### Allmänt

Introduction to the course

Overview of brain imaging methods

MEG/EEG physiology

Explain concisely your expertise and knowledge in

- neuroscience

- neuroimaging

- programming on Matlab and/or Python

- MEG/EEG physiology continued

- MEG/EEG responses

- MEG instrumentation

- EEG instrumentation

This exercises familiarizes you with processing and interpretation of evoked responses.

Download a pre-recorded MEG data set as a Matlab file from here (size is about 220 MB). This file holds three Matlab variables:

**data**: a matrix of MEG-channels (315) by time points (200000)**channels**: a vector (315) of channel names in the same order as the rows in the data matrix**samplingrate**: the sampling rate in Hz (the reciprocal of this value is the time between two consecutive time points in the data matrix)

The data are a continuous (non-averaged) recording of a healthy adult volunteer who received 8 kinds of simple sensory stimuli, one at a time, in a random order. Stimulus onsets are marked with trigger events on channel STI014 (you can ignore other STIxxx channels). There is one trigger code corresponding to one stimulus type, and the onset (transition from zero to a non-zero value) of the trigger pulse denotes the onset of the stimulus. The duration of the trigger pulse is irrelevant.

Your task is to:

1. Compute the average evoked response (separately) for the trials with trigger codes 2, 3, and 16 (just ignore the other trigger codes). Choose a time window which includes, say, a 100-ms pre-stimulus baseline and extends to about 500 ms post-stimulus.

2. Visualize the average responses topographically. You may use the plotEF Matlab function available on this page; see Ex 2: Helper functions.

3. Make your best guess which kind of a stimulus corresponds to each trigger code (i.e., which sensory modality and where presented with respect to the body). Exploit the location and timing information; remember that the planar gradiometers peak right above the source. Note that all the stimuli are rather elementary.

4. Write a report. In addition to the text, include three figures, one for each average response. You are encouraged to search relevant literature for typical characteristics of MEG responses to sensory stimuli. Refer to such studies in your report.

- MEG source modeling

This exercise is about source modelling. The task is to construct and apply a cortically constrained L2 minimum norm estimator to two averaged data sets, visualize the estimate on the cortical surface and interpret the results.

The Matlab data file given below contains all the ingredients:

data1, data2, timeaxis, n_trial two evoked-response data matrices, the latencies of each sample in those matrices, and the number of trials averaged. The datasets are baseline corrected and filtered to 0-40 Hz. Note that the data are rank deficient due to the application of interference suppression methods; the effective number of channels (degrees of freedom) is 64. L the lead field / gain matrix Cn noise covariance matrix, computed from pre-stimulus baseline periods in unaveraged data Cd data covariance matrix, computed from the window 0 - 0.5 s about each unaveraged trial (needed only for a beamformer) anat_decim low-resolution cortical surface mesh for visualizations The regularization coefficient in the L2 MNE inverse operator can set to lambda = tr(L*L') / (tr(Cn) * snr^2) where tr is the

*trace*of a matrix and snr refers to the estimated signal-to-noise ratio of the data. In**unaveraged**data, this can be assumed to be 1. Recall how trial averaging improves signal-to-noise ratio and use that as the snr above.Source covariance is unknown and thus you may use an identity matrix for that.

Matrix inversions that involve noise or data covariance need to be regularized. SVD truncation with tolerance about 1.0e-24 is appropriate. On MatLab, the pseudoinversion function pinv() can do this; check its help page.

For visualization, you can use the provided Matlab function

vis_surface_data(source_estimate, threshold, anatomy)

which displays 'source_estimate' vector on the cortical surface found in 'anatomy'. The function creates separate views to the left and right hemispheres, which you can zoom and rotate using the normal Matlab plot window controls.

In the report:

- explain how you constructed the L2 MNE operator (you may also include the Matlab code snippet)
- include figures of the source estimates of the two data sets
- discuss the differences of the estimates: what can you say about the laterality of the stimuli presented in data1 vs. those in data2?
- discuss how the spatial extent of the estimated activity reflects extent of the underlying neural current distribution

Bonus tasks (one extra point from each):

- Construct a beamformer operator and apply it to the data sets
- Include depth weighting in the L2 MNE operator by modifying the source covariance matrix appropriately

- - data for Exercise 3

- surface visualization function for Matlab - Oscillatory responses

- The data required in the assignment can be downloaded from http://neuro.hut.fi/~jjkujala/FBI_DATA_osc.zip.
Matlab code designed to help in the assignment is given underneath in another tar-file (http://neuro.hut.fi/~jjkujala/FBI_code_osc.zip).

