Task 4

Question 1

Most ERP experiments in social cognition and affective science (SCAN) are built upon a basic behavioural task design, which typically involves the timed presentation of stimuli and the recording of behavioural responses (e.g. on a computer). But compared with behavioural experiments, ERP tasks often include

Answer 1

more trials (e.g. 30–50 per trial type) and longer intertrial intervals (2–4 s).

Question 2

Filtering

Answer 2

algorithms are applied to the analog signal to attenuate frequencies that are not of interest

Question 3

Aligning and averaging

Answer 3

EEG signals are aligned to an event of interest (e.g. a stimulus onset), EEG activity unrelated to the processing of the event will vary randomly across trials and therefore will average to zero, whereas EEG activity elicited by the event will vary according to properties of that event and will stabilize when averaged across trials of the same type

Question 4

2 classes of ERP waveforms are of interest in SCAN:

1- Stimulus-locked waveforms:

Answer 4

arise in response to a specific stimulus, such as a visual image or auditory feedback, reflecting some aspect of perceptual or attentional processing. The earlier, the more likely it reflects an automatic / reflexive response (i.e. N1 or N100, …)

• Often reflect some part of perceptual or attentional processing
• It is assumed that the earlier the deflection emerges following stimulus onset, the more likely it is to reflect an automatic or reflexive psychological process
• Naming conventions refer to the polarity and the ordinal position or the approximate time at which the deflection peaks

Question 5

2- Response-locked waveforms

Answer 5

useful for examining mechanisms associated with the formation and regulation of a behavioural response (i.e. error-related negativity, …).: Aligning EEG epochs to the moment when a behavioral response is made

• useful to examine mechanisms associated with the formation and regulation of a behavioral response
• named according to their polarity and the type of response (e.g. error-related negativity; error-positivity

Question 6

Cacioppo discovered that the P3 (or P300) amplitude increases

Answer 6

a given stimulus represents a category different from that of preceding stimuli

example: The oddball paradigm used here made use of the evaluation of positive vs. negative words (in a negative or positive context) and found that when a target word was evaluatively inconsistent with context words, such as when a negative word appeared within a series of positive words (positive context), a pronounced P3 was evoked.

Question 7

This affective congruency effect is

Answer 7

response conflicts when the response associated with the prime conflicts with the response called for by the target

Previous findings have shown that an affective target word is categorized in terms of its valence (positive or negative) more quickly when preceded by prime words of the same valence (i.e. congruent trials) than by prime words of the opposite valence (i.e. incongruent trials).
Due to their excellent temporal resolution, ERP methods offer the best approach to test many questions central to social cognitive and affective neuroscience

Question 8

the lateralized readiness potential (LRP

Answer 8

a dynamic measure of motor cortex activation associated with preparation and initiation of behavioural responses

Question 9

Face perception:

Answer 9

the N170 component responds to faces.
Research has found differences in N170 concerning the categorization of social groups and race (larger N170 on ingroup and Black faces compared to outgroup faces. If outgroup is perceived as a threat, the processing of the outgroup is enhanced).

• N170 can reveal differences in the extent to which an individual perceives another as a fellow human
• Sensitive to higher level social/motivational factors

ERPs can inform the process through which faces, once encoded, are categorized according to relevant social groups

Question 10

the oddball task

Answer 10

The oddball paradigm is an experimental design used within psychology research. Presentations of sequences of repetitive stimuli are infrequently interrupted by a deviant stimulus

Question 11

the self-regulation of responses to stereotyped targets involves the coordination of two complementary processe

Answer 11

1. An initial conflict monitoring mechanism, subserved by activity in the dorsal ACC, which monitors ongoing responses for conflict (between goal intentions and a race-biased tendency)
2. A regulative mechanism, associated with activity in lateral prefrontal cortex (PFC), which responds to the conflict by strengthening the influence of intentional responses to override an unwanted tendency

