# 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).

Figure 2. Interstimulus Interval (implicit) (Psychstudio trials timeline)

### 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.
See the interval documentation for more detail.

Figure 3. Interstimulus Interval (explicit) (Psychstudio trials timeline)

Figure 4. Editor (Psychstudio trials timeline)

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Ben Howell
Founder, Psychstudio