task 9 - TMS Flashcards
1
Q
How TMs works
A
- brief, high amplitude pulse of current discharged into electromagnetic coil
- produces magnetic field perpendicular to the current
- this magnetic field induces an electrical field in the tissue perpendicular to itself
- electrical field leads to an electrical current in the cortex parallel but in opposite direction to the current in the coil
- leads also to depolarization of underlying neurons
- The greater the rate of change in electric current, the greater the magnetic field induced electrical current in neurons is caused by making them fire
- magnetic field acts as a bridge between an electrical current in the stimulating coil and the current induced in the brain
2
Q
TMS causes neurons to be activated
A
- TMS causes neurons underneath the stimulation site to be activated
- Neurons involved in performing a critical cognitive function -> stimulating them artificially will disrupt that function.
- Therefore neurons are activated from internal source (task demands themselves) and external source (TMS -> disrupts internal activation)
- If you are resting, neurons are just activated (without interrupting anything)
3
Q
Advantage TMS
A
- real brain damage may result in a reorganization of the cognitive system (a violation of the transparency assumption)
- within-subject designs (i.e. with and without lesion) are possible
- can be used to determine timing of cognition
- lesion is focal
- can study functional integration
4
Q
Disadvantages
A
- TMS is restricted in the sites that can be stimulated, i.e. those beneath the skull
- possible that more distant brain structures receive stimulation if they are connected to the stimulation site
- accidents of nature” turn up some unexpected and bizarre patterns (cannot be achieved with TMS)
5
Q
Studying functional integration
A
- how one region influences another or how one cognitive function influences another
- session of focal TMS and then studying how this affects the communication between brain regions using fMRI
6
Q
Repetitive pulses
A
- train of pulses during the task
- more powerful in ability to detect the necessity of a region
- not possible to draw conclusions about timing because it would be unclear which pulse (or pulses) was critical.
- Some tasks may require several pulses for TMS to exert interference
- studies of “higher” cognition (e.g. memory, language) have often used rTMS
7
Q
Finding the right spot
A
- Positions on the head can be defined relative to landmarks (inion, anion, vertex)
- Example: approximately locating area V5/MT (dedicated to visual motion perception) is by marking a spot 5 cm in front of the inion, and 3 cm up from it -precise location is not known before the study, then one could stimulate, say, six different spots lying in a 2 × 3 cm grid,
- Structural and functional MRI can also be used to locate candidate regions
- Alternatively: TMS could be performed prior to a structural MRI scan in which the stimulation sites used have been marked in such a way as to render them visible on the scan
8
Q
Appropriate Control condition
A
- compare performance when the same region is stimulated in critical and non-critical time windows
- compare stimulation in critical and non-critical regions.
- If cognitive function is lateralized, one could use the same site on the opposite hemisphere as a control.
- With above mentioned methods, peripheral effects of TMS can be minimized.
- task control: same region can be stimulated at the same times, but with some aspect of the task changed (e.g. the stimuli, the instructions)
9
Q
Transcranial direct current stimulation (tDCS)
A
- Direct current involves the flow of electric charge from a positive site (an anode) to a negative site (a cathode)
- stimulating pad (either anodal or cathodal) is placed over the region of interest and the control pad is placed in a site of no interest (sometimes on the front of the forehead, or sometimes on a distant site such as the shoulders)
- After a period of stimulation (e.g. 10 min) a cognitive task is performed
- can be compared with sham stimulation, or anodal and cathodal stimulation can be directly contrasted.
- Changes resting state of membrane (so increase or decrease likelihood of firing), you do not induce action potentials like in TMS
10
Q
Use of tDCS
A
- Repeated sessions of anodal tDCS are becoming increasingly used for cognitive enhancement (of normal brains) and neurorehabilitation (of damaged brains)
- Example: tDCS over the primary motor cortex leads to increased cortical excitability and greater hand functionality in patients with motor impairments following stroke
11
Q
Cathodal tDCS
A
- Decreases cortical excitability and decreases performance
- affects the glutamate system (this neurotransmitter has excitatory effects).
- reduces the firing rate
12
Q
Anodal tDCS
A
- Increases cortical excitability and increases performance.
- increases the spontaneous firing rate of neurons whereas cathodal stimulation reduces the firing rate
- affects the GABA system (this neurotransmitter has inhibitory effects)
13
Q
Special specificity TMS
A
- Cortical areas affected by TMS: max. 2-3 cm deep
- TMS may affect remote cortical and subcortical areas via transsynaptic connections -> compromises spatial specificity
14
Q
Safety of TMS
A
-potential to induce epileptic seizures if applied at high frequencies and intensities (depends on parameters, especially frequency)
- kindling: repeated regular application of originally subconvulsive stimuli can culminate in a seizure
- > most likely when stimulating with repeated regular pulses at frequencies above 50 Hz and thus at stimulation parameters that are outside the protocols commonly used for rTMS
- > was also shown at frequencies below 10Hz
-rTMS can cause a disruption of cognitive processing longer than stimulation period itself
> might affect the efficiency of synaptic transmission and thus lead to a suppression of the stimulated area that lasts for hours
- facial nerves may be stimulated, resulting in involuntary twitches
- exclude PP with pacemakers, epilepsy or medical implants
15
Q
Stimulation intensity
A
should be defined according to individual cortical excitability and not in absolute intensity values.
- E..g.: threshold for a motor evoked potential (MEP)
- motor threshold may indeed be a very poor predictor of the effects of TMS on non-motor or ‘silent’ cortical areas
- changing strength in currents to manipulate stimulation intensity
