Showing posts with label Investigations. Show all posts
Showing posts with label Investigations. Show all posts

Tuesday, 12 January 2021

Functional Magnetic Resonance Imaging (fMRI)

Functional Magnetic Resonance Imaging (fMRI)

Introduction

Structural imaging reveals the static physical characteristics of the brain. It makes it useful in diagnosing disease. Functional imaging reveals dynamic changes in brain physiology that might correlate with cognitive functioning, for example. Neural activity consumes oxygen from the blood. This triggers an increase in blood flow to that region and a change for deoxyhemoglobin in that region. As the brain is always physiologically active, functional imaging needs to measure relative changes in physiological activity.

The most basic experimental design in functional imaging research is to subtract the activity in each part of the brain whilst doing one task away from the activity in each part of the brain whilst doing a slightly unfamiliar task. We call this cognitive subtraction.

Other methods, including parametric and factorial designs, can minimize many of the problems associated with cognitive subtraction. There is no foolproof way of mapping a point on one brain onto the putatively same point on another brain because of individual differences in structural and functional anatomy.

Current imaging methods cope with this problem by mapping individual data onto a common standard brain (that is, stereotactic normalization) and by diffusing regions of significance (a process we call smoothing).

A region of activity refers to a local increase in metabolism in the experimental task compared to the baseline, but it does not mean that the region is essential for performing the task. Lesion studies might provide evidence concerning the necessity of a region for a task.

Functional imaging can make crude discriminations about what someone is thinking and feeling and might outperform the traditional lie detectors. However, it is highly unlikely that they will ever be able to produce detailed accounts of another person’s thoughts or memories.

An fMRI measures regional cerebral blood flow. Cognitive functions are region-specific, if a task involves a certain cognitive function, the areas involved will become more active, need more oxygen and more blood. fMRI measures regional levels of blood oxygen by detecting magnetic changes in red blood cells when they become deoxygenated

A man looking at fMRI on his computer




Magnetic Resonance Imaging

Magnetic resonance imaging creates images of soft tissue in the body, which x-rays pass through undistorted (so computerized tomography would not capture well). The density/intensity of the images is water-based, with different amounts of water for different tissues. It enables a 3D image of the layout of these tissues. Structural MRI produces a static image of the brain structure. It has a high spatial resolution. It is used to overlap functional images on to.

Metal items and MRI

We remove metal items before the functional imaging because the strong electromagnetic fields will attract them. Patients who use pacemakers cannot have magnetic resonance imaging or its functional variant.

Underlying Mechanisms of fMRI

The fMRI uses a strong magnetic field to line up protons. It measures oxygenated blood by recording the spin of protons, which have a magnetic charge. After aligning protons in fMRI it sends a radio pulse through the lined-up protons, to record how they resonate.

Different proton resonance patterns.

Different protons (different tissues) resonate differently (magnetic susceptibility), allowing the composition of a tissue image. fMRI uses the differential response of oxygenated and deoxygenated blood for the imaging. Oxygenated blood resonates differently from deoxygenated blood, allowing the composition of an (indirect) image of the brain activity.

It is T1-contrast (measures a different magnetic property to functional scans).

The spatial resolution of fMRI

Although fMRI is not as spatially resolute as MRI, it can record 3x3x3 mm and more detail with a 7T (stronger tesla coil strength) scanner.

Both spatial and temporal T2 contrast rely on tesla strength. Temporal T2 contrast measures a different magnetic property to structural scans.

In structural MRI, the magnetic field aligns protons. It aligns protons in water molecules that have weak magnetic fields, initially randomly oriented, but some align with the external field.  A radio pulse knocks orientation by 90 degrees, which leads to a change in the magnetic field. After this change in the magnetic field, the protons become stead and we can repeat the procedure for fresh slices of the brain. A whole-brain image in 2 seconds (3 mm slices) 1: relaxation time. T1-images structural scans.

It relies on the brain to store a large amount of oxygen and glucose. It does not store oxygen though still consumes around 20% of the body’s oxygen supplies. The brain tissue does not store oxygen; oxygen must be supplied from the fresh blood supply. Active tissue consumes more oxygen compared to less active brain tissue. Oxygen-rich blood is lost in areas of higher brain activity.

Magnetic properties of blood

Oxyhemoglobin is diamagnetic while deoxyhemoglobin is paramagnetic. Hemoglobin molecules resonate differently in these different magnetic states.

Diamagnetic substance

A diamagnetic is magnetic when exposed to the external magnetic field for example oxyhemoglobin.

Paramagnetic substance

A paramagnetic substance is normally magnetic for example deoxyhemoglobin.

