Reason for Inter-stimulus-Interval in psychology studies

Question

In many of the studies that I have read or participated in, I have seen that they show a blank screen between two stimuli. For example, in a study by Cohen et al. (2017), a training paradigm was used where a flanker task was shown, followed by a blank screen for (1000 - Reaction Time) ms, followed by an emotional or neutral image. I've cited this study just as an example, but I've seen similar blank screens in many other studies. My question is:

What is the reason for showing the blank screen? What would have happened if they showed the emotional/neutral image right after the flanker task?

One reason I can think of is that it is shown to keep the starting point of different stimulus constant between different participants. Is there any other reason?

Answer 1

If you've ever tried doing an experiment with no inter-stimulus interval (ISI), you know from first-hand experience that it makes a difference in several ways:

1. If the task is about reaction times, you don't really have time to prepare. I.e. there is an attention-based interference effect. For Flanker tasks, Stroop tasks, go-nogo tasks, etc., you want to measure the optimal performance and hence you add ISI to reduce the interference. In other tasks, you set a short ISI exactly because of the interference effect, e.g. in some conditions of Conner's Continuous Performance Test, on Vigilance tasks to keep challenging attention (see e.g.McCormack, 1958), etc. Low ISIs are also quite useful for Task-Induced Stress.

2. There are other carry-over effects than a reduced/biased attention. For example, repetition priming is substantially reduced for longer ISI's (e.g., Ferrand, 1996). If you are not studying repetition priming, such effects constitute noise or a confounder, so you want to reduce it by having reasonable ISIs.

3. With ISI=0, the next stimulus appears to be caused by your response, so it feels like your key stroke "belongs" to the new stimulus rather than the previous. This can be confusing.

Answer 2

I cannot access the paper due to the paywall, and admitteldy, I am not familiar with the Flanker Task, but I am with the closely related Stroop task. Anyway, the between-trial times may substantially affect the results, because the conflict effect of flanker interference decreases with repeated testing, especially following incongruent/conflict conditions in what is known as the Gratton Effect (Blais et al., 2014). Hence, the method of the cited study (1 second - reaction time) standardizes the inter-stimulus interval, which is apparently a major player in subject performance in a Flanker task (source: Wiki page).

<sub>Reference
- Blais et al., Front Psychol (2014); 5: 1207)</sub>