Task 2

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NIBS protocols, designs and paradigms

Paradigms

Event-related TMS - magnetic pulse is time-locked to the onset of the behavioral task. By applying single pulse TMS at variable times during task execution, it is possible to investigate with a temporal resolution 5-10 ms at what exact time point the neural activity at the stimulation site is critical for successful task performance.

With and without stimulation can be used to assess the role of the area. Causal structure-function mapping.

Disadvantages:

1. No direct way of telling.The only way to know what really happens in the brain during TMS is by directly assessing its neural effects through simultaneous combination of TMS and functional brain imaging techniques.

2. Insufficient information about the time at which an area makes a critical contribution to a given behavior. (rTMS allows the site of stimulation to be establish while later single-pulse experiments can give insight into the chronometry of this critical contribution to behavior).

3. Neck muscle activations.

Types of experiments:

1. Time-locked. You can work with time - locked TMS stimulations to the presentation of the visual stimuli to induce errors in the detection.

2. Interstimulus interval. You can seperate by a variable interstimulus interval to two different brain parts.

3. Using sham stimulation.

Functional imaging should be used with TMS. It is advisable to first identify and localize candidate areas using a functional imaging technique. It could also be localized on the basis of individual anatomical bran scans. Another approach - application of TMS in relation to specific individual scalp landmarks on the 10-20 scalp positioning. But it does not work good individually. This approach is needed because changes of cortical excitation depend non-linearly upon the intensity of tACS.

Parameters are really important in conducting experiments: stimulation durations, intensity, scalp placement. You can also check the behavioral results to understand of your experiment is working.

2 reasons of different intensities:

1. Different cortex excitability

2. Scalp to cortex difference at different sites are correlated.

TMS protocols

1. Single pulsed. Can provide insights into the underlying neural substrates and timing.

2. Paired pulsed. Enable measurements of cortical physiology. Two spatially distinct regions.

3. Repetetive trains. TBS. Can be used where stimulation and task performance are dissociated in time.

4. Patterned stimulation. Have longer lasting effects then rTMS.

5. tDCS. Show prolonged improvements in performance. Or modulate state dependent activity.

6. Triple pulsed.

Paired associative stimulation in which low-frequency median nerve stimulation is coupled with TMS over the contralateral motor cortex. Pharmacological interventions provide more information on TMS.

TMS parameters

1. Intensity of stimulation. 2 factors influence the susceptability of a brain area to stimulation: 1. magnetic field strenght 2. excitability of the cortex. How to use for different individuals: 1. fixed intensity 2. intensity corrected for scalp to brain target distance. 3. Intensity tuning curve 4. behavioral determination 5. functional determination (MRI changes). 0.2 mA resulted in cortical inhibition (motor threshold was increased), 1mA resulted in decreased thresholds. Intermediate intensities had no effect on motor threshold.

2. Frequency of stimulation.

3. Duration of stimulation.

4. Duration of stimulation effect.

Stimulation sites

1. A possible approach is to locate brain areas relative to those that have a reasonably certain position.

2. Use frameless stereotatic system (structural MRI)

3. Brain voyager.

Theta burst stimulation

Protocol - a burst of 3 stimuli at 50Hz (20ms between each stimulus) which was repeated at intervals of 200ms (5Hz). Continous TBS - 40s train of uninterrupted TBS is given (600 pulses) -suppresion. Intermittent TBS a 2s train of TBS is repeated every 10s for a total of 190s a bit of excitation. Intermediate TBS a 5s train of TBS is repeated for every 15s - base level of MEP. All of them of 600 pulses. TBS produces a mixture of facilitatory and inhibitory effects on synaptic transmission, with facilitation building up faster than inhibition.

When experimenters compared a single train of TBS for either 2s (intermittent) against 5s (intermediate). Single train of TBS can lead to a mixture of suppresive and facilitatory effects on MEP`s.

To check for confirmation of TBS effects on the excitability, you can measure SICI.