Problem 4 Flashcards
Event related potentials
Refers to an electrical potential generated by the firing of cortical neurons in response to a specific events
–> Direct measure of neural activity
How are ERP signals extracted ?
From an ongoing EEG wave
–> must filter the signal from the background EEG noise
2 Major classes of ERP waveforms are of interest for SCAN researchers.
Name them.
- Stimulus locked waveform
- -> arise at onset of the stimulus - Response locked waveform
- -> arise when the behavioral response was made
How are ERPs scored for analysis/quantified ?
Determining the time window in which a component of interest emerges
–> then measuring the average voltage within that window for each subject
P3/P300
Refers to a relatively large + positive deflection that peaks between 300 and 800 ms post-stimulus
–> increases when a given stimulus represents a category that is different from that of preceding stimuli
What are the advantages of ERP methods
- High temporal resolution
- Ability to measure psychological processes independently from behavioral response
- Lower costs
- Upright position when conducting it
- -> cloesly mimics how people typically interact in the social world
What are the Disadvantages of ERP methods ?
- Low spatial resolution
- -> insensitive to activity in subcortical structures - The neural source of a particular ERP component is likely involved in multiple psychological functions and a one-to-one mapping of a psychological construct can never be assumed
How was the P3 component used to assess attitudes ?
Participants were presented with images of
a) white peoples faces b) black peoples faces c) positive objects
each of which appeared in the context of negative images
–> white faces increased P2 and N2 compared to black faces
Affective congruency
An affective target word is categorized in terms of its valence more quickly when preceded by prime words of the same valence
ex.: smiling while feeling happy
N170
Responds selectively to faces
–> can reveal differences in the extent to which an individual perceives another as a fellow human
Can N170 component assess in-group favoritism ?
Yes,
They found larger N170 to ones in-group compared to outgroup faces
BUT: only when intergroup competition is absent and one is somewhat indifferent towards outgroups
Can N170 assess racism ?
yes,
as in context of race, the outgroup may be threatening which could lead to enhanced processing of outgroup faces
Which components become active/increase in response to stereotyping ?
- P3/LPP component
- N400/N4 component
- -> when someone violates rather than confirms gender stereotypes
Magnetoencephalography
MEG
Records magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers
–> its traces can also be recorded and averaged to obtain ERPs
BUT: more reliable than EEG
MEG has 2 drawbacks.
Name them.
- Can only detect current flow if it is oriented parallel to the surface of the skull
- Magnetic fields generated by the brain are extremely weak
What is the MEG primarily used for ?
To study
- human brain functions non-invasively at time resolutions of better than a ms
- the way the brain processes signals
- the functional significance of brain rhythms
How does the MEG work ?
- A neuromagnetometer is positioned around the outside of the head to record magnetic fields
- SQUIDs are placed at various points on the surface of the scalp
- Magnetic fields generated by electrically active regions of the brain are transmitted to the SQUIDs
–> magnetic fields must be sampled over a range of locations so the distribution of electrical currents in the brain are accurately calculated
Neuromagnetometer
Refers to a helmet shaped device that covers the whole scalp and measures magnetic field or magnetic dipole moment
–> measure the direction, strength, or relative change of a magnetic field at a particular location
Superconducting quantum interference device
SQUID
Refers to a very sensitive magnetometer used to measure extremely subtle magnetic fields
–> are in the neuromagnetometer helmets and placed around the scalp
Neuromagnetic inverse problem
How can we identify which currents in the brain are responsible for a particular MEG signal by using only info about the magnetic field patterns and the shape of the brain ?
How is the neuromagnetic inverse problem solved and the source of the magnetic fields calculated ?
- Assuming the brain is approximately spherical + its active areas can be represented by single or multiple current dopes
- Computer guesses where the dipoles might be based on the measured distribution of magnetic fields
- Computer calculates the external magnetic field that these dipoles would produce
- Computed + measured field are compared
- Dipole calculation is repeated at different positions until the calculated + experimental results match as closely as possible
Minimum current estimate
Technique to calculate the source of the magnetic fields
–> best as it doesn’t make assumptions about the way in which currents are distributed
EEG vs MEG
- MEG is better than EEG in pointing out where in the brain particular activity takes place
- -> less distortive measurement of electric potentials - MEG only measures the tangentially oriented currents
- EEG can more reliably pick up currents that originate deep inside the brain or are radially oriented
THUS: to obtain the most info one must monitor the brain w/ both MEG and EEG at the same time - but not possible
Normative EEG activity can be divided into … ?
Low EEG frequencies
–> show large synchronized amplitudes (delta + theta)
vs high EEG frequencies
–> show small aptitudes due to high degree of desynchronization
(beta + gamma)
Delta Band
1-4 Hz
Low frequency activity
–> typically associated with sleep and neurological pathology.
BUT: Delta activity is mainly an inhibitory rhythm
Theta Band
4-8 Hz
Prominent during sleep
–> during wakefulness, there are two types of theta activity:
a) Widespread scalp distribution linked to decreased alertness (drowsiness) and impaired information processing
b) Frontal midline theta activity frontal midline distribution, associated with focused attention, mental effort and effective stimulus processing.
BUT: Theta activity may subserve a gating function on the info processing flow in limbic regions
Alpha band
8-13 Hz
Is recorded during states of relaxed wakefulness, especially when eyes are closed
a) Lower alpha
- -> due to stimulus-unspecific + taks-unspecific increases in attentional demands
b) Upper alpha
- -> due to task-specific processing of info
Beta Band
13-30 Hz
Has a symmetrical fronto-central distribution
–> Replaces alpha rhythm during cognitive activity and increases with attention and vigilance
Gamma Band
36-44 Hz
Associated with attention, arousal, object recognition, top-down modulation of sensory processes, and sometimes perceptual binding
–> brain’s ability to integrate various aspects of a stimulus into a coherent whole
What are electrode interpolation methods needed for ?
To deal with artifacts “noise”
Linear interpolation
Corrupted activity/noise is reconstructed through weighted average using data from neighboring electrodes
Spline interpolation (more accurate)
Info from all sensors is used to represent the overall potential distribution on the entire scalp
–> to reconstruct the activity at missing channels
How are EEGs used for Data acquisition ?
EEG activity represents the difference between 2 electrodes
–> active vs reference electrode
How does the EEG work ?
- Performed with 20-256 electrodes embedded in an elastic cap that record signals from the cortex and sub-cortex
- The EEG signal arises from synchronized synaptic activity in populations of cortical neurons
–> higher number of electrodes is not necessarily better as it becomes difficult to ensure high quality data due to poorer signal quality and lower statistical power
- The electrical potential (voltage) is recorded from each electrode, resulting in a separate waveform for each electrode (x-axis: time; y-axis: voltage).
–> This waveform is a mixture of actual brain activity, artifactual electrical potentials produced outside the brain, and electrical activity induced from external sources
- Voltage refers to the potential for an electrical charge that moves between two locations
- -> EEG signal is therefore the voltage between two electrodes
a) active electrode
b) reference electrode that is used for all electrodes
- This voltage is sampled from each electrode channel
- Filters are used to remove very slow voltage changes (<0.01-0.1Hz) and very fast voltage changes (>15-100Hz) as they are likely to represent noise emerging from non-neural sources.