Question 12

Advantages of ERP

Answer 12

1. ERPs are one of the only direct measures of brain activity as it occurs in real time =high temporal
2. The ability to measure psychological processes independently from, or in the absence of, any behavioural response
3. Cheaper
4. Data collection environment that may be less impactful on subtle social and affective processes of interest
5. Participants sit upright during data acquisition
   o More closely mimics how people typically interact in the social world than does a supine position
   o Certain psychological processes, especially those pertaining to approach motivation, do not operate in the same manner when people are lying down compared to when seated or standing

lower. cost

Question 13

Disadvantages of ERP

Answer 13

1. Limited spatial resolution
   o ERPs detect only neural activity of sufficient strength to be measurable at the scalp, rendering them insensitive to activity in subcortical structures
   o The skull and scalp act as a spatial filter on neural activity, reducing the spatial resolution of the signal recorded with scalp electrodes
    It is nevertheless possible to estimate the neural generators of ERPs using source localization procedures that consider the orientation and strength of a dipole signal, as measured at multiple sites across the scalp
    Nonetheless, you can also use it in combination with fMRI for better data collection
2. Interpretational Issues
   - The neural source of a particular ERP component is likely involved in multiple psychological functions and therefore a one-to-one mapping of a psychological construct onto a physiological indicator can never be assumed
   - Readers are cautioned against assuming that, for example, the N2 associated with an ingroup attention bias in social categorization tasks reflects the same neural source or represents similar information processing operations as the prominent N2 often seen in tasks involving response conflict or inhibition

Question 14

Magnetoencephalography

Answer 14

= used to detect the tiny magnetic fields generated from the weak electric impulses transmitted between brain cells

Non-invasive
Good time resolution
Use a neuromagnetometer

The magnetic fields that are generated by electrical currents in a subject’s brain are detected using highly sensitive superconducting quantum interference devices (SQUIDs), placed at various points on the surface of the scalp. The magnetic fields are transmitted to the SQUID by means of a superconducting flux transformer. The magnetic fields must be sampled over a range of locations so that the distribution of electrical currents inside the brain are able to be accurately calculated

+As with EEG, MEG traces can be recorded and averaged over a series of trials to obtain event-related fields (ERFs).
+MEG provides the same temporal resolution as with ERPs, but it can be used more reliably to localize the source of the signal and therefore has a higher spatial resolution.
+Magnetic fields (unlike electrical signals) are not distorted when they pass through all the layers (brain, skull, …)

Question 15

Limitations of MEG:

Answer 15

1) It can only detect current flow if it is oriented parallel to the surface of the skull. For this reason, the neurons that can be recorded with MEG tend to be located within sulci.
2) Magnetic fields are very weak, meaning that to detect them, the person has to be in a toom that is completely shielded from all external magnetic fields, including the Earth’s magnetic field. MEG sensors (SQUIDS) must be held in large containers at a temperature below 4 degrees Kelvin.

Question 16

inverse problem

Answer 16

Inverse problem: how can we identify which currents in the brain are responsible for particular MEG signals, using only information about the magnetic-field patterns and the shape of the brain?

Question 17

to solve the inverse problem

Answer 17

To solve the inverse problem, researchers assume that the brain is approximately spherical and that its active areas can be represented by single or multiple current dipoles. Then the PC gets to work:

1. On the basis of the measured distribution of magnetic fields, a computer first makes an initial guess as to where the dipoles might be
2. The computer then calculates the external magnetic field that these dipoles would produce
3. It compares the computed field with the measured field, before repeating the calculation with the dipoles at different positions until the calculated and experimental results match as closely as possible

b) Minimum current estimate technique: rather than assuming that the magnetic fields are generated by individual dipoles, this technique gives the most probable distribution of the currents in the brain, calculated according to the concept of minimum norm.
- Can be used without any assumptions about the way in which the currents are distributed in the head or whether they occur at the same time

Question 18

MEG versus other techniques

Answer 18

• The magnetic fields measured outside the head with MEG are much less distorted than the electric potentials on the scalp measured with EEG
• For most info: monitor EEG and MEG at same time, but can’t really do this yet

o MEG (pro): only measures those currents that are oriented tangentially to the surface of the brain (in the sulci), even when several areas of the brain are active at the same time.

