Multimodal Brain Imaging

Question 1

what is Single Neuron Activity and Local Field Potential?

Answer 1

single -cell and ( Local Field Potential 140-100,00 neurons - so slightly better spatial resolution EEG)

so AP and PSP

Question 2

for a Continuous Signal the sampling freq is?

Answer 2

(sampling frequency > 8 kHz)

Question 3

settings for single cell?

Answer 3

• High pass filter (> 300 Hz)

* Spike sorting

Question 4

details for LFP

Answer 4

Local Field Potential
• Down-sampling
(not necessary for lower sampling frequency)
• Spike correction
(local interpolation around each spike)
• Low pass filter (< 200 Hz)

Question 5

what was first finding in slides using these methods?

Answer 5

Neuronal Spikes Phase Locked to LFP

one neuron fired more at theta peaks and not troughs

one neuron fired more at theta troughs and not peaks

= phase preference

also - different neurons show diff preferences but all like theta

• Neurons also exhibited phase-locked to oscillations in frequencies other than theta, like delta (1-4 Hz), alpha (10-16 Hz), beta (16-30 Hz), and gamma (30-90 Hz) frequency ranges
• Some neurons show phase locking to multiple oscillations

Question 6

what does this give us

Answer 6

more potential for coding information - like a large number of words in vocabulary

• Neuronal phase-locking phenomenon is present in various brain regions
• Gamma oscillations facilitate a different type of information coding compared with delta or theta oscillations

• Delta and Theta oscillations may facilitate phase-based temporal coding
– Phase Coding Hypothesis: Neurons encode information via the oscillatory phase at which they spike

> Different neurons fire at different phases of ongoing oscillations

Question 7

when combing EEG and fMRI, the one issue is

Answer 7

different time scales (BOLD is sluggish)

Question 8

3 different approaches to combined EEG and fMRI?

Answer 8

1) Separate EEG and fMRI - and subsequent combination
2) Simultaneous - recordings and analysis of high amplitude EEG signals (e.g. alpha oscillations)
3) Simultaneous EEG-fMRI recordings and analysis of ERP and single trials

Question 9

Separate EEG-fMRI Advantage?

Answer 9

Better quality of signals because lesser artifacts

Question 10

associated paper for separate?

Answer 10

Heinze et al (1994) - pet n EEG / auditory visual spatial attention + source localisation

Question 11

Separate EEG-fMRI: Drawbacks ?

Answer 11

• Implicit assumption: Both measures pick up similar neural activity

– Not all activity in EEG result in BOLD responses and vice versa

• Data cannot be combined to investigate emergent brain responses which may not be time-locked to stimulus
• Significant subjective and experimental differences between the two recording sessions

• Other differences – Order effects
– Posture (EEG seated / fmri supine)
– Environmental noise – Level of comfort

Question 12

Simultaneous EEG-fMRI Acquisition: the EEG is …

Answer 12

High amplitude EEG Signals

High amplitude EEG visible to the naked eye – Alphaoscillations
– Epilepticspikes
– SSVEP(steadystatevisualevokedpotential) At low MR field strengths (< 1.5 T)

Question 13

associated Simultaneous paper =

Answer 13

Ives 1993

Question 14

Ives 1993 found

Answer 14

Typical result:
– EEG Alpha power is negatively correlated with BOLD signalin occipital cortex and positively correlated with thalamic activity

Question 15

Simultaneous needs

Answer 15

• Pre-amps inside MR magnet (reduction of cable length)
• Amps must be free of ferro-magentic materials
• Wider dynamic (range of input signal greater to get better recording - you dont want a saturated amplifier, gives no info) range to prevent amplitude saturation

Question 16

What are the Two types of acquisition?

Answer 16

• Interleaved Acquisition

* Continuous Acquisition

Question 17

in Interleaved Acquisition, EEG is acquired….

Answer 17

continuously

Question 18

in Interleaved Acquisition, fMRI is acquired….

Answer 18

intermittently

Question 19

in Interleaved Acquisition, following stimulus presentation…

Answer 19

1-2 seconds of EEG data are captured before a few seconds of fMRI data is aquired

Question 20

This approach helps to reduce ….

Answer 20

interference from scanner noise

Question 21

and is particularly useful in

Answer 21

auditory experiments

Question 22

However, the main artifact xxx is still present

Answer 22

BCG - Ballistocardiogram / pulse artifact

but

it is less a problem as: BCG is most possibly not locked with stimulus-related response, so ERP is usually unaffected

Question 23

what is BCG - Ballistocardiogram / pulse artifact?

Answer 23

Interference related to cardiac cycle and scales proportionately with the magnetic field strength

Question 24

What are the Three possible sources of BCG?

Answer 24

• Small movements of the electrodes and the scalp due to expansion and contraction of scalp arteries between the systolic and diastolic phase

(micro chanages in electrode location)

• Fluctuation of the Hall voltage due to pulsatile changes of the blood in the arteries
• Small cardiac related movements of the body

Substantial variability within and between subjects, so its proper removal is the principal aim of getting a clean EEG signal

Question 25

what is Fully Simultaneous Acquisition?