Blood Oxygenation Level Dependent (BOLD) Signal?

It compares the level of oxygenated with deoxygenated blood derived from the magnetic properties of blood. It is an indirect measure of brain activity.

Factors on while BOLD Signal depends:

1) Cerebral metabolic rate of oxygen (goes up when tissue is active *of genuine interest* more oxygen when spending energy, so de-oxygen goes down)

2) Cerebral blood flow

3) Cerebral blood volume

fMRI compares the differences between magnetic spins of protons in oxygenated blood and deoxygenated

Hemodynamic Response Function.

Initial Dip

Neurons consume oxygen leading to a small rise for deoxyhemoglobin causing reduction of BOLD signal.

Overcompensation

In response to the increased consumption of oxygen, blood flow to the region increases. Increased blood flow is greater than increased consumption >> BOLD signal increased

Undershoot

Blood flow and oxygen consumption dip before returning to original levels. This may reflect a relaxation of the venous system.

Active' areas

Active areas in fMRI refer to a physiological response that is greater relative to some other conditions. To label active areas, we need a baseline response, well-matched to the experimental task. Example: Petersen, Fox, Posner, Mintun, and Raichle (1988) Study brain activity involved in word recognition, phonology, and retrieval of word meaning, cognitive subtraction.

Research designs can exploit this difference by finding two tasks, an experimental task and a baseline task, which differ in terms of a few cognitive components.

Subtraction design

Subtraction is taking a task with the cognitive component in it, and then subtract another task with only that component is taken out

Neuronal structures underlying a single process P

Contrast: [Task with P] [control task without P].

Conjunction

Conjunction requires a set of orthogonal tasks that has a particular component in common. Look for regions of activation that are shared across several subtractions. A test for such activation common to several independent contrasts is called a conjunction. It resembles a factorial design in ANOVA.

Issues with subtraction design

1)    The assumption of pure insertion is the assumption that we can insert a single cognitive process into another set of cognitive processes without affecting the functioning of the rest.

2)    At baseline the brain is always active, and the level of activity is not consistent which makes it challenge where to make comparisons.

Donders coined the term pure insertion as a criticism of reaction time methods. One way to minimize the baseline/pure insertion problem is to isolate the same process by two or more separate comparisons and inspect the resulting simple effects for commonalities. 

Example of this cognitive subtraction in Petersen, Posner 1998

Brain activity involved in word recognition, phonology and retrieval of word meaning cognitive subtraction e.g. contrasts passive viewing of (words vs fixation cross) e.g. (read aloud word vs look at the word) e.g. generate (a word associated with viewed word vs read aloud a written word)

The issue with pure insertion is that adding an extra component does not affect the operation of earlier ones in the sequence.  BUT: interactions are likely to occur– Baseline task: should be as like the experimental task as possible.

Examples of conjunctions and factorial designs by Frith:

1)    Why cannot we tickle ourselves (Blakemore, Rees, and Frith,1998).

2)    Factors touch (felt/not) self-movements (moved/not)

Parametric fMRI design

To get around baseline continuous manipulation of the factor of interest. We treat the variable of interest as a continuous dimension rather than a categorical distinction. Associations between brain activity rather than differences between two or more conditions. passive listening to spoken words at six different rates. Different brain regions show different response profiles to different rates of word presentation. Adapted from Price et al. (1992), and Friston (1997). no baseline necessary.

Functional specialization

Functional specialization: region responds to a limited range of stimuli/conditions. This distinguishes it from the responsiveness of other neighboring regions (no localization).

Functional integration

How different regions communicate with each other. It models how activity in different regions is interdependent. Effective connectivity or functional connectivity between regions when performing a task. Use techniques like the principal component analysis. 

Example:

A word production vs repeating letters in patients with schizophrenia and controls.

Block design

In a block design, stimuli in one condition are grouped. Strong BOLD contrast: higher signal-to-noise ratio simple design and analysis - practice/fatigue effects cannot be used when participants should not know which condition is coming next.

Event-related design

When stimuli are presented completely randomly, we call it event-related (new as temporal difficulties, etc.) design.  This design works with infrequent and random stimuli. If conditions defined by the participant-sorting what happened in a trial (e.g. correct/incorrect trials; biostable percept (Necker cube); the presence of a hallucination - see right). Different stimuli or conditions are interspersed with each other (e fMRI). Intermingled conditions are subsequently separated for analysis. no practice/fatigue effects can be used when participants should not know which condition is coming next: randomization can be used when trials can only be classified after the experiment- weaker BOLD contrast: lower signal-to-noise ratio more complex design and analysis

Session

a scanning session, all the data collected from a participant. Usually comprises a structural scan and several runs of functional scans.

