - produces things such as phosphenes, hand movements
What is the difference between TMS and fMRI/MEG?
- TMS is an interference technique (Aka virtual lesion technique)
- fMRI/MEG measure correlations (BOLD/magnetic fields related to behaviour)
What are the different TMS protocols:
- Single pulse
- Trains of pulses (r(repetitive)-TMS):
e.g. high freq (typically when someone is doing a task - online): 5 pulses at 10Hz
e. g. low freq (typically when someone is not doing a task - offline): 1Hz rTMS for 10 mins
- —————————————-
- Theta burst: very high frequency of rTMS (50Hz; offline)
How are hand muscle movements measured?
Using MEP
TMS stimulation propagates into …
connected and functionally coupled areas, including subcortical areas
Allen et al (2007) - physiological basis of TMS
- Stimulating cat visual cortex (short rTMS trains (1-4s) at various freqs (1-8Hz))
measured activity using single unit recording, haemodynamic recordings, local field potential tissue oxygenation
Key findings:
- Fx more pronounced with longer trains and higher freq:
- spontaneous activity (unrelated to stimulus) up by up to 200%
- Activity resulting from stimulus in vis cortex suppressed by up to 60%
- TMS disturbs phase relationship among neural responses
- neural spikes were decoupled from ongoing oscillations - Tight coupling between TMS-evoked neural responses and changes in cerebral haemodynamics
- initial increase and longer lasting decrease in tissue oxygenation and haemoglobin concentration
Paus et al (1997) - TMS: spatial resolution
TMS & PET
- positive correlation between CBF and number of trains at the stimulation site
- Changes 1-2cm in diameter
- Observed also distal (away from focal point, connected regions) CBF changes
Ilomoniemi et al. (1997) - TMS: spatial resolution
- EEG responses spread to other hemisphere within 20-30ms (distal changes)
- Difference between physiological TMS effect and functional TMS effect on behaviour (not strong to affect behaviour related to other hemisphere for example)
Critical time for TMS delivery coincides with time at which _____ can be recorded (___)
single unit responses, (earlier than ERPs - would have to use it before a known ERP response occurs)
Effects of a single TMS pulse may last up to ___ (but…)
70ms (but not all physiologically active 9 i.e. visible effect on behaviour)
In behavioural studies, single TMS pulse lasts at least ___ (Ashbridge et al., 1997)
10ms
Cooling is ____ technique
an interference
What are the two features of best TMS studies?
- control tasks (e.g. interfering with language production, shouldn’t interfere with singing when stimulating the same area)
- control sites (e.g. predicting speech would be interfered by stimulating a certain area, there should be another area which you could stimulate and it wouldn’t interrupt speech)
What is Brainsight 2?
Localising target location for TMS based on MRI
what does fMRI-guided TMS neuronavigation involve?
Using location of a peak of fMRI activation contrast
MRI-guided neuronavigation
Using MRI image to locate the area of interest (e.g. anterior IPS)
TMS neuronavigation based on group coordinates
based on fMRI peak activation of a whole group
TMS based on the 10-20 EEG system (anatomical landmark approach)
Using for example the location of a P4 electrode
Functional TMS localiser as a method of localisation for TMS sites
Stimulating the site which provides maximal disruption in a different task
Sack et al. (2008)
compared the methods of localisation of sites for TMS using a number stroop task
- 3 TMS pulses when numbers displayed
- had to choose physically larger number (L or R?)
- CONGRUENT (larger number physically larger) FASTER RT than incongruent (smaller number physically larger)
using fMRI to localise caused greatest reduction in RT fx following TMS (Cohen’s d = 1.13)
- 5 participants needed with fMRI localisation for a sig effect
- EEG you need at least 50
!!!!!!!!fMRI provided the best TMS site for disrupting behaviour
Silvanto et al. (2007)
- ## GY has no perception of posphenes when stimulated over left V1Uni/bilateral TMS over V5/MT applied, can he see something where he usually cannot perceive?:
Controls: can see moving posphenes with both uni & bilateral stim
GY: Stimulating left V5 alone - still didn’t see posphenes
!!!!!!with stim of bilateral V5 GY reported awareness in the right visual field
V1 necessary for awareness!
Older visual pathway
Eyes -> brainstem -> higher order vis processing
- concerned with reflexive behaviour, orienting towards something important
Newer visual pathway
eyes -> thalamus -> visual cortex
- conscious visual perception
Which visual pathway is not working in blindsighted people?
Newer pathway (Eyes>thalamus>visual cortex)
Number processing & TMS (Gobel et al., 2001)
Visuospatial (angular gyrus)
vs
motor attention areas (supramarginal gyrus)
** (head maths vs finger maths) **
2 areas in parietal cortex:
- angular gyrus = visuospatial processing area
- supramarginal gyrus = motor attention area
Task: is the 2-digit number bigger or smaller than 65?
!!!!RT gets higher following TMS the closer to 65 the numbers are (distance effect)
!!!!left angular gyrus (vis-spat processing area) + rTMS = even higher RT, especially for larger numbers (two digit numbers)
!!!!supramarginal (motor attention area) + rTMS = same effect but with smaller numbers (5-10)
!!!!!Think of counting small numbers on fingers when you are little, thus greater connection to the MOTOR attention area
THIS IS A DOUBLE DISSOCIATION
Grotheer, Herrmann & Kovacs (2016): Number Form Area (NFA) Discovery
Where was it found?
Found a Number Form area in BOTH hemis
- activation was higher for numbers compared to false numbers, letters and objects
There is also a Letter Form area found only in the LEFT hemi
Grotheer, Ambrus & Kovacs (2016): Disrupting the RIGHT Number Form Area (NFA) to investigate functionality
Decided to disrupt RIGHT number form area, this disrupted accuracy for Numbers
BUT also,
disrupted accuracy for Letters to a lesser extend (the previous study did not find activity for Letters in the RIGHT hemisphere)
O’Shea et al (2007): Combining TMS & fMRI (Select vs execute task)
r dorsal Premotor cortex: activity in stroke patients:
- reorganisation of function?
In the SELECT (Choice) task, the participants had to press buttons with two different fingers, depending on which combination of shape-size they saw.
In the EXECUTE (No choice) task the participants had to press a button when they saw a circle or a square.
- SELECT task: higher activation of the dorsal premotor cortex…
If Left DPC is damaged (stroke), the contralateral signal is going UP
- Scanned healthy N - fMRI
- Simulated the Left DPC lesion with TMS (15 min)
- Scanned again - fMRI
!!!This means that changes in stroke patients in right dorsal premotor cortex are DUE TO REORGANISATION OF FUNCTION
!!!!and not only due to changes in brain structure resulting from stroke
In Goebel et al. (2001) DOUBLE DIGIT number comparison was significantly impaired when rTMS was applied over ___________________.
left angular gyrus
The size of the magnetic field generated with a TMS coil can be up to
2.5 Tesla
What did the TMS experiment with the patient GY show?
GY experienced TMS induced qualia (conscious experience) in his blind field only with bilateral stimulation
The first person in the UK to report magnetically induced phosphenes was
Thompson (1910)
Who discovered electromagnetic induction?
Faraday (1832)
