HC 9 Flashcards
What are causal methods?
They are not real imaging, but induce changes in brain activity. This is done directly, by influencing physical properties of neural tissue. There are also more indirect methods to change brain activity.
Imaging is correlational evidence. Can be relevant to infer causality, but not with certainty. Even so, causal methods cannot infer causality with certainty!
What are stimulation methods?
What is being applied at physical level. It does not equal having an effect on neural processing. Both excitation and inhibition is possible.
What are intracranial recordings?
Invasive electrophysiological techniques used to measure electrical activity within the brain. Electrodes are placed directly onto or into the brain tissue.
What is invasive electrophysiology?
It refers to techniques used to study the electrical properties of neurons and neural circuits within the brain by directly accessing the nervous system. Electrodes are surgically implanted into the brain or other parts of the nervous system. Electrodes are removed prior to surgery.
The choice of the electrode depends on clinical need and anatomical objective. This technique is especially used in epilepsy patients not responding to medical treatment. It is mostly done in the limbic and frontal regions.
What are some invasive electrophysiology techniques?
- Single-Unit recording
- Multi-Unit recording
- Local Field Potentials (LFP)
- Intracellular recordings
- Patch-Clamp recording
- Deep Brain Stimulation (DBS)
- Electrocorticography (ECoG)
What is ECoG?
It stands for electrocorticography. The use of electrodes is placed directly on the exposed surface of the brain to record electrical acitivity from the cortex.
What is SEEG?
It stands for stereo-electroencephalography. Recording electroencephalographic signals via depth electrodes.
What are the advantages and disadvantages of intracranial recordings?
They are similar to EEG, but with much better signal to noise ratio and very high temporal and spatial resolution.
However, using these techniques depends on available subjects, limited time available, invasiveness and lack of control set.
What is microstimulation?
The electrode is inside the neural tissue. Only performed when electrodes are crucial for clinical purposes.
A small electrical current is used. This influences the activity of the neurons near the tip. The larger the current, the wider the affected area and stronger the influence on nearby neurons.
Current too strong results in damage, strong results in action potentials elicited and weak results in membrane potential change.
What is DBS?
It stands for deep brain stimulation. It is kind of a microstimulation. Electrodes in deep structures of the brain, more permanently implanted.
It can be used to treat Parkinson or severe depression or OCD, through enchancing, inhibiting or synchronizing activity.
A neural pacemaker, which can induce repetitive and chronic stimulation across a long period of time is used.
What is FUS?
Focused ultrasound stimulation and non-invasive. The ultrasound used has frequencies of a few hundred kHz, which is far beyond hearing range.
It is a focused volume of space in which waves propagate. It is applied transcranially. Bones reduce the amplitude dramatically (*1/4). Bone does not scatter sound waves.
It is typically used with anatomical MRI.
How is the resolution of FUS?
The spatial resolution is a few milimetres and is higher than other non-invasive stimulation methods in lateral directions.
What is low-intensity FUS?
It leads to neural ‘stimulation’. It uses low-intensity ultrasound waves to target specific areas of the brain.
Why is a substantial minority unresponsive to FUS?
- Individual anatomical differences
- Acoustic paramters
- Biological factors
- Technical limitations
- Subject-specific factors
- Psychological cognitve factors
What is TMS?
It stands for transcranial magnetic stimulation and is non-invasive. It uses electromagnet induction based on Faraday’s principle.
There is less effect on deeper brain regions, so then you should use DBS.
The strength of the magnetic stimulation has an effect on the signal and can cause side effects.
How is TMS applied?
There is an electric current in coil. This generates transient magnetic field perpendicular to coil. This induces a second current in the brain that flows parallel, but in opposite direction.
This stimulates or depolarizes neurons.
What does the shape of the coil in TMS do?
It determines the shape magnetic field and induced current. The best shape is the figure of 8 coil design, because you het a more focused magnetic field and more precise location of altered neural acitivty.
What is TRNS?