o	EEG (pro): measure currents deep inside the brain or that are radially oriented well
	EEG and MEG get their results from partially different neurons (sulci and parallel vs. gyri and radial) and therefore give partially different information

o MEG has a much higher temporal resolution than PET or fMRI
o However, measurements on the brain with MRI, fMRI and PET provide useful constraints to the neuromagnetic inverse problem

Question 19

Delta band (1–4 Hz)

Answer 19

• Low frequency  sleep in healthy humans and neurological pathology
• In adults: delta power increases in proximity of brain lesions + tumours + during anaesthesia + sleep
• There is an inverse relationship between delta activity and glucose metabolism in pathological and normal people  subgenual prefrontal cortex (low sugar consumptions means high delta activity)
• Delta is the predominant activity in infants during the first 2 years of life
  o Then the faster alpha and beta bands increase linearly across the life span
   Inhibitory rhythm

Question 20

Theta band (4–8 Hz)

Answer 20

• Prominently seen during sleep
• Wakefulness: 2 different types of theta activity in adults
  1. Widespread scalp distribution and has been linked to decreased alertness (drowsiness) and impaired information processing
  2. Frontal midline theta activity: frontal midline distribution and has been associated with focused attention, mental effort, and effective stimulus processing
  o Anterior cingulate cortex (ACC) = generator of frontal midline theta activity  positive correlations between theta current density and glucose metabolism
• Generation of theta oscilliations:
  o Septo-hippocampal system
  o Cingulate cortex
   Gating function on the information processing flow in limbic regions

Question 21

Alpha band (8–13 Hz)

Answer 21

• Healthy adults: amplitude between 10 and 45μV
• Relaxed wakefulness
• Greatest amplitude over posterior regions, particularly posterior occipito-temporal and parietal regions  best seen during resting periods, eyes closed
• Diminished rhythm by eye opening, sudden alerting, and mental concentration = alpha blockage / desynchronization
• Can be attenuated when alertness decreases to the level of drowsiness  accompanied by a decrease in frequency
• Role: unknown, maybe visual system functions emerging in the absence of visual input?
• Evidence suggests that different alpha sub-bands may be functionally dissociated
  o Cognitive tasks, lower alpha desynchronization (suppression) has been associated with stimulus and task-unspecific increases in attentional demands
  o Upper alpha desynchronization appears to be task-specific + linked to processing of sensory-semantic info, increased semantic memory performance, and stimulus-specific expectancy

Question 22

Beta band (13–30 Hz)

Answer 22

• Adults: amplitudes between 10–20 μV  symmetrical fronto-central distribution
  o Replaces alpha rhythm during cognitive activity
• Increases with attention + vigilance
   Increases reflect increased excitatory activity, particularly during diffuse arousal and focused attention

Question 23

Gamma band

(36–44 Hz

Answer 23

• Attention, arousal, object recognition, top-down modulation of sensory processes, and, in some cases, perceptual binding (when we integrate things into a coherent whole)
• Associated with brain activation
  a) Increased during perception and learning
  b) Dose-dependent decreases of gamma activity have been described during anesthesia
  c) Systematic decreases in gamma activity have been described throughout the sleep-wake cycle
  o Highest during wakefulness, Intermediate during REM sleep, Lowest during slow wave sleep
  d) Gamma had the highest number of positive correlations between current density and glucose metabolism
• Role: reflect large-scale integration of and synchrony among widely distributed neurons, particularly in states of diffusely increased vigilance
• Physiological mechanisms have been implicated in the generation of gamma:
  1. Intracortical circuitries, in particular those involving distant brain regions
  2. Synaptic interactions among the cortex, thalamus, and limbic structures
  3. Brainstem-thalamic cholinergic activation
• Gamma and theta oscillations can be functionally coupled both during activated (task-related) and resting (task-free) states
   Gamma bursts occur within periods of the theta phase

Question 24

benefits of saline solution

Answer 24

a) The electrode application time
b) Subject’s discomfort
c) Risk of infections

Question 25

Interpolation methods:

Answer 25

1. Linear (nearest neighbour): corrupted activity is reconstructed through a weighted average using data from neighbouring electrodes
   o Pros: mathematically simpler
   o Cons: a) edge electrodes cannot be accurately estimated;
   (b) only a few electrodes are used;
   (c) maxima and minima of activity are always located at electrode sites
2. Spline: info from all sensors is used to represent the overall potential distribution on the entire scalp, and thus to reconstruct the activity at missing channels
   o Pros: greater accuracy

Question 26

Electrode locations and high-density recordings

Answer 26

• Electrode distance of 2–3 cm is required to prevent distortions of the scalp potential distribution = allow resolution of spatially focal EEG patternsMin 60 (preferably more) equally distributed electrodes are required for accurate spatial sampling of scalp activities

Question 27

Reference-free transformations

Answer 27

a) Average reference approach: at each moment in time, EEG signals are re-derived against the average value across all electrodes
b) Hjorth method / source derivation / Laplacian transformation: the average potential difference between each electrode and the nearest 4 electrodes is computed; By computing the density of local radial currents, this approach acts as a spatial high pass filter and emphasizes shallow cortical generators

c) Current source density approach / surface Laplacian: computed as the second derivative of the voltage surface; by acting also as a spatial filter, they can be helpful in identifying focal patterns
o Cons of these methods:
 Require a relatively high number of electrodes (ex: 32 electrodes or more)
 Need a homogenous electrode distribution across the scalp for reliable estimates

• Although the choice of the reference electrode greatly influences waveform analyses, the reference choice is completely irrelevant for any source localization
  o As source localization relies on the spatial distribution of the scalp EEG and ERP, different reference montages (e.g., average reference, linked mastoids) lead to identical estimates of intracerebral sources

Question 28

Nyquist Theorem

Answer 28

the sampling rate should be at least twice the highest frequency present in the signal under investigation

Question 29

aliasing

Answer 29

the introduction of spurious low-frequency components into the signal 
This occurs when a signal is sampled at a rate that is too low, and introduces irreparable distortion to the digital waveform

• Avoid aliasing by either:
  1. Removing frequencies higher than half the sampling rate
  2. Use high sampling rates

Question 30

• There are 2 classes of artifacts

Answer 30

a) Biological: movements, muscle activities, blinks, eye movements, heartbeat, and sweating
 We can record the source of these and then correct for them
b) Non-biological: interferences from power lines (50/60 Hz  fluorescent lights), additional electrical noise, poor subject grounding, and poor electrode contact
 We can take precautions for these (shield the room, …)
- Diminish the influence of nonbiological artifacts:
o Notch filters (50/60 Hz)
o Proper subject grounding: improve the quality of signals and avoid leakage of current from the EEG system to the subjects
o Shielding of the recording system

Question 31

the Fast Fourier Transform (FFT

Answer 31

o The Fourier coefficients indicate the strength of the signal at a given frequency

• Goal: estimate the contribution of various frequency components on the measured EEG signal
• For a given frequency or discrete EEG band, the square of the amplitude (= Power) is used to quantify its contribution to the measured EEG signalPros: provide important information about the frequency compositions of EEG oscillations
• Cons: can’t provide any information about when in time such frequency shifts occur

Question 32

fft method

Answer 32

• Method:
  o When selecting the length of the artifact-free EEG segments, it is important to understand that this variable determines the maximal frequency resolution available for the analyses
  o Assumption: EEG is a stationary signal
  o Accordingly, segments entered in FFT analyses cannot be too long because of potential violation of the stationarity assumption  need to be short
  o To allow reliable estimation and to reduce the impact of second-to-second variability in EEG signals, at least 60 sec of artifact-free data should be used (more than 3.5 sec)
  o To avoid leakage of spurious frequencies arising from abrupt changes in EEG signals at the beginning and the end of the EEG segments, a taper transformation should be used
   use a Hanning (cosine) window: tapers the beginning and end of the EEG segment to zero, whereas the middle of the segment retains 100% of its amplitude

Question 33

o Research suggests frontal EEG asymmetry is associated with

Answer 33

a) State-dependent emotional reactivity
b) Individual differences in emotional reactivity
c) Individual differences in risk for a variety of emotion-related disorders
o Left prefrontal regions: appetitive behaviour and certain forms of affect that are approach-related

