Projector and screen
The visual presentation is done with a Propixx projector outside of the MSR, projecting though a periscope into the MSR onto a back-projection screen that is placed in front of the chair. There's a pair of markers on the floor where the screen should be positioned for correct focus. The matt side of the screen should be facing the projector, as this serves to minimise reflections.
The projected image is approximately 33 by 57 cm. The distance from the top edge of the helmet to the screen, which is about as far as the eye of the avarage subject, is 120 cm. The preferred screen resolution is 1024 * 768.
Take care that your stimulus presentation script takes the refresh rate into account. Visual stimuli will not be displayed with millisecond accuracy, but in multiples of a refresh cycle (e.g. ~16.7 ms at 120Hz). Your software may round latencies up or down. Look here for an example.
Important note on procedure
Note: Please NEVER turn the STIM machine on without the projector being switched on first. 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, using the 0 and 1 buttons on the remote. Just do not use the main switch on the side. There's a sign with some help on how to use the projector on the wall in the control room.
Latency
Using a light diode, we have measured a 9-10ms delay (as of 07/02/2024) 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 will therefore need to be added to the times of trigger codes in the MEG data. The EN acquisition software will actually enable you to specify this delay for a visual event in the acquisition setup. When specifying this the corresponding trigger will be delayed by the correct amount of time, and will be in sync with the actual presentation again.
Maximum refresh rate
The current set refresh rate is 120 Hz (as of 07/02/2024, before that date it was 60Hz with a delay of 17-18ms).
The propixx projector and STIM28 monitor are capable of up to 144Hz presentation but according to propixx documentation this is restricted to specific stimuli presentation otherwise you may experience dropped frames, erratic display.
Changing the refresh rates on the system is not allowed without the MEG lab manager/Head of methods approval.
Projector specifics
The central wavelengths for the red, green and blue 'guns' are 610nm, 550nm and 472nm respectively.