StimulusDetails - Meg Wiki

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In general, we are trying to make the Stimulus Delivery as similar as possible to the MRI (see http://imaging.mrc-cbu.cam.ac.uk/imaging/CbuStimulusDelivery). However, there are important differences, which we will expand here.

Visual

Important note on procedure

Note: Please do NOT turn off the projector. (If the stimulation computer is turned on when the projector is off, it messes up the video settings as the computer will not see the projector anymore, so will set the LCD monitor to be the main one.) It can, and should be set to stand-by, of course, with the 0 and 1 buttons at the back. Just do not use the main switch on the side.

Latency

Using a light diode, we have measured a 34ms delay between the trigger codes sent from the stimulus PC (synchronised with a screen refresh) to, eg, the MEG machine, and the actual visual change on the screen in the MEG room. This time seems to reflect two screen refreshes (at 60Hz) that are buffered by the projector. This time will therefore need to be added to the times of trigger codes in the MEG data.

Maximum refresh rate

The maximum refresh rate that the projector will produce is 60 Hz. You should not try to produce displays at a higher rate. Although the projector will accept the signal from the computer at higher rates (75 Hz) and produce a display, it drops frames to downsample the output to 60 Hz, making the exact timing of the stimulus unpredictable.

We also found out that changing the resolution can affect the refresh rate. The default resolution on the stimulus presentation computer is 1024 * 768 at 60 Hz. When you tell E-Prime to use a resolution of 1280 * 1024, the refresh rate also changes to 75 Hz for no apparent reason. As mentioned above, the projector is not actually able to do this.

The advice is to stick to 1024 * 768 and 60 Hz and to not change these settings.

Also, you may want to have E-Prime display the refresh rate it has detected at the beginning of the experiment. Simply add a text object containing: " Refresh Rate: [Display.RefreshRate] "

Auditory

Headphones

We haven't got a proper pair of headphones at the moment. The ones supplied by Elekta Neuromag are magnetically quiet, but are not able to produce decent sound quality. We've tried out another pair, which sound much better, but these produced some magnetic noise.

The headphones will always be in-ear types. Use a new pair of eartips for each subject, and dispose of used tips immediately. It is easier to insert the eartips before the volunteer goes into the MSR, as this can be fiddly. Just disconnect the first section of tubing from the headphones fro this, and reconnect them when the subject is seated inside the MSR.

Be very careful with the volume control, as it is easy to set this too loud. Always turn it all the way down and then turn up the volume slowly while asking the volunteer to report on the loudness.