Modulo Cona Flashcards
What does the EEG signal reflect?
EEG reflects a summation of post synaptic potentials (PSPs), which are electrical potentials generated in the extracellular fluid of post-synaptic neurons, that have a duration of tens-hundreds ms and are confined to the dendrites and cell bodies of cortical pyramidal cells that are:
• co-activated (synchronous activity)
• co-aligned (similar cortical orientation, parallel to each others )
• aligned perpendicular to the surface of the cortex
Excitation of the postsynaptic neurons creates an extracellular voltage near the neural dendrites that is more negative than elsewhere along the neuron. In this way the neuron become a dipole which came be summed to other dipoles (with some requisites) and give rise to an electrical signal that can be recorded
What’s a dipole?
A region of positive charge separated from a region of negative charge by some distance.
Is it a single neuron’s dipole enough to be measured?
No, a single neuron’s dipole is too small to be measured as far away as the scalp. The sum of many individual dipoles in an area is measurable as a single dipole. Electrodes will measure the sum of both the positive and the negative ends of dipoles in the brain.
Is it always possible to detect neurons’ dipoles?
No, in order to produce a measurable (nonzero) signal, a large number of neurons must be both:
• arranged in a parallel fashion (otherwise they could cancel each other out, or in case of random arrangement, no clear dipole emerges) and perpendicular to the cortex
• synchronously active
Only in this way neurons can create an electrical signals large enough to propagate through the brain, dura, skull, and skin
Which type of connections are thought to drive the synchronous activity that gives rise to EEG?
A combination of thalamocortical and corticocortical
What’s volume conduction?
Volume conduction is the process by which a pool of ions repels nearby ions of the same charge, resulting in a “wave” of charge that travels through the extracellular space
Volume conduction is responsible for the propagation of the EEG signal within the brain.
Once the volume-conducted signal reaches the edge of the volume it is traveling through, volume conduction can no longer occur.
Which are the steps for physiological recording?
• Electrode or transducer placement
• Amplification and filtering
• Analog (polygraph) or digital (computer) recording
What does the electrode measure?
An electrode’s output is a single voltage per measurement typically taken every 1–2 ms.
A measurement of voltage at any electrode on the scalp consists of the sum of influences from many sources of electric fields because the brain is full of dipoles
What’s the impedance?
Impedance is a measure of the opposition that
an electronic component, circuit, or system
offers to the flow of an alternating current at a
particular frequency. It is typically measured in
ohms (Ω)
Which are the characteristics of the electrode?
- Skin surface electrodes
- Good electrical conductors (metal discs)
- Silver-silver chloride (Ag/AgCl) electrodes are
preferable (smaller bias potential and minimal polarization)
How to do the electrode application?
First you have to do skin preparation by cleaning the skin with alcohol or water and rub it a little bit (abrasion) to reduce the impedance to < 5-10 kΩ. Then an electrolyte paste or gel
has to be applied. Since the gel is a conductor, the signal reaches the electrode with less attenuation than if the gel were not present.
Why is Electrode Gel Necessary?
While cerebral spinal fluid and various ion-filled
substances in the brain are very good conductors, they are separated from the electrode by several layers of poor
conductors, including the skull, dead skin cells, hair, and air in the interstices of hair.
The highly conductive electrode gel will saturate the space beneath an electrode, filling in the air pockets between hairs and thus providing a conductive path from scalp to electrode
What’s the International 10/20 System?
A standard system for electrode placement, electrodes are placed at sites 10% and 20% of the total front-back or left-right distance of the skull, based on four anatomical landmarks:
- nasion and inion
- periauricular points
Each electrode’s position is identified with a letter and a number, the letters refer to areas of the brain, while numbers indicate laterality and degree of displacement from the midline (Z).
Which types of montage there are?
Bipolar montage: both electrodes are placed on active sites, used in EKG, EMG. The potential difference between two active sites located in the area of interest is recorded. This configuration is mostly used to record limited changes in potential (e.g. EMG) or in medical tests (e.g. for epilepsy).
Monopolar montage: one electrode is placed on an “inactive” site used as a (common) reference site and one electrode is placed on an active site, used for EEG. The potential difference is recorded between the electrode placed on the active site (of interest) and the electrode placed on the reference site (reference)
Which types of electrodes?
