NIRS Cookbook

PANDA tutorial, for students and coworkers: a recipe to analyze NIRS data

UNDER CONSTRUCTION

Cookbook

In principle, you need to:

  1. read the NIRS datafile
  2. preprocess the data (remove motion artefacts, bandpass-filter, detrend, downsample)
  3. determine important parameters to analyze (maximum response, time to max, GLM amplitude)
  4. analyze (determine differences between conditions)

1. Load data

The data is stored in an oxy3-file and/or in an xls-file. These files contain mostly the same information. The oxy3-file contains the raw, unprocessed data, while the xls-file usually contains the oxy- and deoxy-hemoglobin concentrations.

The following MATLAB routines can be used to load the data:

  • pa_nirs_read - to read the xls-file
    signal). This sound is most often used for sound localization experiments. See below for example.
  • oxy3read - to read the oxy3-file

For now, we will stick with using the excel files and pa_nirs_read.m:

 
fname = 'nirs-subject1-0001.xls'; % filename 
nirs = pa_nirs_read(fname); % read the data

The data nirs-subject1-0001.xls can be found on the Biophysics website, here. This data is from a NIRS experiment in whicha children's story is told in a video. The video is broken down in several 20-s blocks, and the subject is randomly shown the video, audio or audiovideo components in each block. Six optodes (2 receivers, 4 transmitters) were placed, three over left and three over right temporal cortex (T3 and T4 in the EEG 10/20 system) to allow for bilateral measurements over human auditory cortex.

Loading this data into Matlab will yield a structure, that above we termed nirs, with the fields:

 
fsample % sample frequency (Hz) 
label % label of recorded channels, e.g. 'Rx1a - Tx1 OHb'
event % contains fields with details on stimulus events
hdr % some header information in a structure
trial % the data in a matrix (number of channels x number of samples + 1 row with sample number)
time % a time vector
cfg % a structure with some configuration information, not used

For example, let's plot the oxyhemoglobin (O2Hb) channel on the right side (Rx1b) with the transmitter (Tx) optode 35 mm (Tx2) away from the receiver (Rx).

 
sel   = strncmp(nirs.label,'Rx1b - Tx2 O2Hb',15);  % first find the correct row of the data matrix
chan   = nirs.trial(sel,:); % and select it, storing it in the chan variable
% then plot this
plot(chan,'k-');
% some pretty modifications
axis square;
box off;
set(gca,'TickDir','out');
xlim([1 numel(chan)]);
% and some necessary labels
xlabel('Time (samples)'); %
ylabel('Oxy-hemoglobin concentration');

This will yield figure 1.

image

This is clearly a raw, oversampled, unfiltered signal, that can be improved by preprocessing (in the next step).

2. Filter

After the generation of a signal, you generally want to filter your signal, either to bandpass your signal:

  • pa_lowpass - low-pass filter the signal (remove high-frequency frequencies that are not heard, to prevent aliasing, or for specific research questions)
  • pa_highpass - high-pass filter the signal (important to remove low-frequency signals that usually end up distorted when played by the speaker)

or to equalize the signal:

3. Ramp

To prevent onset distortions, you need to ramp the data

  • pa_ramp - add a squared-sine onset and squared-cosine offset

4. Level Ramp

Because of an idiosyncrasy in our set-up, we need to pre- and append 20 milliseconds of zeros to the signal. This is because the second TDT RP2.1 DA-channel is used to control the sound level. Switching this on will also create a sudden onset peak, that needs to be prevented. This is solved by a level-ramp within the RCO. We need to consider this level-ramp by applying:

All steps (generation, filtering, equalizing, and ramping) can be done by TDT, but at present this takes precious time.

5. Save wav-file

And when this is done, we need to save the data, so that it can be used by the TDT system:

  • pa_ writewav - saves the stimulus as a wav file for the DA converter (this also applies a normalization)

You can check whether the stimulus generation has succeeded, with:

  • wavread - MATLAB routine to read an existing wav-file in MATLAB format
  • audioplayer - Matlab routine to play sounds (wavplay)
  • psd - Matlab routine, to determine power spectral density
  • pa_getpower - get power spectrum

Some examples

Let‟s generate a GWN stimulus. Stimulus 3.0 sec, bandwidth from 0.5 to 20 kHz, with on- and offset envelopes.
In Matlab:

 
 
  Noise  = pa_gengwn(3); % Default envelope (250 pnts) and filter
 
  % noise is lp-filtered at 20 kHz
 
  % and hp-filtered at 500 Hz
 
  psd(Noise);
Frequencies below 500 Hz are never useful, and produce distortions on speakers, so they should be filtered out with a highpass-filter. This is already done by default in gengwn.
The GWN stimulus is a well-localizable sound eliciting all binaural difference (ITD and ILD) cues and spectral cues. Sometimes, it is desirable to use lowpass- or highpass-filtered noise, to separate the effects of ITD and ILD. These sounds are created by generating a GWN as above, and filtering it with the functions lowpassnoise and highpassnoise:
 
  HP = pa_highpass(Noise); % noise is hp-filtered at 3 kHz
 
  psd(HP);
 
  LP = pa_lowpass(Noise); % noise is lp-filtered at 3 kHz
 
  psd(LP);
 
 

You might want to equalize the signal:

 
  Noise = pa_equalizer(Noise);
  HP = pa_equalizer(HP);
  LP = pa_equalizer(LP);


Then apply a ramp:

 
  Noise = pa_ramp(Noise);
  HP = pa_ramp(HP);
  LP = pa_ramp(LP);
 
 

In the FART1-setup, speakers will have on onset ramp produced by the TDT-system to ramp to a voltage on the speakers. You will have to take this into account, by putting zeros in front of the stimuli (of about 20 ms worth 20*48.8828125 978 samples):

 
  Noise = pa_levelramp(Noise); %This "prepends" the zerovector to the Noise
 
  HP = pa_levelramp(HP);
 
  LP =  pa_levelramp(LP); 

After this, you should save the matrix to a wav-file, that can be played with the TDT-system:
 
  writewav(Noise,"snd001.wav);
 
  writewav(HP,"snd002.wav");
 
  writewav(LP,"snd003.wav");