task 4 Flashcards
pros MEG
- Same temporal resolution as ERP
- More reliable localization
- Magnetic fields are not distorted as they pass though brain, skull and scalp
- Useful tool in neurosurgery: identify focus of epileptic seizures and location of tumors
Con’s MEG
- Detect current flow only if flow is oriented parallel to the surface of the skull -> neurons can be recorded mainly if they are located in the sulci
- Magnetic fields are extremely weak: SQUIDS to detect them
set up MEG
- SQUIDS placed at various points on the surface of scalp
- Helmet-shaped magnetometers: with 206 SQUIDs at 102 measuring sites
- Each of 102 sensor units is equipped with 3 transformers that provide measurements of the magnetic field B
- 2 transformers: gradient coils: record in x and y directions
- 1 magnetometer coil: record z component
- Carried out in a room made of layers of aluminum and mumetal (iron and nickel): external magnetic fields are trapped in it, shielding the room inside
SQUIDs
- To detect the brain’s weak magnetic fields, the sensors = Super-conducting quantum interference devices, are encased in large, liquid-helium-containing cylinders that keep them colder than 4 degrees Kelvin
- can be placed no closer than 3cm from the brain
Inverse problem
-How to identify which currents in the brain are responsible for particular MEG signals using only info about the magnetic-field patterns and shape of the brain?
Calculating source of magnetic fields
Assumption made: brain is approximately spherical, active areas can be represented by single or multiple current dipoles
- Computer guesses where the dipoles might be
- Calculates externa magnetic field that these dipoles would produce
- Compares computer field with measured field
- Repeat calculation with dipoles at different positions until calculated and experimental results match as closely as possible
Femto-Tesla
- measurement of magnetic fields in the femtotesla (fT, 10–15 tesla) range is important for applications such as magnetoencephalography
- only sensors capable of detecting these very small fields have been based on low-temperature superconducting quantum interference devices operating at 4.2 kelvin
Mu-metal
- MEG Carried out in a room made of layers of aluminum and mumetal (iron and nickel): external magnetic fields are trapped in it, shielding the room inside
Fourier transform
• Takes a time-based pattern, measures every possible cycle, and returns the overall cycle recipe (amplitude, offset, rotation speed for every cycle)
Example smoothie:
-filters must be independent, complete and combine-able
- start with time-based signal
- apply filters to measure each possible circular ingredient
- collect full recipe, listing amount of each circular ingredient
Power
= square of the amplitude (easier to observe, because amplitudes are often very similar to each other)
-Used to quantify contribution to measured EEG signal
2 major classes of ERP waveforms
- Stimulus locked and response locked waveform
- The earlier the deflection, the more likely it is to reflect an automatic/reflexive psychological process
Quantifying ERPs
- waveforms are computed and then scored for analysis
- Determine time window in which a component of interest emerges
- Measure average voltage within window for each subject
- Alternative: automated peak-picking
- Peak latency may be quantified
attitude and evaluative processes - P3
- amplitude increases when stimulus represents a category different from that of the preceding stimulus
- Target word was inconsistent with context words -> P3 was evoked
Measure of implicit attitudes
P2
N2
- Pictures of white faces elicited larger amplitudes of 2 components: P2 and N2
- Response of white faces were more evaluatively discrepant from the negative context than black faces
- The higher the amplitude, means more processing
Affective congruency effect
-Affective target word is categorized in terms of its valence (positive or negative) more quickly when preceded by prime words of the same valance (congruent trials) than by prime words of the opposite valence (incongruent trials)