Answer 25

* Both EEG and fMRI are acquired continuously | * EEG amplifier must have a wider dynamical range so that the amplifiers do not saturate during fMRI acquisition

Question 26

however... Fully Simultaneous Acquisition contains XXX and needs XXXX

Answer 26

* EEG signal contains large artifacts * Sophisticated algorithms are necessary for artifact elimination (mainly on post-hoc basis) * Very high sampling frequency (>10 kHz/channel) is required (to record the artifact really well, in order to remove the artifact!!!! incredible) (to prevent aliasing of scanner artifacts into the EEG signal and to detect and to remove gradient artifacts) • More useful for clinical purpose (localizing seizure foci), but problematic for ERP study (low SNR)

Question 27

3 main ways of removing BCG artifacts?

Answer 27

Removal of BCG artefact: 1. Averaged artifact subtraction (AAS) 2. Adaptive Filtering 3. ICA techniques (independent components analysis)

Question 28

what is AAS Procedure for BCG Artefact Removal?

Answer 28

* Detect the QRS peaks in the ECG (electrode on the chest) signal * Align EEG activity time locked to these peaks * The averaged template is then subtracted from the EEG

Question 29

What is a gradient artifact?

Answer 29

Caused by rapidly switching magnetic field gradients during fMRI acquisitions so to stop information loss through peaking - you use that siper braod amp to record it in order to reduce it

Question 30

how do we Removal of Gradient Artifact (GA)?

Answer 30

Estimating magnetic gradients and subtracting the sum of these responses from the actual EEG Estimating average artifact template - Time averaging of noisy EEG sections to obtain a template - Subtracting the template from the channel - EEG amplifier must capture the large amplitudes introduced by the GA (high dynamic range) - Accurate trigger timing is essential (high sampling frequency)

Question 31

what are some EEG-fMRI: Task Design Issues?

Answer 31

• Event-related design may be better than blocked design • Fast event-related designs are more used – Shorter (1-7 sec) ITI (inter-trial-interval) – Faster data acquisition – Reduction of the impact of cognitive processes due to long ITI – Performances of EEG near optimal (EEG is rapidly decorrelated) – Successive BOLD responses can overlap but be reduced offline

Question 32

what are the Performance characteristics of experimental fMRI design ?

Answer 32

– Estimation efficiency (the ability to make an estimate of BOLD) – Detection power (the ability to detect brain activation) – Conditional entropy (measure of the randomness of the design)

Question 33

the trade off is....

Answer 33

Block design offers good detection power but minimal estimation efficiency, whereas event-related design offers maximum efficiency but poor detection power

Question 34

two main methods to integrate data?

Answer 34

1. Using ERPs as predictors of fMRI response | 2. Dipole based biophysical modelling for integration

Question 35

when Using ERPs as predictors of fMRI response, ....

Answer 35

• fMRI activation is correlated with the amplitude of a specific ERP component • Group level analysis – ERP amplitude and task-related contrast image per participant are subjected to a correlational analysis Maps brain responses that are specifically correlated with a chosen ERP component – No information about the temporal order of responses that led to the chosen ERP component • Individual level analysis – More powerful approach – ERP amplitudes are used as covariates in a general linear model based analysis of the fMRI data

Question 36

in fMRI Constrained Dipole Modeling...

Answer 36

the data can be single-trial or spontaneous EEG data, not necessarily time locked to an external event • Clinical applications – Epilepsy – Sleep research

Question 37

what is – Spike triggered fMRI?

Answer 37

fmri pulse is triggered by eeg potentials

Question 38

what does resting - state fmri use?

Answer 38

continous recording - recall mind wandering / default mode network

Question 39

early problem with TMS and EEG?

Answer 39

artifacts caused by strong magnet field from TMS - they got around it by shocking one hemi and recording the other

Question 40

what tactic do we use these days?

Answer 40

sample and hold

Question 41

what is sample hold?

Answer 41

Stimulus Artefact Suppressor temp protect EEG system from the magnetic environment - TMS pulse is brief - so we briefly turn off EEG (few ms) - Sample the signal - Hold it constant during the stimulus

Question 42

what do you also need to achieve this ...

Answer 42

TMS Compatible EEG Amplifier - to electrically isolte the EEG system (temp shut down)

Question 43

what artifacts in TMS / EEG?

Answer 43

* DC shift of the electrodes just under the coil, arising from the movement of the electrodes due to coil vibration * Reflex activity from cranial muscles • Unwanted auditory activation due to the coil click (Bone conducted sound)

Question 44

using ERP and TMS - Massimini showed TMS-evoked Response & State of Vigilance, what was the result?

Answer 44

Breakdown of Cortical Effective Connectivity in sleep connectivity and cross talk when awake

Question 45

TMS and Brain Oscillations

Answer 45

Thut! - always with the TMS Posterior a-Oscillations and Visual Perception tms to visual cortex = phosphene in vision = less alpha Dynamic coupling between tACS frequency and Effectiveness of the Stimulation = most likely to occur in the darkness alpha /// when light on, most likely in beta

Question 46

BOLD does indeed correlate with .....

Answer 46

electrical neural activity Summary • BOLD certainly reflects an increase in neural activity (almost linear correlation) * BOLD correlates more with LFP and especially with high frequency oscillations in the gamma range * SNR of neural response is higher than SNR of BOLD (fMRI analysis probably underestimates the neural activity)