Neurostimulation II Flashcards
What is the TMS device like?
Device with electrical box (big capacitor, can strore energy and discharge it rapidly), sends current into copper coils and creates a mag field tha eaks at 3,5 T for micro seconds so dont have safety worries like with MRI
Have really high temporal res bc the pulse width is so short, can probe things almost instantaneously, can plot out
temporal resolution of info flow
What different ways can you use a TMS device?
There are many diff ways of using a TMS device: stim neurons multiple times, at diff intervals, with diff repetitions etc. Cause plastic changes, stim motor cortex can cause mvmts output
* Motor evoked potentials (MEPs)
* Cortical silent period (CSP)
* Intracortical inhibition
– Short latency (SICI)
– Long latency (LICI)
* Intracortical facilitation
* Intercortical inhibition
* Intercortical facilitation
* Afferent inhibition
– Long latency
– Short latency
* Repetitive TMS (rTMS)
– Low frequency
– High frequency
– Intermittent theta burst (iTBS)
– Continuous theta burst stimulation (cTBS)
* Paired associative stimulation (PAS)
* Single pulse
* Paired pulse
* Triple pulse
* Quadripulse
* Biphasic
* Monophasic
* Sham stimulation
What different ways can you use a TMS device?
There are many diff ways of using a TMS device: stim neurons multiple times, at diff intervals, with diff repetitions etc. Cause plastic changes, stim motor cortex can cause mvmts output
* Motor evoked potentials (MEPs)
* Cortical silent period (CSP)
* Intracortical inhibition
– Short latency (SICI)
– Long latency (LICI)
* Intracortical facilitation
* Intercortical inhibition
* Intercortical facilitation
* Afferent inhibition
– Long latency
– Short latency
* Repetitive TMS (rTMS)
– Low frequency
– High frequency
– Intermittent theta burst (iTBS)
– Continuous theta burst stimulation (cTBS)
* Paired associative stimulation (PAS)
* Single pulse
* Paired pulse
* Triple pulse
* Quadripulse
* Biphasic
* Monophasic
/* Sham stimulation
What effect do the TMS coils have?
Brain is a really good conductor (neurons are like wires filled with water), if current created by mag field is strong enough, itll depolarise neurons and cause PA
Shape and position of coils allows us a bit of spatial selectivity, to get stronger mag field need bigger coils but is less spatially selective
What is the time course of corticospinal excitability?
Record EMG button press and then stim TMS randomly then line them up to show the build up of info processing in motor cortex
90 ms before you see mvmt, there starts to be a build up of
excitability in motor cortex (begin info processing)
How can we measure inhibitory control of motor function?
Cortex is made of many layers, pyramidal neurons are in layer 5 so all the cells above
them are stimed too when we try to stim them
There are inhi inter neurons in layers 2 and 3 so when stim brain with TMS, dont just get
exci output neurons but also inhi neurons
Can stim lightly to exci inhi neurons and then stim for output a bit later to see how strong the inhi effect was (see excitability of inhi neurons)
With larger ISI, get more inhi
If give benzos, get larger inhibition with ISI
Pharmacologically that this technique has a neurochemical basis
Map out time course/measure strength of inhi ctrl bc normally when study this ask ppl to withhold their response but when withhold their response, theres ntg to measure in the brain, TMS allows us to measure smtg
When get closer to go condition, get more excitability but when get closer to stop signal, get reverse of the excitation
Use paired pulse approach to record inhibitory ctrl (small pulse to activate inhi and big pulse to activate inhi and exci)
Could ask someone to contract (tonic activation) then zap them and get big activation but then get period where muscle goes silent
Excitatory fctions, if stim with TMS and then stretch jaw, the size of stretch reflex inhibition is suppressed by motor cortex stimulation
Give ppl stop signals, show that after stop periods, can measure the amount of inhi coming from the inhi inter neurons
What is chronometry?
Advantage of TMS
Timing the contribution of focal brain activity to behavior
Interfere with a part of the brain at a very specific pt in time
Role of “visual” cortex in tactile information processing in early blind subjects
Large chunk of the brain thats not performing what its meant to do
Blind ppl dev other methods to enhance their other senses, Instead of processing braille as a tactile stim, they start to process it as almost visual
If give someine braille to read ans stim diff parts of the brain that could be involved in perception of braille with TMS, if stim S1 20 ms after stim presentation, get drop off of performance but TMS stim doesnt affect it after that, when TMS stim V1 get no change in detection but 50 ms after S1,
identification performance drops (blind ppl use V1 to identify braille)
How can TMS cause virtual lesions?
