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While going through the below-mentioned paper, I came across some plots which were said to be "locked" with reference to either response or stimulus. Does the locking refer to the initiation of the measurement with starting cue being the presentation of the stimulus or the response of the subject?

References

  • Philiastides, M. G., & Sajda, P. (2006). Neural Representation of Task Difficulty and Decision Making during Perceptual Categorization: A Timing Diagram
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Does the locking refer to the initiation of the measurement with starting cue being being the presentation of stimulus or the response of the subject?

More or less, yes.

When measuring brain activity, you usually make a long, continuous recording during which you expose your study participants to a task over and over again. There's a lot of noise in the recordings, so you need to average over many instances of a stimulus/task event to get an idea of what it does to the brain. To average, you need to cut the recording into trials (also called epochs).

You need to decide what to base these epochs on, i.e. what to cut around. Stimulus-locked measurements happen when you cut an epoch relative to the stimulus onset. The average over all trials will then give you a good impression of the sensory response to the stimulus.

However, there are some higher order cognitive processes going on as well, and they will happen at different speeds over trials and over different subjects, so they will smear out in the average. If you want to look at these, it's wiser to do response-locked measurements, i.e. to cut the epochs around the moment that the study participant responded to the task. This way, just prior to the button press (in the plot), you might get to see some evidence-accumulation or similar cognitive process going on. You can also see the motor response very well.

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I wanted to comment on @Ana answer but I don't have the reputation. I wanted to add to her answer by adding that if you are doing a literature review on this topic the terms target-locked, stimulus-locked and cue-locked are often used interchangeably. Example of components that are extracted from this form of locking is the N2 and P3. Whereas the two most common response-locked ERP components are the ERN (Gehring et al 1993) and Pe.

I also wanted to add that the majority of EEG systems (such as the dense array electroencephalographs from EGI) will take very precise measurements on when the stimulus is presented. Ergo, depending on the component you are trying to evaluate you will lock to either the stimulus or the response. When these programs get out of sync it can be an absolute nightmare to find the source of the minute error.

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  • $\begingroup$ Welcome to Psychology.SE. Can you please provide some details on Gehring et al. (1993)? $\endgroup$ – Chris Rogers Oct 17 '18 at 14:55
  • $\begingroup$ Hi Chris, Are there any particular details you are looking for? The original article by Gehring in 1993. In the 25 years since the original discovery, there have been substantial leaps in the understanding of the ERN. I would recommend this review $\endgroup$ – Patrick Oct 19 '18 at 3:23
  • $\begingroup$ When referencing a journal article or book, mentioning something like (Gehring, et al. 1993) means nothing. What's the title of the article? What journal can I find it in? Which volume and issue number? Is there a doi number I can use to go straight to it? All this will help the reader find the source to what you are referring to and do some background reading. - see this post for guidance $\endgroup$ – Chris Rogers Oct 19 '18 at 6:28
  • $\begingroup$ Hi Chris, not a problem. If you click on the link I provided the article from 93 is included. However, if you don't have time to read it here is the full information. The DOI is: doi.org/10.1111/j.1467-9280.1993.tb00586.x $\endgroup$ – Patrick Oct 19 '18 at 10:00

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