Transcranial random noise stimulation. It is a non-invasive brain stimulation technique that uses alternating currents to deliver random electrical noise through electrodes places on the scalp.
It has a higher chance at finding effects than TMS.
What are some determinants of properties that effect electrical current?
-Physical factors: distance to coil
*Effect gradually dissipates with longer
distance.
*Restricts application to superficial
structures.
-Biological factors
*Conductivity of tissue
*Orientation of axons
Physical parameters are easier to characterize than biological parameters. Predicting effect on neural activity is difficult.
How is the resolution of TMS?
The spatial resolution is 1cm and it has a very good temporal resolution, however it is limited by the duration of pulses and number of time-points tested.
What are the variants of TMS?
-rTMS
-Single/Double pulse TMS
Sometimes, the two are combined. rTMS to establish a causal link followed by single/double for better temporal resolution.
What is rTMS?
It stands for repetitive TMS. Multiple pulses are used to specific brain regions and this is repeated. This influences temporal resolution. There is a stronger effect, but the risk is also higher.
What is single/double TMS?
Single pulse is when a single magnetic pulse is delivered to a specific brain region. For double pulse this is two magnetic pulses delivered in quick sucession, with a short interstimulus interval.
What are the control conditions for TMS?
Sham: turn the coil by 90 degrees. This way, the magnetic field does not influence neural activity and differences between participants is easier to detect.
Stimulation of the vertex, which is the highest point of the skull. The assumption is that there is little to no effect on behaviour.
What is the potential of TMS?
Diagnostic value for testing connectivity between motor cortex and peripheral muscles. Especially for motor related disorders and after a stroke.
For clinical treatment. It has tested for many disorders, so efficacy is proven for some.
For major depression, rTMS is used as an alternative for electroconvulsive therapy. The target site is dorsolateral prefrontal cortex.
It can be used for migraine. Single-pulse TMS over visual cortex.
What is TCS?
Transcranial current stimulation. The principles are similar to TMS, but it uses electrical stimulation instead of magnetic. The current flows from anode to cathode.
Depolarization neuron, +, excitatory –> hyperpolarization neuron, -, inhibitory
How is the resolution of TCS?
The effect size of the current strength is at neural level. Side effects include skin sensations at the start and end of the stimulation.
The control condition, or sham TCS, has a current at the start and end of the experiment to trigger skin sensations.
TCS has a poor spatial resolution (5cm). The contact area for electrodes are relatively large. Relatively uniform delivery of current across electrode area.
It also has a very poor temporal resolution. Therefore, within-subjects design is not possible.
What are the variants of TCS?
-tDCS
-tACS
-tRNS
What is transcranial direct current stimulation?
A direct current in one direction. The location of electrodes determines flow current.
What is transcranial alternating current stimulation?
Alternating current at fixed frequency. To test whether particular frequency ranges are involved.
What is transcranial random noise stimulation?
Alternating current based on random frequency spectrum.
What is the difference between TMS and tDCS?
Stimulation principle: Electromagnetic pulses at a sufficient intensity applied over the scalp induce a current in the underlying cortical neurons (TMS), Weak currents between a pair of saline-soaked surface sponge electrode are transmitted through to the cortex (tDCS)
Target precision: Focal (TMS), Non-focal (tDCS)
Type of currents: Large, but brief currents (TMS), weak, but long currents tDCS)
Inhibitory mode: low frequency rTMS (TMS), cathode over the target area (tDCS)
Excitatory mode: high frequency rTMS (TMS), anode over the target (tDCS)
What is potential of TCS?
Small, but robust effect on neural activity in particular circumstances. Can make subthreshold stimulus perceivable. Small effects accumulate over time.
Larger sample sizes needed for accurate estimate of effect size.
TCS on human cadaver heads, no effective current in brain with common protocols.
What is the importance of having clinical groups?
Patients with neurological conditions and disorders allow for a number of cognitive neuropsychology methods that are otherwise not possible.
Lesion studies are not ethical.