Causal link between brain activity and behaviour
Lost ability to read braille after lesion to occipital cortex
Can interfere with ability to read braille by TMS stim of S1 (more errors in sighted than blind ppl) and of V1 (more errors in blind than sighted ppl)
What are 2 different designs TMS studies can have?
- Online vs. offline design?
2 diff way in which can interfere with the brain ex during a task (online) or in a way that causes a lasting plastic effect (offline; stim is seperate from the experimental task) - If online how are the stimuli ordered?
Where in the processing syst is the interruption happening - If offline how long is the experiment and are the conditions distributed evenly within the lesion window?
Have to make sure that the period of the depression lasts for the experimental task
How is coil localisation important?
How put coil in right place, for motor cortex, put it over the M1 representation of the body part are interested in
If are interested in other cortexes, these dont have direct outputs so have to know more avout them
- Find functional effect
M1 - hand twitch (MEP)
V5 - moving phosphenes
- Find anatomical landmark inion/nasion-ear/ear vertex EEG 10/20 system
- Move a set distance along and across (e.g. FEF = 2-4 cm anterior and 2-4 cm lateral to hand area)
- But: not all brains are the same; MRI co-registration Functional and structural scan, Frameless Stereotactic System
Optical tracking syst on the tool and coregister the scan and the tool into the same geometric space so get like feedback of where the tool is in relation to the brain
What 5 things do we have to control for in TMS studies?
- Control for non-specific stimulation effects
Machine stresses ppl out; has strong effects that arent necessarily due to the stim - Control for placebo/ behavioural arousal etc
Stims could wake ppl up or make them fall asleep - Control for sound Auditory
Cortex responds a lot to clicks - Control for extra physiological effects e.g. twitching
Brain will get proprioceptive feedback - Control for task specificity
What are some control conditions? (3)
- Stim part of brain that shouldnt affect the part of brain that is imp for the experimental task
- Put coil upside down or sideways
- Interleave TMS with no TMS trials
What are 7 major advantages of TMS?
- Reversible lesions without plasticity changes
- Repeatable
- High spatial and temporal resolution
- Can establish causal link between brain activation and behaviour
- Can measure cortical plasticity
- Can modulate cortical plasticity
- Therapeutic benefits
What are 6 limitations of TMS?
- Only regions on cortical surface can be stimulated
- Can be unpleasant for subjects
- Risks to subjects and esp. patients
- Stringent ethics required (can’t be used by some institutions)
- Localisation uncertainty
- Stimulation level uncertainty
What 8 safety factors do we need to keep in mind during TMS studies?
- Seizure induction- Caused by spread of excitation. Single-pulse TMS has produced seizures in patients, but not in normal subjects. rTMS has caused seizures in patients and in normal volunteers. Visual and/or EMG monitoring for after discharges as well as spreading excitation may reduce risk.
- Hearing loss- TMS produces loud click (90-130 dB) in the most sensitive frequency range (2–7 kHz). rTMS = more sustained noise. Reduced considerably with earplugs.
- Heating of the brain- Theoretical power dissipation from TMS is few milliwatts at 1 Hz, while the brain’s metabolic power is 13 W
- Engineering safety- TMS equipment operates at lethal voltages of up to 4 kV. The maximum energy in the capacitor is about 500 J
- Scalp burns from EEG electrodes - Mild scalp burns in subjects with scalp electrodes can be easily avoided using, e.g., small low conductivity Ag/AgCl-pellet electrodes.
- Effect on cognition- Slight trend toward better verbal memory, improved delayed recall and better motor reaction time
- Local neck pain and headaches- Related to stimulation of local muscles and nerves, site and intensity dependant. Particularly uncomfortable over fronto-temporal regions.
- Effect on Mood in normals- Subtle changes in mood are site and frequency dependent. High frequency rTMS of left frontal cortex worsens mood. High frequency rTMS of right frontal cortex may improve mood.
