Entrainment

Página 1

Task 4. Entrainment

Entrainment of Brain oscillations by transcrannial alternating current stimulation.

To remove the tACS artifact, which ideally would be removed by subtracting sine wave fitted to the recordings researchers added a constant 10Hz sine wave with similar characteristics

Synchronization to the external driving force should lead to more regular network dynamics during stimulation.

tACS most likely modulate behavioral performance in a phase-dependent fashion.

Stimulation at 10Hz enhances alpha power in the parieto-occipital cortex synchronizes and modifies behavioral outcome.

Alpha rhythm can actively suppress processing of irrelevant information and therefore direct information flow to task-relevant neuronal structures.

Researchers succesfully modulated the inhibitory alpha network. Alpha power reflects increased inhibition of task-irrelevant sites.

Alpha power increase was amplified during stimulation

The peak of alpha band is shifted during stimulation toward the stimulation frequency of 10 Hz

10 Hz tACS increased oscillatory power in the alpha band.

The effects are confined to alpha band.

On the role of pretimuls alpha rhythms over occipito-parietal areas in visual input regulation correlation or causation?

TMS over occipito-parietal areas at alpha frequency before stmulus onset, mimick the natural brain rhythms of attention deployement, and tested for frequency specific perceptual consequences.

Phosphene threshold being increased with a concurrent visual detection task.

Sham normalization consisted of subtracting the mean hit rate of each subject and sham TMS condition from the mean hit rate for that subject under the corresponding real condition.

TMS frequency and target position depended neither on hemishphere not TMS site. This suggests that the 10Hz specific pattern of contralateral suppresion with ipsilateral enhancement generalizes across hemispheres and sites. Contralateral -> suppression, ipsilateral - enhancement.

Parietal alpha TMS can be interpreted as benefiting these internal representations by suppresing input from distracting visual information.

Single occipital TMS pulse delivered at a low, varibale jitter rate triggers alpha oscillations in occipital regions.

When occuring in phase with TMS-initiated alpha oscillation, these subsequent pulses would be expected to reset more and more neurons to oscillate in synchrony with the evolving alpha wave, resulting in progressive entrainment.

5Hz might not induce alpha waves, because of their short intervals. 20Hz might interfere with additive effects because each second pulse occurs in anti-phase with alpha cycle.

Rhytmic TMS causes local entrainment of natural oscillatory signatures

Entrainment supposses

The induction of a distinct entrainment signature

Progressive enhancement of the target oscillation in the course of the TMS train.

Depend on ongoing activity of the target generator

4 conditions:

Stimulation of individual alpha frequency

some number of TMS in jittered interpulse intervals

coil in 90degrees

sham position.

TMS entraining supposes that frequency-tuned rhythim TMS triggers a local oscillation at the target site which cycles at the natural frequency of the targetd generator.

Widespread broadband responds were observed in a second half of the train, where conditio-specific responses emerged over pulses 3-5 in a narrow alpha band.

phase-locking (when they get to the same phase) factor quantifies the consistency of the instantenous phase across trials.

Results

No progressive alpha-synchronization with arythmic alpha generation

nothing with sham

nothing with 90 degree coil.

Effects only in higher order are of the brain.

Initial TMS pulses might have phase reset a multitude of parallel generates located within the same stimulated network but each cycling at a distinct frequency.

TMS entrainment can therefore be constructed as a progressive recuitment of neuronal element cycling at the target frequency to phase align their activity to TMS.

With increasing stimulus intensity, entrainment may occur from a progressively larger bandwidth.

Entrainment of perceptually relevant bran oscillations by non-invasive rhytmic stimulation of the human brain

One of the computationla processes that oscillations likely enable is the dynamic rating/gating of information through the synchronization of neuronal elements.

Low frequencies relate to long-distance, area-to-area interactions, higher-frequency to local neuronal communication.

Synchronization means that the oscillating neural population can be modelled as a periodic force acting in a certains direction on the phase vector.

Depends on parameters:

amplitude of stimulation

the phase of stimulation

the duration of stimulation

the frequency of the stimulation.

Requirements of entrainment:

Entrainment requires the involvment of neural oscillator

periodicity in the external force

synchronization

External force influencing by direct interaction.

Rhytmic stimulation can come in:

pulsed forms short, transient stimulus events at regular intervals

continous forms which generate oscillating patterns

sensory input pathways

using TMS techniques that stimulate the brain directly bypassing sensory input.

Entrainment critique

There s no evidence of ongoing brain oscillations (??)

May be caused by periodicity of stimulation.

Are expected to last beyond the end of the stimulation train (but they don`t)

Successful entrainment of neural oscillations leads to shorter reaction times.

No brain rhythms coulde be entrained if applied at an inappropriate frequency

The tuning of stimulation parameters is important to achieve entrainment.

Entrainment is short term

Criteria for entrainment

Synchronization.

Frequency-specificity

spatial specificity

The targeted area can also cycle at the entrainment.

Frequency-specific behavioral consequences.

A cyclong pattern of behavior during rhytmic stimulation.

Modeled oscillations:

1. DC current mainly effects the firing rate.

2. AC stimulation up and down regulates the firing rate in an oscillatory manner without changing the average firing rate over a longer time interval.

3. AC stimulation at the frequency of endogenous oscillations mainly affects spike timing.

4. Even low amplitudes of electrical stimulation corresponding to a cortical electric field of 0.2 V/m result in enhanced coherence between spikes and the driving oscillation.

Story

Synchronization means that the oscillating neural population can be modelled as a periodic force acting in a certains direction on the phase vector. Or locked to the external force.

Entrainment therefore is about driving brain oscillations by external force - rather than disruption of oscillations by these forces.