Run

A continuous period of scanning consists of a specified number of volumes

Volume

A Set of slices taken in succession: a 3D spatial image, with a temporal dimension. Expressed in TR (Repetition Time): how long does it take to acquire a volume.

Epoch

A period when a certain condition is presented. Conditions (epochs) can be grouped (blocked design) or randomly intermixed (event-related design).

Correcting for head movements:

Spatial resolution >> small spatial distortions–Individual differences in brain size and shape stereotactic normalization (adjust the measurement of overall dimensions to the 'standard brain'– Individual head aligned differently in scanner over time due to movements. Regions are harder to detect False-positive results. Physically restraining head (using foam or something) and participant instructions Correction

Smoothing

Spreads some raw activation level of a voxel to neighboring voxels. Smoothing enhances signal-to-noise ratio Compensates for individual differences in anatomy.

Assumption

Smoothing assumes that Cognition does not occur in single voxels. Increases the spatial extent of the active region. more likely to find overlap between participants

Steps of fMRI Analysis

Individual differences “averaging over many participants–Correction for head movement– Stereotactic normalization–Smoothing–Statistical comparison

Stereotactic normalization

Mapping regions on each brain onto a standard brain (brain template is squashed or stretched until it fits). Tailarach and Tournoux (1988).

Brain Atlas (based on one brain), Tailarach coordinates–X left/right–Y-front/back–Z top/bottom.

Alternative: Montreal Neurological Institute (average of 305 brains)—Voxels (volume elements), 3-D coordinates.

Tens of thousands of voxels “capitalization on chance Lower significance level (Bonferroni). Choosing a statistical threshold based on spatial smoothness (random field theory).

Analyze the pre-determined region. Reported, corrected, or uncorrected statistical parameters (ROI?)

We start a stat comparison by dividing up data according to design-then perform stat comparison.

Three points of interpretation:

1)    Inhibition versus excitation

2)    Activation versus deactivation

3)    Necessity versus sufficiency.

Inhibition versus excitation?

Functional imaging signals are assumed to be related to the metabolic activity of neurons, and synapses. However: activity can be excitatory or inhibitory. The BOLD signal is more sensitive to neuronal input into a region than the output from the region. Unclear whether functional imaging can distinguish between two neural functions.

Activation versus deactivation

Activation/deactivation Merely refers to the difference between the two conditions. Does not say anything about the direction of the difference.

Necessity versus sufficiency

Necessity: Are active regions critical to the task? Sufficiency: functional imaging shows us active regions, but these may not be crucial. Use methods in conjunction with other methods. 


Friday, 11 January 2013

Electroencephalography (EEG) Interpretation of Waves, including Sleep Waves, Effects of Medications and Neurological Conditions

Electroencephalography (EEG)

Electroencephalography (EEG) is a non-invasive electrophysiological method to measure the electrical activity of the brain. We can compare it with echocardiography of the heart that measures the electrical activity of the heart.

EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain. Clinically, it refers to the recording of the brain's spontaneous electrical activity over a period, as recorded from multiple electrodes placed on the scalp.

Diagnostic features

Event-related potentials: These investigate potential fluctuations time-locked to an event, such as 'stimulus onset' or 'button press'.

Spectral content: This analyses the type of neural oscillations (popularly called "brain waves") that can be observed in EEG signals in the frequency domain.

 

Uses

Most often we use EEG to diagnose epilepsy. It is also used to diagnose

1.       Sleep disorders

2.      Depth of anaesthesia

3.      Coma

4.     Encephalopathies

5.      Brain death.

 

In the past, they would use it to diagnose tumours, stroke, and other focal brain disorders.

Interpretation of Waves

Delta waves

These are 1-4 Hz waves which we can detect frontally in adults, posteriorly in children, and during slow-wave sleep and in babies. These waves should not be present when awake; presence while awake strongly suggests pathology. 

Theta waves

4-8Hz, generalised. Young children, drowsy and sleeping adults, with certain medications, meditation. We see a small amount in awake adults, an excessive amount when awake may show pathology. Theta waves have a 4-7 Hz frequency. Transient theta components encountered in 15 % of the normal population.

Alpha waves

These are 8-12 Hz waves that we identify posteriorly when the subject is relaxed and when the eyes are closed whilst awake.

