5 Transcranial Magnetic Stimulation (TMS) Flashcards
1.
Transcranial Magnetic Stimulation (TMS)
3 key ideas:
- Stimulation – phosphenes
- Disruption – temporal lesions
- Causal relationships (can explore the importance of certain brain functions)
2
What is TMS?
- Transcranial magnetically induced electrical stimulation
- Electrical current induced in the brain by a brief magnetic field
- The magnetic field is created by discharging a current through a coil held on the scalp
- Sudden change in current – steep magnetic gradient, induces depolarisation of neurons cell membranes
3.
Effectiveness of stimulation depends on which 6 criteria?
- Strength of the magnetic field
- Shape of the pulse
- Its rate of change (!)
- Frequency of the pulses
- Coil configuration
- Coil orientation
4.
Describe TMS - machines
- Charging two or more capacitors, then discharge into the coil
- Single pulse or several pulses (rTMS)
- Simpler machines produce monophasic electric field
5.
Simple TMS - machines
- Simpler machines produce a monophasic electric field
- Pulse lasts about 200 μs, biophysical effect on neurons lasts longer
- Takes a long time to recharge
- Disadvantage: sometimes longer effect desired, but recharge time too long
6.
Rapid pulse TMS machine:
- Up to 30 Hz stimulation
- Biphasic waveform
- Restores half the original stored energy in the capacitors
- Fast recharging
- More effective physiologically, lower currents can be used
7.
TMS - coils
Describe Round Coils
- Round coils: 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
8.
Describe the Figure-of-eight coil:
- 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
9.
Describe safety in single-pulse TMS:
Single-Pulse TMS
–possible: mood change, epilepsy, memory
–not with single-pulse TMS
•rTMS
–Occasional epilepsy under conditions of high intensity stimulation
–no long-term effects
10.
Is comfort an issue with TMS?
Yes -
–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
11.
An important question is how to find out where to put the coil.
TMS - localisation
- Study behavioural effects of closely spaced coil positions
- If only central positions yields effects the effective locus must be beneath the center
- Shifting a coil by a small amount can have a big effect
12.
Give an example of TMS locolisation:
Eg visual search to localise posterior parietal cortex (PPC) (see Ashbridge et al, Neuropsychologia (1997)
13.
What is needed to use the system Brainsight?
A structural brain scan is required to map out where all the areas of interest are followed by markers on the skull to help position the coils accurately.
After this the coil will be tracked accurately as it moves over the participants head and show the position in realtime.
14.
TMS- How Focal?
Even having a program such as Brainsight there is still some uncertainty.
Artificial percepts (phosphenes) can be induced by applying transcranial magnetic stimulation (TMS) over human visual cortex.
The phosephenes were reported by particiapnts as seen below getting larger and larger from A through to H.
15.
TMS – experimental factors: list 5
- Single pulse vs rTMS (repeated TMS)
- Disruption vs facilitation
- Connectivity between brain areas
- Causal relationships!
- Restrictions
16.
Further Caution! Spread of activation and the …………………………….
Further Caution! Spread of activation and the path of least resistance.
17.
Stimulation techniques and possible effects: Name 5 effects?
It is common to have activations in different hemispheres due to the strong connections between them.
- Expected Effect
- Connected Effects
- Paradoxical Effects
- Single Pulse
- Paired Pulse (multiple placement)
18.
What control conditions are employed?
Two hemispheres can be stimulated.
Two seperate sights can be stimulated
sham TMS (perceived as real to participant)
Ineffective coil (same noise - no actual activation - this helps remove the effect of noise on the experiment)
Tilting coil is also sham - not obvious to participant that it is sham. Noise and small tap from coil are very similar to the real thing.
19.
Is TMS clinically useful?
- Pre-operational functional localisation
- Alleviation of symptoms (short term - hard to know truth in it - it is still very early days so we don’t know enough yet)
–Depression
–OCD
–etc
20.
At which point in time is the visual system important for a task of letter recognition? Amassian et al. (1989)
TMS a single magnetic pulse delivered over the occipital cortex
The perceptual task for the subjects was to identify three letters— trigams—briefly presented at the fixation point on a visual display unit (VDU).
Normally a trivially easy task, the subjects were impaired when the pulse was delivered between 60 and 140 ms after the presentation of the trigram. At a delay (called the stimulus onset asynchrony; SOA) of 80–100 ms, their performance was consistent with complete erasure of the percept; none of the letters were perceived.
TMS can effect neurons in the visual cortex for far longer than the duration of the pulse, making the results of Amassian et al. (1989) , Corthout et al. (1999a ,b ) not in the least surprising.
21
Which resource gives a good overview of TMS?
•Alan Cowey, What can trancscranial magnetic stimulation tell us about how the brain works? Phil. Trans. R. Soc. B (2005). (pdf on learn.gold)
This gives a really good overview..
22.
The effect of a single pulse of TMS – visual search
- Pulse short, effect can be quite long
- Pop-out (when target element only differers from distracters by 1 feature - it stands out easily) vs conjunction search (when a combination of features define a target - distractors can be a different colour or orientation)
- Frontal and parietal areas ‘light up’ in imaging studies of conjunction search
- TMS of posterior parietal cortex (PPC)
23.
TMS – visual search
- 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 (the posterior porietal cortex may be particularly important in learning new information).
24.
TMS – interactions between cortical areas
Connectivity questions. TMS applied above V1 and V5 at
various interstimulus delays Pascual-Leone & Walsh (2001)
- Position coils above V1 or V5
- Determined thresholds for developing phosphenes.
- Extablished how strong the pulse needed to be.
The conspicuous result is that when TMS was applied over V1 about 20 ms after the pulse to V5, the subjects no longer experienced a moving phosphene and, occasionally, no phosphene at all.
The feedback connexions from V5 to V1 may provide information about visual motion and that when area V1, necessary for conscious awareness, is briefly disabled while the feedback information arrives, the moving percept is unsustainable.
