Altering presentation duration of visual stimuli and the light sensor patch

Dear staff of Cedrus Community Forums.

I am a user of SuperLab 5 and StimTracker to build and implement experiments.

I am making a experimental program in which the visual stimuli are presented with the light sensor patch to detect the time lag between the locations of the computer screen.

The light sensor patch I am using is the one equipped with the SuperLab that you can activate by ticking a checkbox in a event editor.

I would like to ask whether the presentation duration of the light sensor patch and the target stimuli can be differentiated on SuperLab 5.

For example, I would like to present a target stimulus for 200 msec and want to show the light sensor patch for the first 10 msec (onset is adjusted), remaining 190 msec stimulus without the patch being presented.

This is to reduce the number of TTL signals of each event presentation recorded via StimTracker.

I tried resolving target presentation event into two individual consecutive events (first one w/ patch for 10 msec while second one w/o patch for 190 msec, stimuli list being the same) but that led to a blink between the two events, which is a problem, so I think other measure have to be taken.

I would be pleased if I can receive a solution as to this issue.

Best Regards,

Unfortunately, it’s not possible to have the light sensor patch appear for a different duration than the stimulus. This is by design, because it then allows you to obtain onset and offset information.

Are you using a CRT monitor? I ask because you are concerned about reducing the number of TTL signals. Only a CRT monitor would generate a number of pulses. An LCD monitor would not.

Thank you for the quick reply.

Yes, the equipment that is available for me to present stimuli to the participants is a CRT monitor, which is somewhat old-fashioned, although the original screen from which the screen is transferred to the CRT monitor simultaneously is LCD monitor, which is not installed as a presentation computer.

Is there any software-wise way to reduce, or stabilize the number of TTL signals on CRT monitor?

Under the same presentation condition (e.g. presenting the light sensor patch for 10 msec), there are varied results for different events despite the fact that they are exactly identically programmed (e.g. for some events, one TTL signal is recorded while for other events, three consecutive TTL signals are recorded and yet for other events, no TTL signal is recorded)

No, I’m afraid that there is no way to decouple the light sensor patch duration from the stimulus duration.

As for the other artifacts, it’s hard to tell without me being physically present and troubleshooting your setup. Try looking at the brightness of the monitor; this has been a factor in the past.

I’ll try altering the brightness.
Thank you very much anyway.

You can also get what you want by displaying your own light sensor patch instead of using the built-in feature. This would give you more control.

You would need a trial structure like this:

  • Event 1: show a shape in a corner, say lower-right corner. This would be your own sensor patch and should have “Keep invisible” and “End immediately” options turned on.
  • Event 2: show the main stimulus. This event should have “Erase screen” option turned off so that event 1 appears at the same time with it. For event 2, set the time limit to something small, e.g. 10 milliseconds.
  • Event 3: show a “mask-patch”, which will be drawn OVER TOP of the sensor patch. This event should not erase the screen (or else it would erase the main stim from event 2). It must simply draw a white square (or background-colored square) that will wipe out the sensor patch. This event should “end immediately", but NOT be kept invisible. At this point, the main stim is on screen, but the patch was erased very quickly.
  • Event 4 is a blank text event that does not erase anything. This is the “get response” event.

Thank you very much for a possible solution.
I will definitely try those steps in the near future when I have an access to the experiment room.