Beta waves

12-30Hz, frontally. When busy or concentrating. principally fronto-lateral. Anxiety, alcohol, and drugs (barbiturates, benzodiazepines may enhance these)

Sigma waves

12-14Hz, frontal and central regions 

Sleep spindles 

Bursts of oscillatory activity that occur in stage 2 sleep. Along with K-complexes, they are the defining characteristic of stage 2 sleep.

Gamma waves

30-100 Hz. Meditation

Alpha (8-13 Hz):

These are prominent over the occipital region. Alpha waves are accentuated by eye-closure.  Attention attenuates them. A consistent difference of 1 Hz or more between hemispheres is pathological. We see slowing in early phenytoin toxicity.

Mu waves

These are arch-like, 7-11 Hz waves that we can notice over precentral areas—which are the motor areas. Mu waves relate to motor activity. They are attenuated by contralateral limb movements and increase when a person is at rest.

Lambda Waves

Lambda waves are single sharp waves that we see in the occipital region on the electroencephalograph. Usually, these are associated with visual ‘scanning’ and is related to ocular movements during visual attention. These occurs when eyes are open.

Vertex waves:

Vertex waves are electronegative sharp waves over the vertex that an auditory stimulus evokes.


Electroencephalographic Findings in Neurological Conditions

 

Sporadic CJD

In patients with sporadic CJD, early-on there are nonspecific slowing. Later in the course, they may have periodic biphasic and triphasic synchronous sharp wave complexes superimposed on a slow background rhythm.

 

Delirium

In patients with delirium, there is a diffuse slowing of background activity, decreased alpha, increased theta and delta activity. The diffuse slowing correlates inversely with the severity of the clinical symptoms.

 

Epilepsy

Initial interictal EEG is abnormal in 50-75 %, with repeated recordings, 90-95 % will show abnormalities. 2 % of the normal population have abnormalities considered to be epileptiform.

Absence seizures

Generalised, bilateral, synchronous, 3 Hz spike-and-wave pattern occurs in children and 4 Hz spike-and-wave in juveniles with absence seizures.

Generalised epilepsy Primary generalized tonic-clonic seizures:

Bursts of sharp spikes (25-30 Hz) and wave pattern during the ictal stage is diagnostically significant for generalized epilepsy. There is 10 Hz fast activity during the tonic phase and lower-frequency spike and wave complexes during the clonic phase. Generalized-slowing delta-range characterizes the postictal stage.

Partial epilepsy

In people with partial or focal epilepsy, focal spikes or sharp waves occur during the interictal state. When the patient is seizing, focal rhythmic discharges and periodic complexes occur.  

Myoclonic epilepsy

Generalised polyspike and wave activity. This can help for example, in patients who are on clozapine, where myoclonic seizures often develop before they proceed to generalised seizures.

 

 

Encephalopathy: Diffuse slowing

Delirium Tremens: Compared to delirium due to other causes, patients with delirium tremens have a hyperactive trace, fast activity.

Alzheimer's disease: Reduced alpha and beta, increased delta, and theta

Huntingdon’s Disease: In Huntington’s disease, patients have low-voltage waves and there are no alpha waves (flattening).

Normal ageing: Diffuse slowing, which can be focal or diffuse, if focal most seen in the left temporal region

polymorphic, arrhythmic, unreactive delta, periodic lateralized epileptiform discharges

Herpes simplex encephalitis

CJD (in late stages)

subacute sclerosing panencephalitis

Triphasic waves

liver, renal hypoxia, or metabolic encephalopathies

Frontal intermittent rhythmic delta activity (FIRDA)

metabolic encephalopathy

brain stem dysfunction

 

Alpha coma

Diffuse lesions

Rhythmic slowing, occasionally periodic discharges, Widespread, non-reactive alpha-range activity

Generalised encephalopathy

Burst suppression, High-voltage bursts, followed by periods of extreme suppression. Occurs within bi-hemispheric insult and deep anaesthesia

Personality disorder: increased slow waves (theta) in 31-58 % of psychopaths. Changes more right-sided ‘positive spike’ seen in 40-45 % of aggressive and impulsive psychopaths

Anxiety: Increased beta activity

Hypnosis: Like the normal relaxed, waking state

 

 

Effects of Medications on Electroencephalography Rhythms

Medication

Effect

Typical antipsychotics

Diffuse slowing (a)

Antidepressants

Slowing alpha waves, reduction of beta waves; increase other waves

Atypical antipsychotics

They have variable effects (b)

Lithium

Slowing

Benzodiazepines

Increase beta, decrease alpha (d)

Barbiturates

Increase beta

Stimulants

Increase alpha

Depressants

Decrease alpha

Cannabis

Increase alpha

Chlorpromazine

Increased delta, decreased beta

Phenytoin

Slowing of alpha

Benzodiazepines

Increased fast wave (beta) activity

Alcohol

Increased beta (i.e. Fast wave)

Carbamazepine

Increased fast wave

a) Which can be focal or diffuse, if focal most seen in the left temporal region

b) Clozapine has the most significant effect while quetiapine least has the significant.  Clozapine > Olanzapine > Risperidone > Typical > Quetiapine

i.e., increases the slow-wave activity

Sleep Waves on Electroencephalography

Stage 1

In stage 1 sleep, alpha waves disappear while desynchronised theta and delta activity appear. 