Active electrode that is placed at scalp site where voltage is changing
Reference electrode that should be placed at neutral site.
What are the most neutral sites in our body?
Linked mastoids, earlobes, Cz, Average reference, even if there is no neutral site in our body
How to chosen the reference?
Choice of the reference depends on: the number of electrodes, ERPs
of interests.
Typically:
- 1 electrode on top of the head (Cz)
- Average between electrodes on the two ears
- Average of all connected electrodes (average reference). If many
electrodes
- Average of two mastoid references
- Single mastoid (left or right) reference
-Linked
- Nose reference
New method: REST– reference electrode standardization technique. It’s a
reference that tends to infinite (Yao, 2001)
How does the amplification works?
The amplifier increases the amplitude of the signal to a level (at least 1 V) that can be accepted by the recording system
What’s the amplification factor or gain?
Is the amount by which the very small electrical signals generated by the brain are increased in strength so they can be accurately recorded and analyzed. Is obtained by Vout/Vin
Why is the Amplifier Necessary?
The purposes of an amplifier in an EEG system are to:
1) to maximize the Signal to Noise Ratio of the measured voltage,
2) increase the size of a signal above the size of noise that may be introduced in later elements of the circuit
How does the recording works?
After the analog recording, the signal is converted into a numerical form through
the analog-to-digital (A/D) converter and stored in a computer for further processing
What’s the sampling rate?
The speed at which the converter samples the signal, which should be at least 2-5 times faster than the fastest frequency component in the signal (Nyquist rate)
What is signal?
Signal refers to the portion of measured voltage that reflects the brain
What is noise?
Noise refers to the portion of the voltage that reflects other sources
What is the signal-to-noise ratio (SNR)?
Signal-to-noise ratio (SNR) is a measure of how much signal the system measures compared to how much noise, so a higher SNR reflects a better-quality signal that is contaminated by less noise. S/N ratio is a function of square root of number of trials, so it is important to make sure to have a good number of trials to enhance the signal to noise ratio
What are the EEG bands?
Spontaneous EEG is characterized by neural oscillations at certain frequencies, which are fluctuations in the excitability of populations of neurons. EEG can be divided into frequency bands:
• Gamma
• Beta
• Alpha
• Theta
• Delta
What’s Gamma activity?
• Frequency: 30-80 Hz, centered around 40 Hz
• Amplitude: 2-20 uV
• Integration of information, memory, perceptual binding
• In some cerebral pathologies (e.g. schizophrenia)
• Levels of high cognitive functioning
• Associated with local processing and error signaling. Represents fast error correction and synchronization in cortical networks
MUM ‘match-and-utilization- model’ (Hermann et al., 2004):
- ‘early’ gamma-band response GBR) = reflects matching of bottom-up signals with memory contents (alligning the problem with existing knowledge and experiences)
- ‘late’ GBR = reflects utilization of info for coordinating behavioural performance, for redirecting attention or for storage in memory (effectively applying cognitive resources to solve the problem)
What’s the Beta activity?
• Frequency: 13-30 Hz
• Amplitude: 2-20 uV
• Active waking consciousness, alertness
• Active thoughts, problem solving
• Motor control
• Panic attack
• External attention
• Maintenance of the current sensorimotor or cognitive state
•Whole cortex, basal ganglia
What’s the Alpha activity?
• Frequency: 8-12 Hz
• Amplitude: 50-100 uV
• Relaxed wake state, lack of active cognitive
processing, consciousness
•Thalamus, hippocampus, sensory and motor cortex
What’s the Theta activity?
• Frequency: 4-7 Hz
• Amplitude: 5-100 uV
• Two types of theta
• low level of alertness (drowsiness, sleep)
• attention and active processing (meditation, mental effort, focused attention, retrieval from long term memory)
•synaptic plasticity
•Hippocampus, sensory cortex and prefrontal cortex
What’s the Delta activity?
• Frequency: 0.5-3.5 Hz
• Amplitude: 20-200 uV
• Deep non-REM slow wave sleep (deep sleep), unconscious states, sensory gating, memory
Encephalopaties
• Thalamus, cortex
What’s the Quantitative EEG analysis?
The spectral contents of raw EEG can be
quantified by decomposing the signal into its
constituent frequency components (power spectral analysis)
What’s the external noise?
