Problem 9 Flashcards
Name methods to perturb neural function.
- Pharmacology
- TMS
- TDCS
- Genetric manipulations
How can pharmacology be used to perturb neural function ?
Psychoactive drugs can pass the blood brain barrier and then disturb functions
–> by use of agonist (imitator) or antagonist (blocks ) drugs
Disadvantages of the use of pharmacology ?
Lack of specificity
–> one does not know how much of the drug actually makes it to the side of the brain you actually want to measure
Transcranial magnetic stimulation
TMS
Refers to a method of non-invasively producing focal stimulation of the human brain
How does TMS work ?
It consists of a tightly wrapped wire coil encased in an insulated sheath and connected to a source of powerful electrical capacitors
- Triggering the capacitors discharges a brief high amplitude pulse of electrical current into an electromagnetic coil
- The current in the TMS coil produces a magnetic field
- When the coil is placed on the surface of the skull, the magnetic field passes through the skin and scalp
- This induces a physiological current in the cortex that causes neurons to fire
THUS: based on the principle of electromagnetic induction
Downsides to TMS ?
- Exact mechanism causing the neural discharge is not well understood
- The area of neural activation will depend on the shape and positioning of the coil
- The area of primary activation can be constrained to about 1.0-1.5 cm
Transcranial direct current stimulation
Is a form of neuromodulation, thus it alters nerve activity through targeted delivery of electrical stimulation
–> done to normalize nervous tissue function; can produce changes in behavioral performance
THUS: developed to help patients with brain injuries
How does the TDCS work ?
- Sends low direct current (1-2 mV) through wires to two small electrodes (anode and cathode) that are placed on the target areas of the scalp
–> anode (positively ch.); cathode (negatively)
- Neurons placed under the anode depolarize (elevated state of excitability, more likely to initiate an action potential) =IMPROVEMENT IN PERFORMANCE because neurons excited
- Neurons placed under cathode hyperpolarize (less likely fire).
=HINDER PERFORMANCE
==> stimulates + activates brain cells by delivering electrical signals
TDCS vs TMS
- TDCS affects neural activity over a much larger area than the one that is directly affected by a TMS pulse
- TDCS effects of cathodal stimulation are less consistent than TMS
- TDCS can change neurological conditions
Name the stimulation parameters of TMS.
- Pulse intensity, amplitude, frequency, wave form
- Duration
- Rise time
- Magnetic field distribution
- Peak magnetic energy
–> changes in these parameters can affect different stimulation characteristics in very different ways
e.g. smaller coils produce stronger + more focal fields
Repetitive TMS
rTMS
A train of pulses is applied during the task (rates up to 100 Hz)
–> ability to detect the necessity of a region
THUS: Is more for the purpose of localizing a function of a certain brain area, as temporal resolution decreases
Single pulse TMS
spTMS
A single pulse is delivered at a precise time during a task
–> provides info about the timing of cognition + capable of identifying the temporal order in which specific neural processes are activated
THUS: learning at which moment within a trial a brain area is critically involved in performing a certain task
HIGH TEMPORAL RESOLUTION
Triple-pulse TMS
tpTMS
Refers to a variant of repetitive TMS
–> 3 pulses are delivered
Why is it beneficial to combine TMS with fMRI ?
It gives us info about
- the transient local change of neural activity during the execution of a certain task
- Physiological modulation of this activity by TMS
- Behavioral consequences of the magnetic stimulation
–> enables the navigation of the coil over the intended target region + verifying the actual position of the coil
Limitations of TMS studies
- Interpretation difficulties
–> TMS induced impairment of task performance may be due to many different effects, which leads to a difficulty interpreting results and pinpointing whether the area is indeed relevant for specific cognitive functions
- Technical difficulties
–> accuracy + depth of penetration
Advantages of TMS studies
- Manipulates brain activity as an independent variable
- -> investigate its influence on the performance of different cognitive tasks thus narrows the range of hypotheses on brain-behavior relationship - Allows systematic and independent disruption of neuronal activity in different components of a task- correlated activation network
- Compared to studies that combine functional imaging with neuropsychological deficits occurring in structural brain damage, the functional deficits induced by TMS are far more transient, and therefore its effect is unlikely to bring about functional reorganization
- Combining TMS with functional imaging thus represents the appropriate methodological approach to study the functional contribution of specific brain areas to the performance of a task
Faraday’s principle of electromagnetic conduction
Suggests that if an electrical current passes through one coil, it can induce a current in a nearby coil
Metamagnetism
Involves combining
- TMS
- fMRI
- PET
in behavioral studies
In which way is the size of TMS coils important ?
Smaller TMS coils produce more focal magnetic fields
Figure of 8 coil
TMS
Consists of two circular ring-shaped coils mounted next to each other
–> the focal point of stimulation lies at the intersection of the two loops
- more focused magnetic field distribution than the circular coil
- most commonly used
Circular coil
TMS
Inside the coil a copper conductor is wound in one or several turns into a ring-shaped configuration
–> no magnetic field focus, but a maximum current in the entire outer winding that produces a ring-shaped magnetic field around the coil