Stage 2

Low-voltages and delta waves with sleep spindles and K complexes are the characteristic waves that occur during this stage. 

Stage 3

High-voltage slow-waves appear during stage 3 of sleep. Less than 50% are delta waves. Sleep spindles and K complexes diminish.

Stage 4

During this stage, which we also know as slow-wave-sleep, delta waves grow over 50%.  Sleep spindles and K complexes absent.

Electrode Placement

We place electrodes according to the International 10-20 System, which entails measurements from:

  1. The nasion
  2. The inion
  3. The right auricular depression
  4. The left auricular depression

 

sphenoidal electrodes (between the mandibular coronoid notch and the zygoma) can obtain readings from the inferior temporal lobe

nasopharyngeal leads (in the superior part of the nasopharynx) can get readings from the inferior and medial temporal lobe

Wave characteristics

Amplitudes range from 5 to 150 μV

Frequencies range from 1 to 40 Hz

Spikes are transient tall peaks that last less than 80 ms

 

Sharp waves rise rapidly, fall more slowly, and last over eighty milliseconds

Frequency ranges

 

Normal Electroencephalographic findings

Infants have slower and higher amplitude rhythms. Initially, this is asynchronous and easily disturbing; mature rhythms develop between 2 and 6 years. Adults usually show either alpha posteriorly or beta anteriorly, but generalised low-amplitude beta may be present. 

These are present in all populations by puberty. When the subject is drowsy, alpha becomes intermittent and theta appears. In old age, alpha frequency slows, and delta activity is lower. 

 

 

What are the benefits of electroencephalography (EEG)


It has excellent time-resolution. Cognitive, perceptual, linguistic, emotional, and motor processes are fast and dynamic. For example, consider theta-band (4-8 Hz), rhythm but quiet for our conscious experience. Or consider gamma (30-80 Hz). A direct indicator of neuronal activity Multidimensional (time, space, frequency, power, phase (temporal), connectivity. Portability (observing the brain in action). Inexpensive + advanced analysis techniques on time series, e.g. single-trial classification methods using Fourier transform.


Limitations of EEG


It is not well-suited for precise functional localization. It is not well-suited for measuring deep brain structures (e.g., putamen, thalamus, nucleus accumbens). Sub-optimal method:  in the brain, where does process X occur or is information Y stored. It is also not very well-suited to study very slowly fluctuating process 'infra-slow' with uncertain and variable time course (but fMRI is)from Joy Interpretation issues:1) suffers from interpreting null results absence of proof is not the proof of absence.> ERP does not reveal all of EEG information (single trial)>  ERP does not capture non-phase locked responses2) ERP limited opportunity for linking results to actual neurophysiological dynamics > ERP less understood than oscillatory (is formed) and synchronous.


What type of neuronal activity does EEG capture?


postsynaptic potentials as opposed to action potential


What does EEG reflect?


EEG reflects the differences in electrical potential over time, created by the current flows originating from neuronal populations


What are chemical synapses?


Chemical synapses cause local changes in postsynaptic membrane potentials, through neurotransmitters. Information transmits with some delay about a millisecond.


What are the electrical synapses?


Electrical synapses or gap junctions. Ions flow directly through large channels into adjacent cells, with no time delay.


What is Postsynaptic Potential (PSP)?


An electrical potential started at a postsynaptic site that can vary in amplitude and spreads passively across the cell membrane, decreasing in strength with time and distance.


How is a postsynaptic potential generated?


When AP reaches the presynaptic axon end, it releases a neurotransmitter into the synaptic cleft. The neurotransmitter binds to the receptor of the postsynaptic neuron by opening or closing an ion channel. This led to a graded change in membrane potential.


What two types of postsynaptic potential are there?


Two types of PSP Excitatory PSP (for excitatory synapse)


Inhibitory PSP (for inhibitory synapse)


what it requires for a postsynaptic neuron to fire?