Compared to the brain signal, the amplitudes of noise sources are gigantic.
There are 2 ways to solve the problem of external noise through two general means, reducing the power line noise:
- passive shielding (Faraday cage)
- active electrodes which incorporate an amplifier. This allows the electrode to amplify and filter the signal at the source.
What’s the Internal noise?
Participants produce a number of internal sources of noise
It is particularly insidious because they can’t realistically be excluded while collecting data
Body noise: ∼1 Hz oscillation of the electrocardiogram, the periodic rib cage expansions and contractions of
breathing, muscle tension, blinking
What are the recording artifacts?
Unwanted interference signals that contaminate the signal of interest
Which ocular artifacts there are?
Blinks and Eye movements (vertical and horizontal)
For Horizontal ocular movements the electrodes should be placed in the external side of the eye
For Vertical ocular movements the electrodes should be places below the eye, in line with the pupil
Which are the main recording artifacts?
Electrical artifacts: electrical environment and electronic equipment (AC power lines, 50-Hz noise, 60 Hz US).
Motion and Glossokinetic artifacts: mechanical movements of subjects, electrodes, cables
Muscular activity
Biological artifacts: Heart signals (ECG), Skin potentials
Ocular artifacts: blinks and eye movements
What’s an EEG?
Electroencephalogram (EEG) is the spontaneous and continuous electrical activity of the
millions/billions of neurons in the brain, recorded at the scalp
What’s an ERP?
Event-related potentials (ERPs) are the summation of synchronized activity of thousands of
neurons recorded as small voltage fluctuations in the EEG that are time-locked to external or
internal event
What do ERPs reflect?
ERPs reflect synchronized neural activity that can be observed:
• in response to the presentation of external stimuli
• in association with the development of a specific cognitive process
• in motor preparation and execution
What’s the magnitude of ERPs?
ERPs are smaller than the background EEG (1-10 uV vs. 20-50 uV)
What’s a key method to reduce noise?
Since all sources of voltage are random with respect to time-locked event except the ERP, and the timing of the ERP activity is the same on each trial, you can average the signals detected in each trial, in this way, assuming that the noise in random, the signal should emerge
What are the ERP components?
A series of positive (P) and negative (N) voltage deflections across time classified on the basis of
their polarity and typical latency (e.g., N100, P300), and their scalp topography. These deflections vary also in amplitude and spatial distribution.
However, a component’s latency may vary considerably across experiments, conditions, or
even electrodes.
What’s the amplitude of an ERP component?
voltage difference between baseline and peak or peak-to-peak difference
What’s the latency of an ERP component?
time interval between stimulus onset and peak
What does define a component?
A component is defined by
- Neural source (that leads to a spatial distribution)
- Functional meaning
However an ERP component is not the result of a unique generator, or
reflect a unique process, or defined as a peak
Does the peak define a specific component?
No, a peak does not necessarily correspond to a single underlying “latent” component. An observed waveform could be given by different possible sets of source components
Why is the mean/area amplitude useful?
It is useful for broad components with no real peaks and it is unbiased by noise leve
Are there some techniques that can recover the underlying latent components from the observed
data?
• Source localization approaches
• Principal Component Analysis (PCA)
• Independent Component Analysis (ICA)
What is the latency jittering?
It’s the variability of the latency of the peak of a component. In very different populations, a difference in amplitude depends on a difference in the latency, so we have to be careful in interpreting amplitude. Latency jittering could be an issue in terms of averaging, since when we average we do it under the assumption that the timing of ERPs is always the same across trials.
What does polarity depend on?
Polarity does not usually tell us anything per se, except it can help us determine which component
we are seeing. Polarity depends on a combination of several factors
¤ excitatory vs. inhibitory PSPs
¤ PSPs on the apical dendrites vs. on the basal dendrites/cell body
¤ location and orientation of the dipole(s)
¤ location of the reference electrode
Which type of relationship between a component and a cognitive process?
Correlational. We can say that a component is associated or reflect a given cognitive process, but we cannot say that a process causes an ERP, while with TMS-EEG Co-registration we could do causality inference
Which types of component there are?
• Exogenous components (early): sensory components obligatorily triggered by the
presence of a stimulus
• Mesogenous components (intermediate): still influenced by physical parameters but modulated
by top-down processes
• Endogenous components (late): entirely cognitive processes
• Motor components: accompanying the preparation and the execution of a
motor response
What are the exogenous components?
