= MEG auditory dataset - CTF = ''File: sample_auditory.zip'' <)>> == License == This tutorial dataset (MEG and MRI data) remains a property of the MEG Lab, McConnell Brain Imaging Center, Montreal Neurological Institute, McGill University, Canada. Its use and transfer outside the Brainstorm tutorial, e.g. for research purposes, is prohibited without written consent from the MEG Lab. If you reference this dataset in your publications, please aknowledge its authors (Elizabeth Bock, Peter Donhauser, Francois Tadel and Sylvain Baillet) and cite Brainstorm as indicated on the [[CiteBrainstorm|website]]. For questions, please contact us through the forum. == Presentation of the experiment == ==== Experiment ==== * One subject, two acquisition runs of 6 minutes each * Subject stimulated binaurally with intra-aural earphones (air tubes+transducers) * Each run contains: * 200 regular beeps (440Hz) * 40 easy deviant beeps (554.4Hz, 4 semitones higher) * Random inter-stimulus interval: between 0.7s and 1.7s seconds, uniformly distributed * The subject presses a button when detecting a deviant * Auditory stimuli generated with the Matlab Psychophysics toolbox ==== MEG acquisition ==== * Acquisition at '''2400Hz''', with a '''CTF 275''' system at the MNI, subject in seating position * Online 600Hz low-pass filter, files saved with the 3rd order gradient * Recorded channels (340): * 1 Stim channel indicating the presentation times of the audio stimuli: UDIO001 (#1) * 1 Audio signal sent to the subject: UADC001 (#316) * 1 Response channel recordings the finger taps in response to the deviants: UPPT001 (#2) * 26 MEG reference sensors (#5-#30) * 274 MEG axial gradiometers (#31-#304) * 2 EEG electrodes: Cz, Pz (#305 and #306) * 1 ECG bipolar (#307) * 2 EOG bipolar (vertical #308, horizontal #309) * 12 Head tracking channels: Nasion XYZ, Left XYZ, Right XYZ, Error N/L/R (#317-#328) * 20 Unused channels (#3, #4, #310-#315, #329-340) * 3 datasets: * '''S01_AEF_20131218_01.ds''': Run #1, 360s, 200 standard + 40 deviants * '''S01_AEF_20131218_02.ds''': Run #2, 360s, 200 standard + 40 deviants * '''S01_Noise_20131218_01.ds''': Empty room recordings, 30s long * File name: S01=Subject01, AEF=Auditory evoked field, 20131218=date(Dec 18 2013), 01=run * Use of the .ds, not the AUX (standard at the MNI) because they are easier to manipulate in FieldTrip ==== Stimulation delays ==== * '''Delay #1''': Between the stim markers (channel UDIO001) and the moment where the sound card plays the sound (channel UADC001). This is mostly due to the software running on the computer (stimulation software, operating system, sound card drivers, sound card electronics). The delay can be measured from the recorded files by comparing the triggers in the two channels: Delay''' between 11.5ms and 12.8ms''' (std = 0.3ms) This delay is '''not constant''', we will need to correct for it. * '''Delay #2''': Between when the sound card plays the sound and when the subject receives the sound in the ears. This is the time it takes for the transducer to convert the analog audio signal into a sound, plus the time it takes to the sound to travel through the air tubes from the transducer to the subject's ears. This delay cannot be estimated from the recorded signals: before the acquisition, we placed a sound meter at the extremity of the tubes to record when the sound is delivered. Delay '''between 4.8ms and 5.0ms''' (std = 0.08ms). At a sampling rate of 2400Hz, this delay can be considered '''constant''', we will not compensate for it. * '''Delay #3''': The CTF MEG systems have a constant delay of '''4 samples''' between the MEG/EEG channels and the analog channels (such as the audio signal UADC001), because of an anti-aliasing filtered that is applied to the first and not the second. This translate here to a '''constant delay''' of '''1.7ms'''. * '''Delay #4''': When correcting of delay #1, the process we use to detect the beginning of the triggers on the audio signal (UADC001) sets the trigger in the middle of the ramp between silence and the beep. We "over-compensate" the delay #1 by 1.7ms. This can be considered as '''constant delay '''of about '''-1.7ms'''. * '''Uncorrected delays''': We will correct for the delay #1, and keep the other delays (#2, #3 and #4). After we compensate for delay #1 our MEG signals will have a '''constant delay '''of about 4.9 + 1.7 - 1.7 = '''4.9 ms'''. We decide not to compensate for those delays because they do not introduce any jitter in the responses and they are not going to change anything in the interpretation of the data. ==== Head shape and fiducial points ==== * 3D digitization using a Polhemus Fastrak device driven by Brainstorm ('''S01_20131218_*.pos''') More information: [[Tutorials/TutDigitize|Digitize EEG electrodes and head shape]] * The output file is copied to each .ds folder and contains the following entries: * The position of the center of CTF coils * The position of the anatomical references we use in Brainstorm: <
> Nasion and connections tragus/helix, as illustrated [[http://neuroimage.usc.edu/brainstorm/CoordinateSystems#Pre-auricular_points_.28LPA.2C_RPA.29|here]]. * Around 150 head points distributed on the hard parts of the head (no soft tissues) ==== Subject anatomy ==== * Subject with 1.5T MRI * Marker on the left cheek * Processed with FreeSurfer 5.3 <)>> == Tutorials using this dataset == * [[Tutorials/Auditory|MEG auditory tutorial (CTF)]]