A postsynaptic neuron will fire an action potential if a depolarization that exceeds the threshold reaches its axon hillock. It requires the combined effect of many excitatory synapses for a postsynaptic neuron to fire.


What are the two types of summation?


spatial and temporal


What is the spatial summation?


Is the summing of potentials that come from distinct parts of the cell? If the overall sum of EPSPs and IPSPs can depolarize the cell at the axon hillock, an action potential will occur.


What is temporal summation?


Temporal summation is the summing of potentials that arrive at the axon hillock at contrasting times. The closer together in time that they arrive, the greater the summation and possibility of an action potential.


What are EEG signals then?


EEG signals are primarily produced by the summation of postsynaptic potentials of millions of neurons summed millions of neurons/ firing in phase aggregated millions of PSPs 'note: EEG does not measure action potential


How is the AP aligned?


geometrically and in phase


What is EEG less sensitive to?


It is less sensitive to deep brain structures. Field strength decreases exponentially with distance. Neuronal populations in deeper structures are not arranged in a geometrically parallel fashion.


What EEG cannot measure?


It cannot measure individual molecular or synaptic events, nor it can isolate events that are produced by a specific neurotransmitter or neuromodulator. It is not very suitable to measure to slow (< 0.1 Hz) or very high (> 100 Hz) fluctuations.


What are electrodes made from?


Silver electrodes with a thin coating of silver- chloride, Tin Electrodes, Gold-cap Electrodes


The conductivity should be good between the electrode and the scalp, how?


Gel to reduce the impedance/resistance “Impedance below 5 Kilo Ohms Scalp preparation (removal of dead skin cells)also: Active Electrodes Integrated preamplifier.  Faster preparation time.


How many electrodes?


Traditional 19 Standard 32-64 (sufficient). High-density 128-256 (or more)


What are the pros of having more electrodes?


better spatial sampling better source reconstruction


What are the cons of having more electrodes?


long prep time electrolyte bridge poorer signal quality


What is an electrolyte bridge?


When the gel creates a short circuit between closely placed electrodes.


It amplifies the signal. Why and using what?


It amplifies the signal from a few ¼ Volts to a few Volts. The amplification is done by Differential Amplifiers.


What electrodes are associated with amplification?


Three electrodes: Active Electrode (A) placed at the desired site. Reference Electrode (R) placed elsewhere on the scalp Ground Electrode (G) placed elsewhere on the scalp/body. Elimination of ambient noise • Works best when impedances are the same (low) for A and R• Amplifier gain: 5-10 Ka. The optimal gain depends on the input potential and output range.


What are the usual reference sites?


Preferably a neutral site (tip of the nose, the earlobes, the mastoids, the chin, etc). Three practical criteria: Choose the site that is convenient and comfortable. Choose a site that does not induce hemispheric bias. Choose a site used by other researchers in your fields. Mostly used neutral references: the average of two earlobes average of two mastoids. Other referencing scheme: “Average of all electrodes Current source density maps Reference free method. Requires high-density recordings€¢ Less accurate for boundary electrodes Insensitive to deep sources Laplacia


What is aliasing?


When we are sampling a system (brain) with a sampling freq less than twice the maximum freq of interest. Because we monitor frame by frame - but at what rate? If it is not as fast as the original, then it is a POOR representation, as we haven't sampled enough to capture the information in the actual sample. Nyquist Criterion - sample at least twice as fast as the maximum freq (Sampling frequency (fs) should satisfy Nyquist Criterion fs > 2 fmax (fmax max. frequency of interest)This can be something like x5 the maximum - if we want to do alpha etc.


Why would we need to filter the signal?


to reduce artifacts


What filters are applicable?


Low pass High Pass Band pass band stop, notch


What is a high-pass filter?


0.5 Hz (or 0.1 Hz for slow brain responses)


What is a low-pass filter?


100 Hz


What is a notch filter (band stop)


50 Hz (for removing power line noise; 60 Hz in the USA)


We need to xxx analogue to xxxx


convert - digital


what resolution is EEG


16/24 bit Resolution (216 or 16192 different voltage values can be coded by the ADC)


What is aliasing?


When we are sampling a system (brain) with a sampling freq less than twice the maximum freq of interest.Because we monitor frame by frame - but what rate? If it is not as fast as the original, then it is a POOR representation, as we haven't sampled enough to capture the information in the actual sample.Nyquist Criterion - sample at least twice as fast as the maximum freq (Sampling frequency (fs) should satisfy Nyquist Criterion fs > 2 fmax (fmax   max. frequency of interest)This can be something like x5 the maximum - if we want to do alpha etc.


Why should we avoid aliasing?


