TMS Flashcards
What is TMS?
Transcranial magnetically induced electrical stimulation
How does it briefly work?
Electrical current induced in the brain by a brief magnetic field
How is the magnetic field created?
The magnetic field is created by discharging a current through a coil held on the scalp
What does this do to the brain?
Sudden change in current – steep magnetic gradient, induces depolarisation of neurons cell membranes
What 6 things affect the effectiveness of stimulation?
1) Strength of the magnetic field
2) Shape of the pulse
3) Its rate of change (!)
4) Frequency of the pulses
5) Coil configuration
6) Coil orientation
TMS works by xxxxxxxx two or more xxxxxxx, then discharge into the coil
Charging two or more capacitors, then discharge into the coil
In TMS, there is an option to create
Single pulse or several pulses (rTMS)
Simpler TMS machines produce a….
monophasic electric field
The pulse lasts about …
200 μs, biophysical effect on neurons lasts longer
What is the disadvantage of this?
Disadvantage:
sometimes longer effect desired, but recharge time too long
rTMS machines produce upto….
30 Hz stimulation
How is the waveform
biphasic
rTMS is also
Faster at recharging
More effective physiologically, lower currents can be used
two types of tms coils?
round
figure 8
tms round coils have:
insulated circular winding
• Current created magnetic field, which induces an electrical field in the brain tissue below
• No single focal point
• Placement of the coil crucial, small shifts can have large effects
• Orientation important
tms figure 8 have …
- Figure-of-eight coil: two round coils with current rotating in opposite directions in the two coils
- Strongest field at the centre, where they are contiguous
- Two rings at slight angle
- Smaller coils better for localisations, but too weak
- 8 cm coils has effect of appr. 2 cm below its centre
Single-Pulse TMS safety?
– possible: mood change, epilepsy, memory – not with single-pulse TMS
rTMS safety?
– Occasional epilepsy under conditions of high intensity stimulation
– no long-term effects
TMS causes of general discomfort?
• Discomfort
– TMS can stimulate superficial nerves on the scalp which can produce tingling or pain – avoid sensitive regions
– Coil produces noise (100 dB) with each pulse – participants wear earplugs
When localising TMS, we
• Study behavioural effects of closely spaced coil positions
If only central positions yields effects…
the effective locus must be beneath the center
we can also use xxxx to localise tms
Brainsight TMS navigation:
Brainsight TMS is a revolutionary product used in over 500 laboratories worldwide. It enables a TMS coil to be positioned over a specified target location based upon an individual’s MRI image, MRI generated 3D curvilinear reconstruction of the brain or MNI average head model brain.
MRI structural scan markers
coil position
TMS – how focal?
quite broad (have to listen to audio as missing info)
TMS – 4 experimental factors to consider….
- Single pulse vs rTMS
- Disruption vs facilitation
- Connectivity between brain areas
- Causal relationships!
But a caution, as TMS causes
Further Caution! Spread of activation and the path of least resistance
Control conditions should be located on
1) a diff hemisphere
2) a diff site
3) further away (to have no effect)
4) at angle (to have no effect)
5) or - interleaf no tms trials
what is TMS clinically useful for ?
• Pre-operational functional localisation
• Alleviation of symptoms
– Depression – OCD
– etc
The effect of a single pulse of TMS:
Time between onset of the letter stimulus and the pulse is varied. When the pulse follows the stimulus by x up to x ms, the subject …
70 - 130ms
fails to identify the stimulus.
Note – blinks!
the effect of single pulse is simply…
Pulse short, effect long
What were the effects of TMS on visual search display experiment ?
TMS right parietal cortex at various delays after stimulus.
> Left: no effect in pop-out task
right: conjunction task, stimulation around 90 ms resulted in longer RTs when the target was present (dotted line is no-stim).
Effect of TMS decreases with training, however returned when new conjunctions were included.
TMS – interactions between cortical areas
xxxx need audio
TMS – interactions between cortical areas
xxxx need audio
So what does TMS explain
TMS is a tool that can reveal which cortical areas are involved and even necessary, and when they are involved
What can’t TMS do?
TMS does not explain how the brain works, that is, how it carries out its neural processing.
TMS is particularly useful in conjunction with …
fMRI andEEG
TMS matches the
spatial of fMRI
and
temporal of EEG
Because of its exquisite temporal resolution, TMS can…
determine the order in which different cortical areas are involved in a particular behavioural task and precisely when they are vulnerable to interference
However, The effects of TMS over a particular area of cortex are not necessarily the same as the consequences of brain damage to the same part of the cortex …. why?
in part because the brain shows some reorganisation after damage, but also because TMS affects not only the region beneath it but also more distant regions via propagated neural activity
What don’t we know about the effects of TMS?
is TMS more effective at depolarizing cell bodies in grey matter or axons in white and grey matter?
Are functionally and morphologically and pharmacologically different neuronal cell types equally affected, or is their sensitivity and recovery different?
More than xxxx the brain remains out of range of ‘selective’ stimulation by TMS….. where is TMS most useful?
At present, TMS is most useful in investigating the dorsolateral and dorsomedial aspects of the brain.
Organic lesion vs TMS - 4 benefits of TMS?
TMS
• No reorganisation or compensation
• Lesion is focal
• Lesion can be moved within the same participant
• Can study ‘functional connectivity’ (through time)
Organic lesion vs TMS - 3 benefits of Organic lesion?
Organic lesions
• Subcortical lesions can be studied
• Lesions can be accurately localised with MRI
• Changes in behaviour and cognition are more apparent
what does tDCS stand for?
transcranial direct current stimulation
what does tACS stand for?
transcranial alternating current stimulation
how many electrodes in tDCS/rACS?
• Two electrodes
what are the elctrodes called and what do they do?
– Anodal stimulation increases cortical excitability
– Cathodal stimulation decreases cortical excitability
what does TDCS do?
- Selective inhibit or excite neural populations
- Modulates spontaneous network activity, through changes to the resting membrane potential
- Affects spontaneous firing rates
• After effects up to 1 hour – changes to synaptic
microenvironment, e.g. GABAergic activity
• Electric field displaces all polar molecules (including
neurotransmitters), prolonged neurochemical changes
What does TACS do?
- Applies oscillatory current to the cortex
* Enables stimulation at specific frequency bands (e.g. alpha)
What about the clincical implications of TDCS?
- As substitutive treatment for pharmacotherapy, e.g. in pregnancy
- As augmentative treatment, e.g. after stroke
- Inexpensive, when resources for pharmacological treatment are lacking
