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<p>Would the minimum delay between stimulus presentation and trigger recording not be the most appropriate value to correct the jitter to? The delay between transmission of a trigger by a stimulus presentation machine and recording of the trigger by the recording
machine can be modeled as having a constant lag, plus variable jitter. The constant lag is difficult to quantify absolutely, but in relative terms it should be close to the minimum difference between timestamps for the same events from two machines. Using
the average difference seems to me to also add the average of the jitter, which doesn't make sense given that there's no way that the trigger will arrive at the recording machine before it is transmitted by the stimulus presentation machine.</p>
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<p>Matt <br>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> eeglablist-bounces@sccn.ucsd.edu <eeglablist-bounces@sccn.ucsd.edu> on behalf of Steven J Luck <sjluck@ucdavis.edu><br>
<b>Sent:</b> Thursday, September 22, 2016 10:00 AM<br>
<b>To:</b> RICHARDS, JOHN<br>
<b>Cc:</b> EEGLAB List; chao wang; support@egi.com; erplab@ucdavis.edu Listserv; Colin Noe<br>
<b>Subject:</b> Re: [Eeglablist] [erplab] About stimulus presentation delay</font>
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<div>Hi everybody. We are about to release a new version of ERPLAB that contains a routine for shifting event codes to deal with these delays. Of course, you’ll need to measure the delays, but then it will be easy to shift the codes.
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<div class="">On Sep 21, 2016, at 10:04 AM, RICHARDS, JOHN <<a href="mailto:RICHARDS@mailbox.sc.edu" class="">RICHARDS@mailbox.sc.edu</a>> wrote:</div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">By the way, I am sure most of you must be familiar with the lag issue that EGI had with its Netstation amp. An older version of the firmware in the amp was incorrectly calculating
the lo-pass filter and all the points were lagged by a significant amount. This did not show up in the EPrime-stimulus delay, but in the EGI-EPrime synch times. The delay was significant re ERP component timing, so in this case any data that were collected
with this machine should be adjusted. Several people I know had moved from the older version to the newer version, and so had to adjust the delay to some data and not others.</span></div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">I have cc’ed the support at EGI, and they can comment on this if they wish. Also you can find info about this, I googled “EGI latency problem” and found a discussion of this,
<a href="https://sccn.ucsd.edu/pipermail/eeglablist/2015/010427.html" class="">https://sccn.ucsd.edu/pipermail/eeglablist/2015/010427.html</a> . This has been extensively discussed elsewhere also.</span></div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">John
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<div class="MsoNormal"><span class="" style="font-size:10pt; font-family:Consolas">***********************************************<br class="">
John E. Richards<br class="">
Carolina Distinguished Professor<br class="">
Department of Psychology<br class="">
University of South Carolina<br class="">
Columbia, SC 29208<br class="">
Dept Phone: 803 777 2079<br class="">
Fax: 803 777 9558<br class="">
Email: </span><u class=""><span class="" style="font-size:10.0pt; font-family:Consolas; color:blue"><a href="" class="">richards-john@sc.edu</a><br class="">
</span></u><span class="" style="font-size:10pt; font-family:Consolas">HTTP: <a href="http://jerlab.psych.sc.edu" class="">
jerlab.psych.sc.edu</a><br class="">
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<div class="MsoNormal"><b class=""><span class="" style="font-family:Calibri">From:
</span></b><span class="" style="font-family:Calibri"><<a href="mailto:erplab-request@ucdavis.edu" class="">erplab-request@ucdavis.edu</a>> on behalf of John Richards <<a href="mailto:RICHARDS@mailbox.sc.edu" class="">RICHARDS@mailbox.sc.edu</a>><br class="">
<b class="">Date: </b>Wednesday, September 21, 2016 at 11:10 AM<br class="">
<b class="">To: </b>Colin Noe <<a href="mailto:cmn2rice@gmail.com" class="">cmn2rice@gmail.com</a>>, chao wang <<a href="mailto:hnchaowang@gmail.com" class="">hnchaowang@gmail.com</a>><br class="">
<b class="">Cc: </b>EEGLAB List <<a href="mailto:eeglablist@sccn.ucsd.edu" class="">eeglablist@sccn.ucsd.edu</a>>, "<a href="mailto:erplab@ucdavis.edu" class="">erplab@ucdavis.edu</a>" <<a href="mailto:erplab@ucdavis.edu" class="">erplab@ucdavis.edu</a>><br class="">
<b class="">Subject: </b>Re: [erplab] About stimulus presentation delay</span></div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">Nearly everyone has a delay between the EPrime time and the time the stimulus reaches the screen. It should be measured. The latency depends largely on the method that your
computer uses to send a figure in memory to the display (< 1 frame) and the latency of the display. If you are using a “projector” then this may be a long and variable time. A “scope” is nearly instantaneous. We use a computer monitor with a fast display
rate, and our lags are < ½ video frame. For audio, we use the recommended EPRime audio code, and our delays are about 3 ms with almost no variability.</span></div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">If you have a constant or fairly constant lag, AND, if you are very concerned about using latency measures as a DV, then you should definitely adjust. Otherwise, adjusting