Ro get a faithful representation of our sample


What should the sampling frequency satisfy?


Sampling frequency (fs) should satisfy Nyquist Criterion fs > 2 fmax (fmax   max. frequency of interest)


For an EEG signal with a maximum frequency of 70 Hz, aliasing occurs when. A fs 256Hz B. fs 1024 Hz C. fs 512Hz D. fs 128 Hz


128


What are EEG artifacts?


Problems in the EEG signal that need reducing/eliminating.


What are the 5 main artifacts?


1) Saccades 2) EMG (mastoid/jaw muscle/face muscles)3) EKG (pulsation of the heart)4) Skin potentials (leading to blocking)5) Alpha waves (mind wandering etc)


What is the brute force approach to rejection?


Brute force approach: Reject if over threshold (75-100 μV) as the brain doesn't create these frequency artifacts usually have much larger amplitude


Other factors in artefact rejection?


Blink (Check vEOG, Topography, Polarity) -measure the diff• Eye movement (Check hEOG, Step-like wave) - measure the diff• Electrode shift (Shifting of potentials)• Muscles (High frequency) beta - gamma Heart (Mostly in mastoid electrodes, Low frequency)


Three issues with artifact rejection?


Loss of a significant portion of data. Some participants are very prone to certain artifacts • Some tasks essentially call for artefacts. We lose TOO many trials


What is the alternative to artefact rejection?


artifact correction


What are the simple methods for artifact correction?


Subtraction method (variance-based) Filtering


What are the advanced methods for artifact correction?


Mathematical approaches:1) Dipole/Source modeling procedures2) Independent Component Analysis (ICA)


A brief description of Independent Component Analysis (ICA)?


a computational method to separate the sources of artifacts -  identifying the troublesome parts by certain characteristics and individual weights - then reverse the weights (reducing artifacts)


how to practically minimize Artifacts?


Electrical screening of the testing space (Faraday cage). Careful instruction of participants to minimize movement; blink pauses. Ensuring the participants in a relaxed condition (to reduce muscle activity). Careful electrode application to minimize impedance. Maintaining the cool temperature and low humidity level inside the lab (to reduce slow drift). Filtering (e.g., high-pass filter to remove slow-shifts [i.e.,low-frequency fluctuations in the EEG], and low- pass filter to avoid aliasing band-pass filter)


What are the 5 standard frequency bands?


Delta: < 4 Hz Theta:4-7Hz Alpha: 8-14 Hz Beta:15-30Hz• Gamma > 30 Hz5 frequency bands (FRE BAND in THE GAY BED)


What is the Fourier Analysis?


Transformation of the EEG into sine (sinusoidal) functions of various frequencies like a freq histogram (strength of particular freq)


What does Fourier Analysis lead to?


Leads to a power spectrum: power as a function of frequency


What are the applications of spontaneous EEG


Cognitive Research. Experiments with long-duration stimuli (i.e. task requiring sustained attention, ecologically appropriate stimuli)> Perhaps Mind-wandering? Monitoring sleep stages.  Clinical Research.  Epilepsy. Detection of seizures.  Localization of focus/foci. Prediction of seizure onset. Monitoring the level of anesthesia. Detection of brain death. Measurement of drug effects. Detection of cerebral pathology, e.g., through blood supply problems. Sleep disorders. Almost all neurological disorders have EEG correlates.


What is an event-related potential / evoked-potential?


The general class of potentials displaying a stable time relationship to a definable reference event.


What is an ERP reference event?


Reference event Onset/offset of a stimulus Motor response Decision moment. 


Who uses the term EP and who ERP Terminology?


EP: Perception and clinical research. ERP: Experimental cognitive research.


What characterizes an ERP?


ERPs are waveform characterized by a series of positive (P) or negative (N) deflections at different latencies  ERP Components Exogenous Components: Modulated by external characteristics of stimuli Endogenous Components: Modulated by internal characteristics


What is the ERP hypothesis?


ERP Hypothesis: ERP is a signal (s) that appears superimposed and without interaction on the background or ongoing EEG, which is considered random noise (n).


Assumptions of ERP?


ERP is uncorrelated with background EEG. Background EEG is random. ERP is invariant across trials (same ERP is repeated over trials - take the average as invariant) • Background EEG varies (randomly) from trial to trial. 


So how do we get an ERP?


Signal averaging


How does signal averaging work then?


After averaging across trials, the noise will cancel out and only the event-related EEG response will remain. Background signal cancels out and leaves with the ERPs, which are invariant.


What are the advantages of ERP?


