The trial_animation function is used to display a sequence of images at a fixed rate.

trial_animation(
stimuli,
frame_time = 250,
frame_isi = 0,
sequence_reps = 1,
choices = respond_any_key(),
prompt = NULL,
post_trial_gap = 0,
on_finish = NULL,
data = NULL
)

## Arguments

stimuli A vector of paths to the image files How long to display each image, in milliseconds How long is the gap between images, in milliseconds How many times to repeat the sequence A character vector of keycodes (either numeric values or the characters themselves). Alternatively, respond_any_key() and respond_no_key() can be used A string (may contain HTML) that will be displayed below the stimulus, intended as a reminder about the actions to take (e.g., which key to press). The gap in milliseconds between the current trial and the next trial. If NULL, there will be no gap. A javascript callback function to execute when the trial finishes A javascript callback function to execute when the trial begins, before any loading has occurred An object containing additional data to store for the trial

## Value

Functions with a trial_ prefix always return a "trial" object. A trial object is simply a list containing the input arguments, with NULL elements removed. Logical values in the input (TRUE and FALSE) are transformed to character vectors "true" and "false" and are specified to be objects of class "json", ensuring that they will be written to file as the javascript logicals, true and false.

## Details

This function is used to specify an "animation" trial in a jsPsych experiment. An animation trial displays a sequence of images at a fixed frame rate and the sequence can be looped a specified number of times. The participant is free to respond at any point during the animation, and the time of the response is recorded.

### Stimulus display

The only required argument is stimulus, which should be a vector of paths to the image files, one per frame. The file paths should refer to the locations of the image files at the time the experiment is *deployed*, so it is often convenient to use the insert_resource function to construct these file paths automatically. The images will be displayed in the order that they appear in the stimulus vector.

The behaviour of an animation trial can be customised in various ways. The frame_time parameter specifies the length of time (in milliseconds) that each image stays on screen, and the frame_isi parameter controls the inter-stimulus interval (that is, the gap between successive images) during which a blank screen is shown. The sequence_reps argument specifies the number of times the sequence repeats.

### Response mechanism

Because animation trials typically require precise timing, they are designed to accept key press responses only, and choices argument is used to control which keys will register a valid response. The default value choices = respond_any_key() is to allow the participant to press any key to register their response. Alternatively it is possible to set choices = respond_no_key(), which prevents all keys from registering a response: this can be useful if the trial is designed to run for a fixed duration, regardless of what the participant presses.

In many situations it is preferable to require the participant to respond using specific keys (e.g., for a binary choice tasks, it may be desirable to require participants to press F for one response or J for the other). This can be achieved in two ways. One possibility is to use a character vector as input (e.g., choices = c("f","j")). The other is to use the numeric code that specifies the desired key in javascript, which in this case would be choices = c(70, 74). To make it a little easier to work with numeric codes, the jaysire package includes the keycode() function to make it easier to convert from one format to the other.

### Other behaviour

The prompt argument is used to specify text that remains on screen while the animation displays. The intended use is to remind participants of the valid response keys, but it allows HTML markup to be included and so can be used for more general purposes.

Like all functions in the trial_ family it contains four additional arguments:

• The post_trial_gap argument is a numeric value specifying the length of the pause between the current trial ending and the next one beginning. This parameter overrides any default values defined using the build_experiment function, and a blank screen is displayed during this gap period.

• The on_load and on_finish arguments can be used to specify javascript functions that will execute before the trial begins or after it ends. The javascript code can be written manually and inserted *as* javascript by using the insert_javascript function. However, the fn_ family of functions supplies a variety of functions that may be useful in many cases.

• The data argument can be used to insert custom data values into the jsPsych data storage for this trial

### Data

When this function is called from R it returns the trial object that will later be inserted into the experiment when build_experiment is called. However, when the trial runs as part of the experiment it returns values that are recorded in the jsPsych data store and eventually form part of the data set for the experiment.

The data recorded by this trial is as follows:

• The animation_sequence value is an array encoded in JSON format. Each element of the array is an object that represents a stimulus in the animation sequence. Each object has a stimulus property, which is the image that was displayed, and a time property, which is the time in ms, measured from when the sequence began, that the stimulus was displayed.

• The responses value is an array encoded in JSON format. Each element of the array is an object representing a response given by the subject. Each object has a stimulus property, indicating which image was displayed when the key was pressed, an rt property, indicating the time of the key press relative to the start of the animation, and a key_press property, indicating which key was pressed.

In addition, it records default variables that are recorded by all trials:

• trial_type is a string that records the name of the plugin used to run the trial.

• trial_index is a number that records the index of the current trial across the whole experiment.

• time_elapsed counts the number of milliseconds since the start of the experiment when the trial ended.

• internal_node_id is a string identifier for the current "node" in the timeline.