depends on the resolution of your ERP components. We study the “Nc”, which is a rather slow-wave component in infants lasting from 350 to 700 ms. So being off by < 1 frame (or even several frames) will not affect the mean or max ERP measures. We also study
the “N290” (an infant analog of the N170). In this case the latency of the component is a DV in many studies, so adjusting is important. The peak latency of the N290 is fairly variable across participants, so we could do the ERP analysis of peak ERP at peak
latency (and mean around peak, or mean from nn to nn ms) and not be too badly off with no delay adjustment. But since we want latency meausres also, we adjust.</span></div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">If you have an extremely variable lag, say greater than the resolution of the ERP component you want to study, then you are out of luck. The avg latency lag will be the latency
lag for NO stimulus presentation, and the ERP will be noisy. If the variablty of the stimulus lag is too great re your EEG/ERP analysis resolution, then you should fix your display.</span></div>
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<div class="MsoNormal"><span class="" style="font-size:11.0pt; font-family:Calibri">John</span></div>
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<div class="MsoNormal"><span class="" style="font-size:10pt; font-family:Consolas">***********************************************<br class="">
John E. Richards<br class="">
Carolina Distinguished Professor<br class="">
Department of Psychology<br class="">
University of South Carolina<br class="">
Columbia, SC 29208<br class="">
Dept Phone: 803 777 2079<br class="">
Fax: 803 777 9558<br class="">
Email: </span><u class=""><span class="" style="font-size:10.0pt; font-family:Consolas; color:blue"><a href="" class="">richards-john@sc.edu</a><br class="">
</span></u><span class="" style="font-size:10pt; font-family:Consolas">HTTP: <a href="http://jerlab.psych.sc.edu" class="">
jerlab.psych.sc.edu</a><br class="">
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<div class="MsoNormal"><b class=""><span class="" style="font-family:Calibri">From:
</span></b><span class="" style="font-family:Calibri"><<a href="mailto:erplab-request@ucdavis.edu" class="">erplab-request@ucdavis.edu</a>> on behalf of Colin Noe <<a href="mailto:cmn2rice@gmail.com" class="">cmn2rice@gmail.com</a>><br class="">
<b class="">Date: </b>Wednesday, September 21, 2016 at 10:49 AM<br class="">
<b class="">To: </b>chao wang <<a href="mailto:hnchaowang@gmail.com" class="">hnchaowang@gmail.com</a>><br class="">
<b class="">Cc: </b>EEGLAB List <<a href="mailto:eeglablist@sccn.ucsd.edu" class="">eeglablist@sccn.ucsd.edu</a>>, "<a href="mailto:erplab@ucdavis.edu" class="">erplab@ucdavis.edu</a>" <<a href="mailto:erplab@ucdavis.edu" class="">erplab@ucdavis.edu</a>><br class="">
<b class="">Subject: </b>Re: [erplab] About stimulus presentation delay</span></div>
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<div class="MsoNormal">I recognized the issue ahead of time, and programmed a delay into my marker code so that it lined up. There is a way to move marks in the EEG though.
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<div class="MsoNormal">As for acceptability, that is a question for the journal editors and experts in this forum.</div>
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<div class="MsoNormal">On Tue, Sep 20, 2016 at 9:12 PM chao wang <<a href="mailto:hnchaowang@gmail.com" class="">hnchaowang@gmail.com</a>> wrote:</div>
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<div class="MsoNormal">Thanks for your quick reply! So, how do you segment data with respect to stimulus onset?</div>
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<div class="MsoNormal">My delay is a little complex, about 60% ~ 75% are within the average +/- 5 ms. I want to use the average value to segment EEG data or shift ERP component latency. But I am not sure if this is acceptable in ERP analysis? </div>
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<div class="MsoNormal">Thanks!</div>
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<div class="MsoNormal">On Wed, Sep 21, 2016 at 9:58 AM, Colin Noe <<a href="mailto:cmn2rice@gmail.com" target="_blank" class="">cmn2rice@gmail.com</a>> wrote:</div>
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<div class="MsoNormal">I have a jitter of +- 3 ms with mine (along with a 50 ms delay), which I assume is better than most (who likely have not checked timing).</div>
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<div class="MsoNormal">On Tue, Sep 20, 2016 at 8:40 PM chao wang <<a href="mailto:hnchaowang@gmail.com" target="_blank" class="">hnchaowang@gmail.com</a>> wrote:</div>
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<div class="MsoNormal">We're recording EEG data and we use E-Prime to present stimuli. During recording, stimuli are presented to subjects via a projector. The projector induces a delay between stimulus presentation on E-Prime computer and in front of subjects.
We measured this delay and found that the delay was not constant across trials but instead jitters within certain range around the average. I would like to know is there a typical range of the jitter that can be accepted in ERP experiments? And in this situation,
what should I do to segment EEG data with respect to stimulus onset? I mean, use the average delay to determine the real instant of stimulus presentation, or any other ways exist? Forgive me if my words is not clear :)</div>
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<div class="MsoNormal">Thank you in advance!</div>
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<div class="MsoNormal">Chao</div>
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