The assignment consists of two parts, and of writing a report describing and showing the main findings.

**1. Spontaneous oscillatory activity**Analyze how spontaneous (continuous) oscillatory activity is modulated when the subject opens his/her eyes. Identify the peak frequency of the modulation, identify which of the two data-sets (data1, data2) represents eyes-closed condition, and show the effects on the MEG sensors.

The data and related variables are contained in the file P1_spontaneous_data.mat

- chanNames=names of MEG sensors
- sff=sampling
frequency) .

Visualization of the spectra across sensors can be conducted with the function:

plotSpectra (e.g., plotSpectra(f,spectra,spectra2,'coils','grad2'),

where f contains the frequency bins where the spectra have been estimated and spectra1/spectra2 the estimated spectra; selection of visualized frequencies and the scaling of the visualization is controlled via options 'xlimits' and 'ylimits', e.g., plotSpectra(f,spectra,spectra2,'coils','grad2','xlimits',[2 25],'Ylimits',[-5e-10 3e-9]) )

Tips:

- To estimate the spectra, use
function pwelch. Compute the spectrum for one channel at a time (in a
loop).
- Select a reasonable frequency
resolution; around half-a-hertz will suffice; see matlab help for
pwelch (nfft).
- When visualizing, concentrate on
frequencies below 30Hz so that the possible high-frequency noise does
not hamper the detection of the sought power peak for alpha frequency
(about 10Hz).

**2. Induced responses, time-frequency representation**Analyze the event-related modulation of rhythmic activity via time-frequency representations (TFR) during an attentive visual task. Identify the frequency range where the main modulation occurs and show the effects on the MEG sensors.

The data and related variables are contained in the file P1_event-related_data.mat;

the file contains a single structure (epochs) with several fields (

- chan_names= names of recorded channels,
306 first contain the MEG data;
- peri_int=time-interval of the
epochs/trials;
- Fs=sampling frequency;
- zero_sample=sample number at
time 0;
- ns_pre=number of samples before stimulus presentation;
- ns_post=number of samples after stimulus presentation;
- ns_ep=length
of epochs; data=cell array with the recorded data for all 120
epochs/trials).

TFRs per trial and channel can be computed with the function energyvec (e.g., TFR=energyvec(f, time_series, Fs),

where f is a frequency of interest, time_series the MEG data for a single channel, and Fs the sampling frequency).

Visualization of the TFRs across sensors can be conducted with the function

plotTFR (e.g., plotTFR(freqVec,timeVec,TFRS,'layout','NM306planar.lay','labels','on','baseline',[-0.3 0]);,

where time/freqVec are vectors containing the time instances/frequencies where the TFRs have been computed).

Scaling of the visualization is controlled via options 'xlimits', 'ylimits and 'zlimits' (e.g., plotTFR(freqVec,timeVec,TFRs,'layout','NM306planar.lay','labels','on','baseline',[-0.3 0],'xlimits',[-0.1 1],'ylimits',[3 20],'zlimits',[-1e-23 1e-23]);

The baseline-subtraction can be performed also outside the visualization with the function TFRbaseline (TFRs=TFRbaseline(timeVec,freqVec,TFRs,[-0.3 0]); )

Tips:

- TFRs need to be computed first
separately for each frequency, channel and epoch with the function
energyvec
- it is best to focus on
reasonable frequency range and resolution, e.g., freqVec=[3:30];
- to see when and at what frequency-range the bright spot in some of the channels really is, use function imagesc to visualize a single channel. In this you will need the provided TFRbaseline-function since imagesc does not perform a baseline correction (in plotTFR the baseline correction is built-in)

- chanNames=names of MEG sensors
- Basics of MRI

- Physiology of fMRI

In this exercise you should do a simple statistical analysis on fMRI data using the general linear model, as implemented in matlab (glmfit). Write a report describing and showing your findings.

The data is from a single subject performing a simple auditory experiment. The auditory stimulation was words presented in a block design (to both ears) at a rate of 60 words per minute. 96 acquisitions (or scans) were made with a TR (repetition time) of 7s. There were two experimental conditions: auditory stimulation and rest. The conditions were presented in blocks of 42 seconds (6 scans), giving a total of 16 blocks. The condition for successive blocks alternated between rest and auditory stimulation, starting with rest. The first 12 scans (one task and one rest block) were discarded, leaving 84 acquisitions.

