Sound Card
The stimulus delivery computer has a SoundBlaster ZxR soundcard.
Sound Latency
Up to December 2014 we used the Xonar Essence STX soundcard, which produced a latency of 33ms with a jitter of +/- 3ms. From January 2015 the soundcard is a SoundBlaster ZxR. Between January 2015-December 2022 the latency was 26ms +/- 2ms jitter. The length of tube was increased December 2022 and the current latency is 28ms with +/- 2ms jitter. These latencies are measured at the inserts, so include the delay caused by the 3.5 meter of tube the sound has to travel through (which is approximately 3.5m / (343 m/s) (speed of sound in air), ie ~10ms).
Headphones
The headphones supplied by Elekta Neuromag are not able to produce high quality sound. On top of that they aren't completely free of magnetic artifacts either. We do not use them anymore. Instead, we've installed a set of higher quality headphones outside of the MSR, with 3.5 meter of tube attached. These tubes enter the MSR on the right side of the MEG machine.
In addition the sound output of the stimulus computer can be recorded in the fif output file of the MEG acquisition system. To enable this you only have to add the channel 'MISC5' to your setup.
The headphones are in-ear types. Use a new pair of inserts for each subject, and dispose of any used inserts immediately. It is easier to insert the eartips before the volunteer goes into the MSR, as this can be fiddly.
It is very important that you make sure that your soundfiles are created as loud as is possible, without distorting the sound. Make sure you adjust your recording level correctly during the actual recording, and adjust the level in a sound editor if needed.
Measurements for the headphone system are here: MEG tube phones with silicone inserts.pdf. These still used the previous Xonar Essence STX soundcard, but we do not expect the new SoundBlaster ZxR card to have much of an effect on this.
Sound test
A script, called 'FindThresholds', is available on the desktop of the stimulus presentation computer to test if the headphones are inserted properly. The subject will need a pair of button boxes and when the script is started sounds are played to both ears for a couple of minutes while the subject is asked to press the button on the side of the sound when they hear one.
When the script terminates it will report two numbers, one for the right ear and one for the left ear. These numbers are the attenuation, in dB, that can be applied to the test tone before the subject is unable to hear it. Normal values are from 45 to 75 dB. The difference between the left and the right ear should ideally not be more than 10 dB. The default test tone used is a 1000 Hz pure tone of about 1 second at maximum loudness.
During an MEG recording, the sound output is recorded in channel Misc005.
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