tms Flashcards
What principle is TMS based on?
the principle of electromagnetic inductions
TMS basics what is happening at the level of the coil
- Electricity flows through the coil
- induces a magnetic field
- (This magnetic field induces a transient magnetic field that affects the membrane potential of neurons underneath) the
- field is capable of inducing action potentials of neurons underneath
TMS - does it have excitatory or inhibitory effects
both - it can both elicit activity and interfere with ongoing electrical activity (virtual lesion)
what are the different types of TMS
- single pulse TMS
- repetitive pulse (rTMS)
single and repetitive - online vs offline effects?
- single pulse - just one pulse - very transient process. online.
- repetitive pulse TMS - online and offline.
online effect
Looking at behavioural effects WHILE the pulse is being delivered.
repetative pulse stimulation
more than one pulse a second.
- up to many pulses
- 100Hz would be 100 pulses a second
- online - looking at current effects of the pulse
- offline - looking at after-effects of the pulse
rTMS offline effects - what effects the stimulation after-effects
the frequency at which you stimulate n
- low freq repetitive TMS (< 1HZ) - decreases cortical excitability
- high freq rTMS (> 5Hz) - increases cortical excitability
the effecrts ofTMS depend upon the frequency at which you stimulate
Why was theta burst stimulation ( TBS) created. what are the subtypes?
to enhance the after-effects (longer duration) of repetitive TMS
- Intermittent TBS - facilitates MEP - increase cortical excitability
- continuous TBS - inhibits MEP - decrease cortical excitability
- intermeiate TBS - unaffects MEP
MEP is the motor evoked threshold - how much activity is needed to elicit a behaviour.
whats the difference between TMS and TDCS?
- TMS - electromagnetic induction
- TDCS - direct electrical stimulation - but with very very weak currents (~1mA)
TDCS - how is the weak current delivered?
the weak current is delivered to the cortex using 2 electrodes (damp sponges) placed on the scalp
- anode - red - current enter through anode
- cathode - blue. - travels through any cortex it goes through and exits through the cathode
so this means wif you change these round you are changing which neurons the current passes through.
how does TMS look vs TDCS
TMS
- swimming cap placed on the head with red spot somewhere
- pair red spot with the mid-point of the coil
- deliver pulse
TDCS
- two electrodes placed on scalp - anode and cathode
- held in place with adjustable band
- electrode leads attached to stimulator
- stimulator - lightweight electronic box
- very portable
TDCS - what convection is it
conduct the behavioural task you are interested in e.g., visual search task either
- during stimulation
- and/or immediately after it
Could have stimulation parameter that lasts 15 minutes. might leave it to start having an effect after 5 minutes then test n the remaining 10.
or you could stimulate for 15 minutes and not do anything then as soon as stimulation ends go - right now do the task
TDCS electrods - sizes
large (7x5 cm)
small ( 3x3cm)
so they aren’t particularly small electrodes
What does the efficacy ofTDCS depend on?
different parameters
- current density - this determines the ELECTRICAL field strength
- the strength of the field is dependent on both - the stimulation density chosen and the size of the electrode (size of area being stimulated)
so higher intensity + smaller electrode size will increase the ELECTRICAL field density
small intensity + large electrode - you spread the stimulation across large area
differences in the lasting effects of excitablity with TMS and TDCS
TMS - 1 pulse effects activity, soon as stimulation stops activity reverts back to normal
TDCS - induces longer lasting changes in excitability (depending on stimulation parameters)
- might return back to normal if TDCS is applied for less than 10 mins
- if it’s applied for 15 mins - excitability changes that last up to 90 minutes
two types of TDCS
Anodal stimulation
- anode covers a region of interest
- enhances cortical excitability
cathode stimulation
- cathode over region of interest
- decrease cortical excitability
describe a set up of anodal stimulation
- if we were interested in the right posterior parietal cortex - place the anode there
- place cathode over the contralateral subra-orbital region. (the forhead)
if anode is over the region of interest it should enhance cortical excitability here.
Why does placing the anode over the region of interest INCREASE cortical excitability?
Excitatory (anodal TDCS) causes locally reduced GABA
Stagg et al., (2009)
Why does placing the cathode over the region of interest DECREASE cortical excitability?
- Inhibitory (cathodal) stimulation causes reduced glutaminergic neural activity
- with highly correlated reduction in GABA
Stagg et al., 2009
What did Staggs paper find out when looking at the anodal and cathodal stimulation underlying mechanisms?
- used magnetic resonance spectroscopy to look at the effect of TDCS on NT’s
- found anodal TDCS caused locally reduced GABA
- while cathodal stimulation reduced glutaminergic activity
Other than the reduction of GABA/Glutaminergic NT’s what other things could modulate the effects of TDCS?
some evidence that TDCS effects are modulated by glial cells.
