# Intertrial Interval (ITI)

Article by Ben Howell Figure 1. Intertrial Interval

Intertrial Interval is defined as the duration of time between the onset of one trial and the onset of the next trial. In (Figure 1) the duration between the onset of the stimulus `S1` in trial `T1`and onset of the stimulus `S1` in the next trial `T2` is the Intertrial Interval. The ITI therefore measures the entire duration from the start of one trial to the start of the next trial, including the duration of fixations, stimuli, responses and the like. Intertrial Intervals are commonly used as an experimental factor in classical conditioning experiments.

## Intertrial Interval in Psychstudio

### Method 1 (implicit)

To introduce padding to the start of a trial (thus extending the overall ITI), we can delay the onset of the first construct in the trials timeline by setting a start time greater than zero. Note: padding can also be introduced at the end of the timeline but this is not often possible if we have constructs with Infinite duration (i.e. items that end upon some event, such as a response key press).

In the example shown in (Figure 2) we can calculate the ITI using the following formula:
`Fixation onset delay` (20ms) + `Fixation duration` (20ms) + `S1 duration` (20ms) + `S2 duration` (20ms) + `Response key duration where duration > 20ms`. Figure 2. Intertrial Interval (implicit) (Psychstudio trials timeline)

### Method 2 (explicit)

In the example shown in (Figure 3), we have added an interval construct `Onset interval` to the timeline to explicitly define the onset delay.

In the example shown in (Figure 3) we can calculate the ITI using the following formula:
`Onset interval` (20ms) + `Fixation duration` (20ms) + `S1 duration` (20ms) + `S2 duration` (20ms) + `Response key duration where duration > 20ms`.

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. Intertrial Interval (explicit) (Psychstudio trials timeline) Figure 4. Editor (Psychstudio trials timeline)