Exogenous components are modulated by stimulus physical parameters, very early components (< ~100 ms) reflecting exogenous sensory activity coming mainly from the primary sensory brain
areas and pathways. They are of interest in studies of psychiatric and neurological patients, to
determine whether the disorder impacts sensory input to subsequent cognitive processing.
What are the mesogenous components?
Early components (~ 100-250 ms) still strongly influenced by stimulus physical parameters, they are modality-dependent. However, they are modulated by higher-level, top-down processes, such as selective attention, vigilance, state of arousal.
Visual P1/N1, N170, N2, Mismatch Negativity
What are the mesogenous components?
Early components (~ 100-250 ms) still strongly influenced by stimulus physical parameters, they are modality-dependent. However, they are modulated by higher-level, top-down processes, such as selective attention, vigilance, state of arousal
What are the endogenous components?
The late components (> ~ 250 ms) entirely reflect task-dependent cognitive/psychological processes. They are not directly influenced by the physical properties of the eliciting stimulus. “Invoked” by the psychological demands rather than evoked by the external stimulus.
P300, N400, P600, slow cortical potentials, readiness potential, contingent negative variation
When does an amplitude increase of P1/N1 reflect?
Enhanced P1/N1 amplitudes at lateral-occipital electrodes reflect spatial focusing of attention across a variety of tasks, and are associated with speeded reaction times and improved detectability of target signals
Which type of stimuli elicit larger P1/N1?
stimuli at attended locations elicit larger P1 (80-130 ms) /N1 (120-200 ms) components than at unattended locations
What is the difference between P1 and N1?
P1 is more related to processing N1 facilitation and N1 to attentional orienting
What does the N170 reflects?
N170 reflects perceptual processing of faces/activation of face representation
What does the N170 reflects?
N170 reflects perceptual processing of faces/activation of face representation
What’s the distinction between N1 and N170?
Some ERP researchers refer to an N1 component to objects vs. an N170 to faces, whereas others refer to an N1 which is particularly enhanced for faces
Which are the components of N2 family?
- N2c, greater for unfrequent stimuli. Related to visual stimuli
- MMN, uditory. Perception of discrepancy in the environment
- N2pc, posterior contralateral. Allocation of spatial attention
What does the MMN reflect?
Preattentive, automatic response (160-220 ms) to a change in the sensory (auditory) memory trace. A mismatch with a perceptual template. MMN arises from violation of an automatically
formed, short-term neural model or memory trace of physical (or more abstract) regularities. Often isolated with a deviant-standard difference wave
What does the P300 reflect?
It reflects stimulus categorization, allocation of attentional resources and working memory updating.
It is sensitive to target probability, both overall and local probability, probability of a task-defined stimulus category, not probability of a physical stimulus
It is not sensitive to the time required for response selection end execution after categorization
Additive effects of 3 independent factors
- amount of information transmitted
- subjective probability
- stimulus meaning
How a stimulus can be considered as meaningful?
Stimuli can be made meaningful by linking them to a task (e.g., counting) or can be meaningful in and of themselves (e.g., salient, emotional stimuli)
Which are the characteristics of P3a?
frontally maximal, earlier (250-400 ms), elicited by unexpected, novel stimuli
What are the characteristics of P3b?
parietally maximal, later (300-600 ms), elicited by “meaningful” and/or attended stimuli
What do P3a and P3b have in common?
Both are elicited by infrequent stimuli, but the P3b is present only when stimuli are task-relevan
What does the N400 reflect?
Typically seen in response to violations of semantic expectancies
An index of semantic processing, associated with the semantic integration of words in a sentence
context
It reflects the associative strength in semantic memory
What does the P600 reflect?
Language processes and Syntactic violation
What’s a slow cortical potential?
Negative or positive event-related cortical activity that lasts from 300 ms to several seconds
Linked to functional regulation of the threshold of cortical excitability, leading to excitatory (increased neuronal activity) or inhibitory (decreased neuronal activity) mobilization
Slow surface negativity might reflect cortical facilitation (e.g. readiness to act) and slow surface positivity might reflect cortical dysfacilitation (e.g, interruption of cortical excitability
for memory storage and updating of information)
What’s the readiness potential?