Question 34

• Advantages of asymmetry metrics

Answer 34

1. Control for individual differences in skull thickness (causes differences in scalp power values)
2. Increase statistical power by reducing individual differences in overall activity, and limit the number of statistical tests performed
3. Internal reliability of the alpha power frontal asymmetry index is high across studies and the test-retest reliability is acceptable

Question 35

time frequency analysis

Answer 35

A. Short-time Fourier Transform (STFT): for computation of an FFT-based time-dependent spectrum (so-called spectrogram)
- EEG signals are viewed as a composition of sine waves with varying frequencies

B. Wavelet analyses: allow a more adaptive time-frequency approach affording flexible resolution

• Ability to accurately resolve EEG waveforms into specific time and frequency components
• EEG signals are viewed as shifted and scaled versions of a particular mathematical function

Question 36

Most ERP experiments in social cognition and affective science (SCAN) are

Answer 36

built upon a basic behavioural task design, which typically involves the carefully timed presentation of stimuli and the recording of behavioural responses (e.g. on a computer).

Question 37

The ERP signal must be extracted from background EEG (noise)

Answer 37

), which is done by filtering the EEG signal, aligning to events of interest and averaging:

Question 38

Mechanisms of affective priming

Answer 38

Previous findings have shown that an affective target word is categorized in terms of its valence (positive or negative) more quickly when preceded by prime words of the same valence (i.e. congruent trials) than by prime words of the opposite valence (i.e. incongruent trials). This affective congruency effect is conceptualized as incongruent trials eliciting response conflicts when the response associated with the prime conflicts with the response called for by the target.
Therefore, the amplitude of the lateralized readiness potential (LRP), a dynamic measure of motor cortex activation associated with preparation and initiation of behavioural responses, shows evidence of response activation following the onset of the prime even before the target appears, as shown by ERP research (participants activated the response suggested by the prime – they prepared to press the “positive” key). Incongruent prime trials on the other hand should reflect response conflict as participants attempt to overcome their initial prime-driven tendency. This response conflict was demonstrated through an amplified amplitude of the N2 component (indicative of conflict monitoring in the ACC).
 Affective congruency effect does not happen because of the semantic network between the words but rather because of the conflict that occurs between incongruent words.
 Affective congruency effects, at least within this classic paradigm, result from conflict during the response output stage rather than from ‘spreading of activation’ effects within a semantic network.

Question 39

This proves again that stereotype-based categorizations occurs very rapidly.

Answer 39

Findings show that P3/LPP amplitude was enhanced to sentences containing definitional as well as stereotypical incongruities. Similar effects on P3 amplitude during processing of racial stereotype–incongruent trait words were demonstrated. Another component, the N400 (or N4) was found to be sensitive to violations of semantic processes and therefore also semantic social knowledge (i.e. social stereotypes).

Question 40

meg&Erp

Answer 40

MEG has the same temporal solution as ERPs, but it can be used more reliably to localize the source of the signal because magnetic fields are not distorted as the pass through the brain, skull and scalp (great spatial resolution)

Question 41

MEG

Answer 41

MAGNETOENCEPHALOGRAPHY (MEG): The electrical current associated with synaptic activity produces small magnetic fields that are perpendicular to the current
Drawbacks:
1. able to detect current flow only if that flow is oriented parallel to the surface of the skull (for this reason the neurons that can be recorded with MEG tend to be located within sulci, where the long axis of each apical dendrite tends to be oriented parallel to the skull surface
2. the magnetic fields of the brain are extremely weak, so MEG requires a room that is magnetically shielded from all external magnetic fields including the Earth’s magnetic field

Question 42

Stereotyping

Answer 42

To investigate the rapid activation of stereotypes, ERP components that respond to the semantic relationship between words in a sentence have been examined

xample 1: pp read sentences that violated definitional or gender-stereotypical (our aerobics instructor gave ‘himself’ a break)sentences  P3/LPP amplitude was enhanced to sentences containing incongrueities independent of pp overt judgements
Example 2: N400 (sensitive to semantic processes) is sensitive to words that violate the semantic contex (pizza is too hot to cry/eat)
- larger N400 when subjects read sentences that violated rather than confirmed stereotypes
Stereotyping occurs very rapidly

