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Assessing stimulus presentation accuracy in E-Prime

Validating your experiment timing is a key step in launching a new study or reactivating an older one. To assist you with this step, we want to share with you the results of our recent testing of Net Station 5.4 with E­ Prime 2.0 so you will have a baseline for evaluating the timing accuracy you see on your system when you upgrade.

In a nutshell, these timing tests allow us to record a file that will allow the comparison between:

1. The time when E­-Prime states that the stimulus is presented.

2. The time that our AV Device physically detects the presentation of the stimulus.

We know that the beauty of Event-­Related Potentials is their exquisite temporal resolution, and so by quantifying the expected stimulus presentation offset through Timing Tests, we can have confidence in the timing characteristics of our experiment and take account of this offset during signal processing. 

The calculation of the stimulus presentation offset is achieved by running your file through the Event Timing Tester, which you can find in your EGI Utilities folder. The output is shown below, and in this example we are comparing the stimulus event code associated with a visual stimuli and the DIN generated by the AV Device’s photocell. When reviewing these results we know that there is going to be an offset present — that in itself is not a problem — but rather we are looking for two characteristics: 

     1. That offset values do not drift across the experiment. We judge this in two ways: 

          -We review the Maximum and Minimum offset values to gauge whether there are any outliers within the data set. We want to see these values be tightly centered around the Average offset.

          -Then, by scrolling through all the offset values recorded, we check to see how consistent the offsets values are across the recording. We want to see that the values remain constant and not show any indication of drift across the timing test. 

     2. That 99% of offset values are within +/­3 ms of the average offset value.

We configure our E-­Prime computers so that we can achieve stimulus presentation accuracy of +/­ 3 ms in 99% trials. This means that, in 99% of the trials within the timing test, the stimulus offset recorded will be within +/­ 3 ms of the Average offset value. As you can see in the example shown, which was taken from a timing test run in our Quality Department during our Net Station 5.3 regression testing, we have achieved this criteria: in 82.83% of trials there is no variation around the Average, and in 17.17% of trials the offset value is within +/­ 1 ms of the Average offset value. The reason why we are able to control stimulus presentation timing to this extent is that we use Network Timing Protocol (NTP) in order to synchronize the two computers — your E-­Prime PC and your Net Station Acquisition Mac — and therefore have more control over timing. When NTP is not used, for example when PsychToolBox uses our ECI, the stimulus presentation accuracy achieved is that 97% of offset values are within +/­ 5 ms of the Average offset value. 

13 Timing Tester Results


Additional Info

  • Product Type: E-Prime
  • Information Type: Application Advice
Last modified on Wednesday, 07 June 2017 21:32

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