ERPs are simple, fast to computed ERPs require very few analyses or parameters. ERP has high temporal precision and accuracy. ERP literature is quite mature. ERP provides an excellent quality check.


What is an ERP component?


We can simply define an ERP component as one of the component waves of the more complex ERP waveform. So one part could be the One and another the Pe. An ERP component is a part of a waveform with a circumscribed scalp distribution (physiological substrate) and a circumscribed relationship to experimental variables (functional substrate).


What is an example of an ERP component?


Examples: MMN (mismatched negativity, 160-220 milliseconds at central sites) N170 (face-related potential at occipital sites)


Why do we study ERP components?

Common language linking diverse experiments, paradigms, etc 2. The base for integrating ERP with other measures of brain activity 3. Structure-function information


What is the baseline period?


In averaging, all trials are set (arithmetically)to have the same zero voltage at stimulus onset, so that only deviations from the baseline voltage are seen in the ERP, after stimulus presentation. if Baseline subtraction (mean of the baseline period is subtracted)


EEG


How many trials for electroencephalography?


As many as possible. The number of trials depends on signal-to-noise characteristics; the effect size and the type of analysis to be performed SNR (signal-to-noise-ratio) increases as a function of the square root of the number of trials. Practical suggestions. 50 trials / condition / participants. Similar number of trials for all conditions. Phase/power produce positive bias with fewer trials). If not possible, match trial count. Select the first N trials from each condition (N-the number of trials in the smallest condition). Select N trials at random. Select N trials based on some relevant behavioural or experiment variable (i.e. reaction time)


What is a VEP?


visual evoked potential


What is an AEP


Auditory evoked potential


What is an SEP


Somatosensory Evoked Potentials (SEP)


What is Contingent-Negative Variation (CNV)


Indicator of learning paired stimuli (Get Set – Go)• Reflection of attention, concentration &amp; readiness to S2 • Index of neuronal excitability


What ERP accompanies semantic violations ?


N400 in Semantic Violations


What accompanies violations in music


ERAN


What are the two major limitations of ERP?


The first concerns interpretational issues, particularly regarding interpreting null results - the absence of proof is not the proof of absence. An ERP reveals little EEG information (single-trial)– ERP does not capture non-phase-locked responses The ERPs provide limited opportunities for linking results to actual neurophysiological dynamics. ERPs are less understood compared to the neurophysiological mechanisms that produce neuronal oscillations and synchrony.


If ERP is evoked activity, what other types of activity are there?


Dynamic Brain Oscillations


And what two types of Dynamic Brain Oscillations are there?


Evoked oscillations have a strict phase relationship regarding the stimulus (every time brain respond same)Induced oscillations do not have a strict phase relationship (not always the same time-varying latency – e.g. (cognitive control / top-down / attention)


So what are the limitations of ERP regarding oscillations?


ERP takes the average (keep the same latency) but induced the response gets lost. The disadvantage as ERP only captures stimulus time-locked relationships.


Dynamic Brain Oscillations advantages


Logical interpretations.  Neurophysiological mechanisms. Ubiquitous oscillations.  Neuronal oscillations are the most promising bridge linking findings from multiple disciplines. Covers a more comprehensive multi-dimensional space. 


Methods for dynamic brain oscillation analysis?


short term Fourier transform of time-frequency time series or wavelet needs freq + time!!!


Oscillations in complex cognition 


then give them a hint ....brain oscillation structure - over occipital-parietal region - when higher (hint couldn't help) but if gamma is lower... then the hint is successfully utilised. Only worked in specific brain states (receptive) characterized by the oscillatory state if alpha was high in temporal - then also was more likely to solve the problem (when we focus on something, gamma increases, when diffuse attention, alpha is high. And for these types of problems, being too focused rarely works.  Need to be open (alpha) not fixed (gamma)so posterior gamma   focused attention (fixation)posterior alpha   diffuse (open to new solution)


Object Perception and feature binding


Gamma Band Synchrony


Visual binding in adults


in gamma band comparing kaniza triangle bind diff features - new perception represented by 40hz  - and is phase-locked


Attention to music in musicians


Gamma SYNCHRONY


Cognitive Insight 


Posterior Beta and anterior Gamma


Aha Moment! 


Posterior Gamma for sudden solutions


Which ERP for Semantic Violations?


N400


Music violations?


MRAN


Early Evoked potentials: AEP –SEP – N10CEP –Chemosensory has no early ERP


later Attention P1 / n1 p2


Early Evoked potentials: SEP


n10


Early Evoked potentials: CEP


Chemosensory has no early ERP


 


 


 

 

 

 

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