How is timing measured in Superlab?

[ChrisM]

I have attached a small test experiment and recorded data that was used to check the accuracy of Slab timing using the Blackboxtoolkit system (www.blackboxtoolkit.co.uk), BBTK for short. Essentially, 4 images were presented and for each one a reaction time was recorded when a button on an RB830 button box was pressed.
My problem is with the reaction times recorded with SuperLab compared to the actual times measured with the BBTK.
I was expecting to see a difference, but a constant difference. However, it varies with each stimulus presentation and my initial thought is that SuperLab may not be synchronising the button box timer start with the screen refresh.
I know that a TFT panel has quite slow response times that must be considered, but this is a constant for any one experiment.
I also know that the the threshold level of the opto-detectors is manually set and will introduce an error in itself, but again this is a constant for any one experiment.
In short, how does the timing work when using images and a button box, and why do I get this variability?

[Hisham]

Hi Chris,

I am familiar with the BBTK and know Richard Plant well.

With SuperLab 4.0, stimulus presentations are synchronized with the refresh rate of the screen on Windows XP and 2000, and on Mac OS 10.4, but not on Mac OS 10.3.9.

The only thing that might explain the variability is the response pads' USB drivers. We'll be checking on that very thoroughly but we're waiting another few weeks for our own StimTracker (http://www.cedrus.com/stimtracker/) product prototypes to be ready. One key difference with the BBTK is that StimTracker will do its own time stamping in hardware with a 50 microsecond resolution vs. BBTK doing it in the computer itself (Richard is well aware of this upcoming product and has even seen the front panel's design :)).

Hisham.

[ChrisM]

Thanks Hisham,
I am still left with this timing dilemma though and BBTK itself seems to add to the confusion. I am sending event pulses from SuperLab to a BrainVision EEG system to define the EEG recording epochs so I need more accuracy and consistancy than I can currently achieve. As far as I can see the acquired EEG data may not correlate with the timings defined by Superlab and would not be reliable enough for research purposes. I hope you can de-mystify SupeLab timing for me in due course.