Slow cortical negativity developing about 1500 ms before the execution of a voluntary movement
What happens in the early phase of readiness potential?
In the early phase (BP1, up to -500 ms), it is symmetrical, more prominent at Cz, and reflects
increase in cortical excitability of brain areas involved in motor preparation and programming
What happens in the late phase of readiness potential?
In the late phase (BP2, up to -50 ms), it becomes lateralized and reflects the implementation of
specific movement parameters, including the selection of the effector
Why is readiness potential involved in the free will?
Libet’s experiments showed RP starts before we become conscious of deciding to act, suggesting unconscious processes might drive decisions. This challenges free will by implying our brains “decide” before we do. However, critics argue RP may reflect preparation rather than commitment, and that conscious vetoing is still possible.
What’s the Contingent Negative Variation (CNV)?
Slow cortical negativity developing in the foreperiod between a warning (S1) and an imperative (S2) stimulus, to which a behavioral response is required.
Most prominent at Cz and bilaterally symmetrical
It reflects expectancy, anticipatory attention, motor preparation, but also non-motor processes,
including temporal processing
• facilitation of S2 stimulus processing
• reduced RTs to S2 with larger negativities
• It has been recently demonstrated that a CNV can occur even when no specific action or task is require
Pros and Cons of ERPs?
No invasive, undefined/poor spatial resolution, excellent temporal resolution, inexpensive
Which are the steps of a typical ERP study?
- experimental idea
- experiment planning and programming
- data collection
- data pre-processing
- statistical analysis
- writing paper
While are triggers importante while designing the experiment?
- To identify correct, wrong, missed response
- To identify all the possible conditions (then EEGLAB can merge the conditions)
What are the epochs?
Epochs are time segments of EEG data extracted around specific events, such as a stimulus or a response. These epochs help isolate brain activity related to the event by averaging multiple trials.
Types of Epochs:
1. Pre-Stimulus Epoch – Captures brain activity before the event (e.g., baseline activity or anticipatory processes).
2. Post-Stimulus Epoch – Covers brain activity after the event, where ERP components like P300 or N400 are analyzed.
3. Baseline Period – A short pre-stimulus window (e.g., -200 ms to 0 ms) used to normalize the signal.
Epochs reduce noise and improve signal detection, making them crucial for analyzing cognitive and sensory processes in ERP research.
How to decide the number of epochs?
The decision of the number of epochs is based on your ERP of interest
- Large and stable components (eg P3): 30-60 epochs
- Medium amplitude ERP (e.g., N2): 150-200 epochs
- Little amplitude ERP (e.g., P1): high number of epochs (400-800)
How to decide the timing of stimuli?
It’s important that inter-trial interval is variable to avoid that the ERPs are not in phase with other EEG components
What’s the problem with the alpha waves?
The problem with the alpha waves (10 Hz) is that they have frequencies in the same range of the ERP
What’s the problem of the fixation cross?
Even if it reduces ocular movements, the fixation cross could elicit another ERP
Which are the strategies for an experimental design?
- Focus on one (or few) ERP components: Isolate a component in a given exp and keep all the other component unchanged among the different experimental conditions
- Use experimental manipulations already consolidated: easier to interpret them
- If possible, explore large components: less affected by artifacts
- Isolate components creating ‘difference waves’.
- Focus on components that are easy to isolate (eg. LRP, N2pc)
- Avoids confounds linked to physical stimuli.
- When the differences related to the stimulus cannot be avoided, let’s create a control exp
- Be careful to compare ERPs evoked by a different number of epochs/trials
- Be careful when there is a difference between the presence or the timing of a response.
- If possible, vary the experimental conditions trial-by-trial rather than per block
Which are the Non-invasive functional brain imaging techniques?
- Hemodynamic techniques:
- Positron emission tomography (PET)
- Functional magnetic resonance imaging (fMRI)
- Functional Near-Infrared Spectroscopy (fNIRS)
These ones have an Excellent spatial resolution (~1-2mm) and Limited temporal resolution (~1sec)
- Electro-magnetic techniques
-Electro-encephalography (EEG)
-Magneto-encephalography (MEG)
These ones have a Limited spatial resolution (~1cm) and Excellent temporal resolution (1msec)
What’s a difference between EEG and MEG?
MEG has a Spatial resolution better than EEG
