Interstimulus Interval (ISI)

Article by Ben Howell

Figure 1. Interstimulus Interval

Interstimulus Interval is defined as the duration of time between the offset of one stimulus and the onset of another stimulus. In (Figure 1) the duration between the offset of the stimulus S1 and onset of the stimulus S2 is the Interstimulus Interval. Interstimulus Intervals are commonly used as an experimental factor in priming experiments.

Interstimulus Interval in Psychstudio

Method 1 (implicit)

In the example shown in (Figure 2) we have a trials timeline containing an Interstimulus Interval of of 20ms as the difference between the finish time of S1 (60ms) and the start time of S2 (80ms) is 20ms. Setting stimulus start time and duration is done in the trials timeline editor as shown in (Figure 4).

Method 2 (explicit)

In the example shown in (Figure 3) we have a trials timeline containing an Interstimulus Interval of of 20ms as the difference between the finish time of S1 (60ms) and the start time of S2 (80ms) is 20ms. Additionally, we have placed an interval construct ISI on the timeline to explicitly define the interval.

Using an explicit interval construct provides the following benefits:

• explicit capture of start time, end time and (actual) duration.
• ability to offset the start of the interval in relation to the ending of another item on the trials timeline.
• variable duration intervals.
• intervals defined by factors and levels.