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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. |
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
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.
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.