Stimtracker and Biopac - Timing?

Could do with some advice from others who are using stimtracker with Biopac.

We have recently purchased a Stimtracker and I am in the process of testing it out with our Biopac MP35 system with a view to using it to send event markers and by raising and lowering lines to mark onset and end and sending pulses to convey info about different types of stimuli rather than by using the photocell. However, I seem to be getting some small timing errors transmitted to Biopac (between 0.1 and 0.3 secs) when I mark the onset/ offset of the picture stimuli - this seems to be the case whether I use the photocell or the raise/ lower lines event option.

When just sending event markers however they seem to be the right length (with only a timing difference of perhaps 0.01 sec - see attached S/Lab file which shows how I am sending the markers).

Anyone come accross this? Is this likely to taking account of screen drawing time also or am I seeing something else? I have tried 2 different Biopacs and two different PCs (both for sending and recieving) and am still seeing the same anomolies. Any help gratefully recieved! What would be particularly useful would be an example file which someone has used to mark the display of stimuls which has bang on timing in Biopac.

just_stim_events.sl4 (5.72 KB)

Timing will definitely be better when sending a pulse during a single event rather than raising the lines in one event and lowering it in another. This is due to the extra “housekeeping” that SuperLab needs to do.

With StimTracker, I recommend using the photocell for best marking of stimulus onset, while sending markers via USB mainly to describe the trial condition.


Thank you for your reply, although I find it a little disappointing.

I am getting timing errors whether I use the photocell or the lines up/ down option for marking. The error is (as I said in first post) the same in both cases. Using a button box to output event markers via the accessory connector to a BIOPAC SS2L lead is proving to be a much more accurate solution.

Is it possible for me to get a copy of the files that you used to test stimtracker with Biopac and the Biopac template that you used? that way I will be able to investigate this issue further. It may be that this is an issue with BIOPAC MP35 recieving data via the 25pin I/O port in which case I’ll need to come up with another solution.

I think Stimtracker is potentially a great piece of kit but unless I can solve or work round these apparent timing issues we are going to have difficulty using it with any degree of confidence for either third year project students or in research situations.

There are no files to test StimTracker with Biopac. The timing tests were done using oscilloscopes.

Please see StimTracker’s specifications. The device is very accurate. In fact, we designed it so that the photocells’ input goes out as event markers without any intervention from the on-board microprocessor. The only outside “intervention” is the setting of the photocells’ threshold – which does not affect timing.

I don’t know where the delays are coming from. I’d suspect anything that is computer-based, e.g. USB drivers. But the StimTracker published specs are correct.

Thank you for your answer. I have tried using the photocells without the USB plugged into the PC, therefore eliminating any timing issues caused by USB, and I am still getting the same timing issues. Was the Stimtracker tested with Biopac MP35 prior to release as I’m wondering if that is where the errors is creeping in (at the MP35 end)? Although it seems odd that when I output just markers - with no attached image stimuli the timings are much more accurate.

When you scoped the output of the Stimtracker what type of lead did you use as I may try and replicate that so that I can try and track down the source of the timing issue I’m seeing. At the moment it’s difficult to pin down the issue as all the equipment I’m using is different to our standard setup for sending event markers! Normally we use the button box with lead on accessory connector going directly into channel 3 of an MP30 via a modified SSL2 lead and Superlab 4.07b - here we are using an MP35 and inputting to the 25 pin I/O port, using Stimtracker and Superlab 4.08 so while normally I would just change one thing at a time for testing purposes and track down the error that way that’s obviously impossible in this case! In fact, the only thing that has remained constant throughout are the PCs that I am using. Any help would be gratefully recieved.

The operation of the photocells is completely, 100% independent of USB. It will make no difference whether USB is plugged in or not.

When we tested StimTracker, we used an oscilloscope. We didn’t use SSL2 or other EEG-related leads. Our only concern was to capture the photocell input and put out an event marker in the shortest possible time. Once the event marker leaves StimTracker’s connector, it’s beyond our control.

How did you get the timing info from the Stimtracker into the scope as I presume scope uses a standard BNC input? Did you wire the various pins out to BNCs and test taht way? What I’m hoping here is that I can replicate the back on timing via a scope and therefore will at least have eliminated that from teh equation and will have to look at Biopac…

Anyway, you’ve e-mailed me over this suggesting we discuss on the phone… I’ll mail you back with some possible times and we can take it from there.

I didn’t test it personally. The information is gathered from the on-board chips that are used (all with well known and fast specs), and then tested by our electronics engineer.