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Methods of COGNEU > TASK 9 - TMS > Flashcards

TASK 9 - TMS Flashcards

1
Q

TMS

A

= transcranial magnetic stimulation = non-invasively manipulate brain activity

  • physiologically change brain activity
  • brain activation = independent variable
  • task performance = dependent variable
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2
Q

set-up

A

A. main power-pulse generation unit
- charges a bank of capacitators that can produce high discharge currents
B. electromagnetic Stimulating Coil
- applies magnetic pulse of up to several tesla
- shape of coil determines how focused the induced current is (strength of induced electric field depends on rate of change in magnetic field –> depends on rate of change of electrical current in coil)
- figure-of eight coil and circular coil: achieve peak magnetic field strengths of 1.5 to 2.5 tesla

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3
Q

mechanism

A
  1. change in electric current in a wire (= the stimulating coil) generates a magnetic field (MF)
    - high amplitude pulse of current is discharged into electromagnetic coil
    - produces magnetic field perpendicular to the current
    - -> the greater the rate of change (= how fast the first peaks occurs) in the current the greater the magnetic field
  2. induces a secondary electric current to flow in another wire placed nearby
  3. second current is not induced in a wire but in neurons below the stimulation site
    - MF produces an electric field (EF) perpendicular to itself in the tissue
    - -> current parallel but opposite direction to coil current
  4. neurons fire
  5. if these neurons are involved in a specific function, stimulating them artificially will disrupt that function
    - introduced neural noise overwhelms the organised neural communication
    - if neuron not involved in task, this interference won’t occur
  6. efficacy of pulse depends on orientation of cell bodies with respect to flow of current
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4
Q

TMS pulse

A
  1. very strong current starting in coil
  2. magnetic field with up to 4-5 Tesla
  3. rate of change is maximally at beginning
  4. induce an electric field in brain
  5. induce tissue current
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5
Q

coils

A

A. circular coil
- cupper wire organised in a ring shape
- stimulation less focal: stimulate whole area along edges of the ring –> large area that is equally stimulated
B. figure-8/butterfly coil
- two rings are mounted next to another
- currents run in opposite directions –> at intersection the currents add up and have strongest stimulation

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6
Q

physiology

A
  • if stimulation is strong enough,
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7
Q

single-pulse TMS

A

= event-related TMS = lock stimulation to onset of task

  • chronometry of functional relevance: repeat event-related in different locations
  • causation and temporal relationship: pin point at which exact moment after onset of task, is stimulation causing behavioural change (when is it having a functional role)
  • online TMS: immediate effects
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8
Q

spTMS

- stimulation

A

= immediate, observable responses in motor cortex for example
- when stimulated during resting state –> stimulation

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9
Q

spTMS

- disruption

A

= system is not able to work properly

- when stimulated during action execution –> disruption

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10
Q

repetitive TMS

A

= apply 100-1000 pulses in a specific frequency

1) HF-rTMS (10Hz): LTP = increased excitability, triggered excitatory effect
2) LF-rTMS (1Hz): LTD = decreased excitability, inhibition
- -> effect of HF and LF depend on the state of the area (whether excitatory or inhibitory)
- induce a virtual lesion: area is less responsive
- lasting effects, induce neuro-plasticity

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11
Q

rTMS

- modulation

A

= modulates excitatory threshold of an area beyond period of stimulation
- after stimulation, no confounding effect and testing becomes easier

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12
Q

TMS

- process

A

TMS stimulation –> virtual lesion–> measurement of task performance –> specific impairment in behaviour/cognition –> insights into the function of the inhibited area

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13
Q

pre-localisation

A
  • use pre-defined landmarks (10-20 system)
  • if precise location unknown
  • -> stimulate different spots on a grid and check behavioural effects
  • -> determine localisation of behaviour by means of subtractive inference
  • -> use MRI/fMRI (reduces assumptions about temporal + spatial aspects made by TMS)
  • beforehand: identify position on skull
  • after: show the used site in scan
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14
Q

control condition

A
  1. compare performance when same region is stimulated in critical + non-critical time windows
  2. compare critical + non-critical regions
    - extra cues about spatial size of interest when using adjacent regions
    - when function believed to be lateralised use opposite hemisphere as control
  3. sham TMS: no TMS at all; placebo
  4. task control
    a. change some aspect of the task while keeping stimulation
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15
Q

spatial resolution

A
  • MF not spatially focal but EF has a spatial resolution of a few millimetres
  • unlike fMRI & PET the number of sites that can be compared is more restricted
  • -> separate hypothesis for every comparison
  • limited to superficial cortical regions: the further you distance from coil, the weaker the signal –> the less likely to stimulate
  • stimulating deeper structures may also stimulate overlying cortex
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16
Q

temporal resolution

A
  • at the point of maximal TMS activation, the stimulated area has its lowest SNR with respect to the task it’s trying to perform
  • unlikely that the time at which TMS has its max effect corresponds with peak times reported in ERP/MEG studies
  • ERP peak represent the contribution of more than one neural event with a group maximum at the peak
  • -> if signal represents neural event that is essential to the task, the time of TMS interference will usually precede ERP peaks
  • -> when TMS is applied over the areas that contribute to this signal, it may disrupt processing of the individual components that may be maximal before/at/after the reported peak
17
Q

Sack/Dücker 2002

A
  1. established with fMRI which areas were active during mental clock test
  2. applied rTMS to that area
    a) LF-rTMS to the right parietal: impairment in spatial imagery
    b) LF-rTMS to the left parietal: no impairment
  3. fMRI patterns showed 2 activation patterns: early (bilateral activation) + late (only right)
  4. hypothesis: right PPC can compensate the loss of the left PPC
18
Q

Sack/Dücker 2005

A

= combine rTMS and event-related TMS; at what time point is the right PPC relevant for the task

  1. LF-rTMS to right PPC, no LF-rTMS to left side (sham TMS = “placebo”)
    - disruption has an effect after longer time (where spatial comparison happens)
  2. LF-rTMS to right PPC, LF-rTMS to left side
    - induced lesion of left PPC prior to stimulation of right
    - right PPC becomes relevant before: has to compensate the tasks of left PPC
19
Q

TES

A

= transcranial electrical stimulation = use electrical stimulation instead of magnetic

  • cheap, easy, portable
  • consists of battery, two electrodes, current running from anode to cathode
20
Q

TES

- physiology

A
  • never strong enough to trigger an action potential –> modulate the resting membrane state/potential
  • shift threshold (positively/negatively) of membrane resting potential to make area less or more excitable for inner or outer stimulation
21
Q

TES

- tDCS

A

= direct stimulation

  • constant current
  • -> 1 end is sending and 1 receiving the current
22
Q

tDCS

- physiology

A

= you can increase and decrease excitability, similar to rTMS

1) anodal tDCS:increase excitability level
2) cathodal tDCS: decrease excitability level
- -> place either anodal or cathodal end over the target area

Methods of COGNEU (9 decks)

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