The data has been preprocessed: The functional images were realigned (motion corrected), and coregistered to the anatomical image. The images have been normalized to the MNI template brain. Finally, the functional data was smoothed with a 6-mm FWHM Gaussian kernel.

For the purpose of your analysis (and to reduce the amount of data posted on the Moodle page) I have extracted the preprocessed data from three different regions of interest (ROIs): the left auditory cortex, the right auditory cortex, and an unrelated region in the left posterior cortex (~150 voxels in each). Your task will be to:

1) Create and show a 'boxcar' model of the experiment based on the description above.

2) Take the shape of the BOLD response into account and create a better model for your experiment. Use the shape of the hemodynamic response function (sampled at 1s) from the mat-file containing the hrf response. Plot the result.

3) Perform a statistical analysis on each voxel in the three different ROIs using the general linear model. Can you find statistically significant results? A p-value of 0.001 (for the gain) can be considered significant for the uncorrected data.

4) Correct for multiple comparisons. You may use Bonferroni correction. Do your results survive correction? A corrected p-value of 0.05 can be used.

5) How well does your model fit the data in i) a voxel with a significant result and in ii) a voxel where you did not find a significant result? Plot the responses, the fitted models, and the residuals.

6) How could you further improve your model? Describe (in the text) at least one nuisance factor that you could include in your design matrix to get a better fit.

Useful matlab commands: conv, glmfit, glmval

- MEG and fMRI connectivity
**Exercise: Functional connectivity**The data required in the assignment can be downloaded from http://neuro.hut.fi/~jjkujala/FBI_data_conn.zip. Matlab code designed to help in the assignment is given in another zip-file (http://neuro.hut.fi/~jjkujala/FBI_code_conn.zip).

The assignment consists of two parts, and of writing a report describing and showing the main findings.

1. Functional connectivity, real vs. spurious interactions

Demonstrate how the selection of an appropriate vs. inappropriate reference region affects the estimates of cortico-cortical coherence in MEG; principally, show how field spread/spatial leakage properties change when a reference region displays activity compared to a situation where it does not. First, identify the frequency showing the most prominent rhythmic activity and the source locations showing activity at that frequency, and show what kind of coherence estimates are obtained at the same frequency when one of the two brain regions active at that frequency is taken as a reference vs. cases where the reference is placed elsewhere. The data and related variables are contained in the file P1_connectivity_data.mat; the file contains the beamformer estimates of cortical time-series (funct_data) in 29 locations along a single axis (x-coordinates given in variable locations) and the sampling frequency of the data (sff). Two of the 29 locations contained simulated sources that were mutually coherent; in the rest of the regions, no activity was simulated.

-Spectral estimation can be conducted with the function pwelch (to be applied separately in each location). Frequency resolution of ~0.5Hz is sufficient, and for visualization it is best to focus on frequencies < 30 HZ. When you have identified the frequency of interest, the spatial profile of power will be clearer the power values are averaged e.g. within a 2-Hz wide window

-Compute coherence (with function mscohere) first from one of the identified active regions to all other regions, and the from at least one region were no activity was detected. Similarly to the power-mapping, the results will look clearer if the computed coherence values are averaged within a 2-Hz wide window.

2. Estimation of directed interactions using Granger Causality

Estimate the appropriate model order for Granger Causality analysis, and evaluate the directed influences two time-series have on each other. The main goal is to identify which time-series is the driving one, and at which frequency. The data (variable X containing 2 time-series) and its sampling frequency (s_freq) are given in P2_granger_data.mat.

Tips:

-The appropriate model order can be identified using the function cca_find_model_order. The function returns the appropriate order based on both The Akaike and Bayesian information criterion. Either one can be used in the subsequent analysis. It is sufficient to consider model orders < 20.

-The spectral Granger causality estimation can be done using the function cca_pwcausal. The function can be called using the syntax [GW,COH,pp,waut,cons]=cca_pwcausal(X,Nr,Nl,nlags,Fs,freq, STATFLAG) (note the variable order), where the number of realizations is in this case 1 and STATFLAG can be set to 0. Regarding the examined frequencies, it is sufficient to consider frequencies below 50Hz. As regards the main output (GW), one should note that the terms are estimated such that the column variable is Granger Causing the row variable.

- Experimental design