Question 43

Self-regulation

Answer 43

The process of coordinating goal-directed responses: bottom-up (corrective) vs. top-down (proactive) aspects of control and ERP research has been instrumental in delineating the specific mechanisms involved
Self-regulation of responses to stereotyped targets involves the coordination of two complementary processes:
1. an initial conflict monitoring mechanism, subserved by activity in ACC (monitors ongoing response for conflict)
2. regulative mechanism: associated with activity in LPFC (responds to conflict by strengthening the influence of intentional responses to override an unwanted tendency)
Example 1: response locked ERN component is always large for incorrect responses. PP completed a stereotype- inhibition task:
ACC is sensitive to unwanted stereotype-driven response tendencies, suggesting a role for conflict monitoring in the regulation of racial bias
- Monitoring of these responses occurred implicitly and operated independently of the regulative control process
Example 2: alcohol manipulated control of pp  intoxicated subjects exhibited regulative control but no conflict monitoring
- Amplitude of FSW (frontal negative slow wave) on stereotype-inhibition n trials correlated with pp ability to withhold stereotypic response

Question 44

meg Compared to EEG:

Answer 44

1. although the same currents are responsible for the signals in EEG and MEG, magnetic field measured outside the head with MEG is less distorted than the electrical potentials on the scalp measured with EEG
2. MEG only measures currents oriented tangentially to the surface of the brain
   - However, some currents (radially oriented or deep in the brain) are better picked up by EEG
   - It would be good to combine EEG and MEG measurement, but a straightforward way to do so is not yet available

Question 45

EEG Compared to PET and fMRI:

Answer 45

Better temporal, worse spatial resolution

- fMRI and PET can solve the inverse problem

Question 46

The spatial resolution of EEG/MEG is constrained by several factors

Answer 46

distorting effects of the head volume conductor (from source to electrode), low signal-to-noise ratios and limited spatial sampling due to practical limits on the number of electrodes that can be utilized, and the inverse problem is ill-defined

Question 47

Development of localization techniques, especially when used together with high-density EEG recordings, realistic head models and other functional neuroimaging techniques has shown that

Answer 47

spatial resolution of EEG may be higher than previously thought, opening new opportunities for investigating spatiotemporal dynamics of brain mechanisms underlying mental processes
EEG GENERATION: THE ROLE OF THALAMO-CORTICAL

Question 48

: Interaction between thalamic and cortical networks are assumed to play a key role in various rhythmical EEG activities

Answer 48

In animals: neurophysiological evidence has shown that several thalamic, thalamocortical and cortical neurons display intrinsic oscillatory patterns, which in turn generate rhythmic EEG oscillations- Thalamus has been described as a key player in the generation of alpha and beta oscillations
- thalamic oscillations in the 7.5-12.5 frequency range have been shown to activate the firing of cortical neurons. The associated depolarization that occurs in the cortical layer IV, in turn, creates dipolar source with negativity in layer IV and positivity in superficial layers.
- Cortical alpha power was found to be inversely correlated to glucose metabolism in the thalamus (consistent with the assumption that the thalamic activity in response to sensory or cortical input may lead to alpha suppression)
- Corticocortical and thalamocortical interactions during information processing have also been postulated in the generation of oscillations at higher frequencies (beta band 13-30 Hz)
- Thalamus is also implicated in the generation of delta waves (1-4Hz) which possibly arise through interactions between deep cortical layers and the thalamus that are normally inhibited by the ascending reticular activating system
- Septohippocampal system and limbic regions are involved in the production of theta oscillations
EEG oscillations depend on interactions between the cortex and the thalamus, which both produce intrinsically rhythmical activities
- Thalamus is implicated in the pacing of such rhythmical activities
- Cortex provides the output in response to thalamic input and generates the majority of oscillations that can be recorded