- Glial cells don’t have an action potential or anything
- they are structural cells within the brain that have many important functions like functional and nutritional support for neurons
Ruhenonen and Karhu 2012
What things have people argued modulate the effects of TDCS in the brain?
- the reduction of NT’s
- glial cells
Disadvantages of TDCS
- shit spatial resolution - cant tell you wheresomething is happenign in the brain
How long does a typical TDCS experiment last for
About 15 minutes
- probably set electrodes up 5 mins before that
- because you want to let electrodes become body temperature
- electrodes + saline might initially be cold
- to improve conductivity you want them to settle
- less of a problem when on skin but if over someone’ s hair it can be hard to get nice good clean contact between electrode and scalp
- so leave for 5 mins to produce this nice contact
- then 15 mins of stimulation
if you are delivering TDCS and the stimulator that monitors the stimulation going in says “high impedance” what does this mean?
you’ve lost contact in some way, someone might have moved and knocked electrode further away from scalp
OR and what normally happens - is it’s dried out. so to prevent this inject the electrodes with saline syringes to keep the impedence low throughout the 15 mins of stimulation
What effects do the reference electrode appear to have?
Baudewig et al., 200
- TDCS and then neuroimaging
- found the effects of TDCS on cortical excitability are supported by concurrent neuroimaging findings - after cathodal stimulation
- MRI sees a decrease in activity in areas underlying the electrode.
see a less task-related activation following cathodal stimulation directly under where the electrode was placed
are the effects of TDCS restricted to the site of the electrode
- no, they extend to other functionally relevant and widespread areas.
- target the wider processing loops relevant to task
- particular strength of the technique - good for questions about network, if you want to investigate the communication between sites
how does TDCS help us investigate networks in the brain
- because the effects of TDCS aren’t restricted to the site under the electrode
- if your’e interested in the communication between sites - networks in the brain TDCS is good for those
- deliver stimulation to one point in the brain - this then spreads through the network of functionally connected areas
Describe 3 studies using TDCS
- lyer et al (2005) - verbal fluency. 2mA stimulation of left PFC. anodal improved while cathodal decreased performance.
- Fregni et al., (2005) - working memory. left DLPFC anodal stimulation improved performance.
- Vines et al., (2006) - auditory pitch memory. cathodal stimulation over left supramarginal gyrus had a detrimental effect on performance.
What is the basic point of TDCS and TMS?
- functional localisation
- investigating the contribution of a particular brain area to a particular task
- and whether that particualar area is involved in the task
When would TDCS be prefereable over TMS in cortical functinoal localisation studies
if you are interested in sound.
- tms audible clicking sound would interfere with any task of auditory processing
- silent TDCS has this advantage
if you’re interested in PFC
- stimulating this with the TMS coil would just stimulate someones face
- get huge twitches in the face - periferal nerve side effects
- so as n do task they would just be twitching their way through it
key things about TMS
- transient process
- limitted to cortical stimulation
- has somatosensory (tingling) and auditory effects
Describe TMS set up
- electrical capacitor stores electrical activity
- sends this electrical activity to the stimulator
- stimulator sends pulse to coil
- current flows round coul producing electromagnetic field of 2 tesla
What different types of coil used in TMS
- figure of 8 coil
- one with different geometry
Describe the mechanics of TMS figure of 8 coil
- 2x power in the middle - more focal stimulatio
- copper inside, good bc it has high heat capacity.
- as the current flows it generates heat, In the past the coil would burn n scalp if too hot.
- now theres a built in thermometor that shuts coil off if it gets too hot
- also copper has good tensile strength
Why might someone not decide to use the figure of. 8coil
- if they wanted to stimulate deeper regions
- the geometry of other coils allow the stimulation to go down the interspheric fissure
Why might someone not decide to use the figure of. 8coil
- if they wanted to stimulate deeper regions
- the geometry of other coils allow the stimulation to go down the interspheric fissure
Describe the evolution of TMS
- in the past - single pulse delivered every 4 seconds
- this is bc after the pulse - a lot of the energy was being lost as heat
- so we had to wait for capacitor to recharge after each pulse
- then we found a way to reuse the energy allowing for repetative pulses back to back without the waiting period between
- led to development of rTMS - quicker stimulation
what are the stimulation effects of TMS
- difficult to say bc it depends on a lot of parameters
- where is stimulated, what parameters are used, current neural activity of the region
- but in general TMS causes depolarisation of resting neurons - increased likelihood they fire
TMS can either elicit activity or interfere with ongoing activity. Describe how.
- if pulse delivered to resting neurons - generates action potential and elicits activity
- if pulse delivered during ongoing activity - interferes with activity and disrupts performance.
- this slows responses, increases the number of errors made.