Question 49

EEG GENERATION: THE ROLE OF LOCAL SCALE AND LARGE SCALE SYNCHRONIZATIOn

Answer 49

Many neurons must be activated synchronously in order to detect an EEG signal at the scalp
Synchronization of oscillation is a key mechanism for neuronal communication between spatially distributed brain networks. Oscillatory processes might
1. Bias input selection
2. Temporally bin neurons into assemblies
3. Foster synaptic plasticity

Higher frequency oscillations (e.g., gamma) appear to originate from smaller neuronal assemblies, while low frequency oscillations (e.g., theta) span larger neuronal populations

Question 50

NORMATIVE EEG ACTIVITY

Answer 50

: the millisecond temporal resolution of EEG allows scientists to investigate not only fluctuations of EEG activity (increases/decreases) but also to differentiate between functional inhibitory and excitatory activities
- Low frequencies (delta and theta): show large, synchronized amplitudes
- High frequencies (beta and gamma) show small amplitudes due to a high degree of desynchronization
In adults, the amplitude of normative EEG oscillations lies between 10 and 100 millivolt (more commonly between 10 and 50)

Question 51

electrodes

Answer 51

Irrespective of their material, EEG electrodes should not attenuate signals between 0.5 and 70 Hz. To allow comparison among studies, it is important to adhere to standardized electrode locations (10-20 system by Jasper; 10-10 system; 5-5 system)
In recent years, high resolution EEG systems with numbers of electrodes ranging from 64 up to as many 256 have been introduced with the goal of increasing the spatial sampling of the EEG
- studies have suggested that an electrode distance of 2-3 cm is required to prevent distortions of the scalp potential distribution and thus allow resolution of spatially focal EEG patterns
- studies assessing the role of spatial density on source reconstruction have shown an improved spatial resolution with high density recordings
o improved source localization accuracy increased linearly from 25-100 electrodes, but reached a plateau after 100 electrodes
Uniform and homogenous coverage of the scalp is of paramount importance for reliable measurements of the scalp potential field.
Both simulation and experimental studies suggest that at least 60 (preferably more) equally distributed electrodes are required for accurate spatial sampling of scalp activities

Question 52

ELECTRODE INTERPOLATION:

Answer 52

ELECTRODE INTERPOLATION: The importance of spatial sampling for source localization is associated with the issue of how to best deal with corrupted EEG signals due to noise
Source localization depends on scalp potential distribution, which can be distorted by uneven spatial sampling  omitting corrupted electrodes is not feasible, and interpolation method are needed
1. linear interpolation method: corrupted activity is reconstructed through weighted average using the data from neighboring electrodes
disadvantage: edge electrodes cannot be accurately estimated; only a few electrodes are used, maxima and minima of activity are always located at electrode site
2. Spline interpolation method: information from all sensors is used to represent the overall potential distribution on the entire scalp and thus to reconstruct the activity at missing channels
Recording reference choice: EEG waveforms represent the differential voltage between a given electrode an the reference electrode
- Reference electrode should be electrically inactive and thus allow the measurement of an absolute EEG value  BUT this is never the case

Question 53

solution

Answer 53

Solution: Reference-free transformations have been proposed
a) Average reference – at the moment in time, EEG signals are re-derived against the average value across all electrodes
b) Radial current flow/ Source derivation/Laplacian transformation – average potential difference between each electrode and the nearest four electrodes is computed. By computing the density of local radial current, this approach acts as a spatial high pass filter and emphasizes shallow cortical generator
c) Current source density/surface Laplacian – computed as the second derivative of the voltage surface; by acting as a spatial filter, it can be helpful in identifying focal patterns
All of these methods often require a high number of electrodes, as well as homogenous electrode distribution across the scalp for reliable estimates
The choice of the reference electrode influences waveform analysis, the reference choice is completely irrelevant for any source localization

Question 54

Nyquist theorem

Answer 54

the sampling rate should be at least twice as high as the frequency under investigation. This prevents aliasing, the introduction of spurious low frequency components into the signal
- Occurs when the signal is sampled at a low rate
Solutions:
1. Frequencies higher than half the sampling rate should be removed from the EEG before digitization occurs
2. High sampling rates could be used