Describe how r/TMS might elicit or inhibit activity.
- if pulse delivered to resting neurons - generates action potential and elicits activity
- if pulse delivered during ongoing activity - interferes with activity and disrupts performance.
- this slows responses, increases the number of errors made.
Are TMS and TBS delivered at similar intensity?
- no
- TMS is delivered at a higher intensity - 1.3 Tesla
- TBS is delivered at a lower intensity - 1 Tesla
Describe theta burst stimulation
- short bursts but heavily packed
- burst of 50 Hz! (which is gamma frequency)
- then repeated bursts of activity are in the theta frequency (5 Hz)
- this coupling between gamma and theta frequency mirrors what happens in the neocortex during a cognitive task
What parameters are used in TBS?
- initiall Hz = __ ?
- repeated Hz = __ ?
lots of different combinations exist but most people replicate the parameters set by Huang et al., 2004
Different types of TBS
what do we use TMS for?
- functional localisation
- and timing of neural activity
Functional localisation: Visual study. Ellison and Cowey (2006)
- TMS delivered to right lateral occipital cortex (region important for shape discrimination)
- n did 3 tasks - shape, distance, colour discrimination tasks.
- TMS slowed the reaction time in the shape discrimination task
- tells us this regin is important for shape perception
Functional localisation: Visual study. Beckers and Zeki (1995)
- motion perception - dots move actoss screen n say which direction they moved in
- stimulated V1 and v5
- 60ms (before stim onset) and 100 ms (after stim onset)
- most interference = TMS to V5 - 10ms and +10ms after stim onset
- n were 100% accurate, now 50%
- now we know V5 is crucial for motion perception
Functional localisation: Visual attention study. Walsh et al., (1999).
- conjunction task
- visual search find line that satisfied 2 criteria - orientation and colour
- requires both serial search and visuo-spatial attention deployment
- TMS delivered simultaneously with stim onset for 500ms at 10Hz
- stimulation increased RT by 100ms
Functional localisation: Visual attention study. Ellison, Rushworth and Walsh (2003)
- feature search task - 1 feature
- could just do parrallel search - stim practically jumps out
- is the R.PPC still involved?
- NO. TMS had no effect
- then gave them 2 conjunction tasks - an easy one (solvable by parallel search) and a harder one
- is the R.PPC involved in both?
- YES. TMS had an effect
- meand rPPC detects location of a combination of features, conjunction tasks.
- findngs support feature detection theory (T and G) that say R.PPC integrates feature and spatial information (finds objects that meet the criteria and locates them)
Functional localisation: Visual attention study. Ellison et al., (2004)
- hard feature, easy feature and hard conjunction
- while stimulating R.PPC and the right superior temporal gyrus
- TMS to RSTG decreased performance in the hard feature task
- this area is related to task difficulty
Timinng: Movment study. Schluter et ak., (1998)
- task - respond to a stimulus as qiuck as possible
- stimulate premotor cortex vs motor cortex
- premotor cortex stim - most interference when stiulated 140ms after stim onset
- motor cortex stim - most interference when stimulated 220-300ms after stimulus onset
- gives us insight into the timing of region involvment
- premotor involved earlier, region probably related to initiation and panning of movment
- motor = involved later, proobably related to execution of movmnet
Timing: language study. Stewart et al., (2001)
- task: say - supracalifragalisticexpyalidocious
- While TMS delivered to brocas area
- n cant do it - region related to speech production
Investigating the interaction between areas: Ellison, Lane and Schenck (2007)
- explore interactino between R.PPC and V5
- TMS delivered during task completion - 500ms at 10 Hz
- Various visual search tasks given: colour/shape conjunction task and mvment conjunction task
- n press button when believe they see the stimulus
- results: double dissociation
- PPC stimulation had big effect on colour/shape task
- V5 stimulation had big effect on motion task
PART 2
- now stimulated both areas and give n a motion conjunction task
- PPC stimulation = no effect
- V5 stimulation = yes effet
- V5 and PPC stimulation = massive effect
- V5 and vertex = yes effect, same as V5 alone
- so not just V5 paired with something bc - v5+vertex had a normal effect. it was specifically when paired with PPC when it had the biggest effect
why ?
- because V5 strips motion task bare
- now a feature search task
- but we stimulated and impaired v5 so you cant do this anymore and have to serially search the items looking for the conjucted stimulus (PPC job)
- since PPC also impaired your basically fucked
What are the theraputic effects of TMS
Approved for the treatment of
- depression
- schizophrenia
- chronic pain
- neglect
Saftey guidlines of TMS
- be careful with intensity and frequency (if using rTMS)
Excusion criteria - both n and experimenter
- anything in head
- epilsepsy
- neurological conditions
- medication
- pregnant n
- hearing loss/tinitus
