Specify a video trial with button response

The trial_video_button_response function is used to play a video stimulus and collect a response using on screen buttons.

trial_video_button_response(
  sources,
  trial_ends_after_video = FALSE,
  width = NULL,
  height = NULL,
  autoplay = TRUE,
  controls = FALSE,
  start = NULL,
  stop = NULL,
  rate = 1,
  choices = c("button 0", "button 1", "button 2"),
  button_html = NULL,
  margin_vertical = "0px",
  margin_horizontal = "8px",
  prompt = NULL,
  trial_duration = NULL,
  response_ends_trial = TRUE,
  post_trial_gap = 0,
  on_finish = NULL,
  on_load = NULL,
  data = NULL
)

Arguments

sources	Path(s) to the video file. Videos may be specified in multiple formats (e.g., .mp4, .ogg, .webm)
trial_ends_after_video	If TRUE the trial will end as soon as the video finishes playing.
width	The width of the video display in pixels (if NULL, natural width is used)
height	The height of the video display in pixels (if NULL, natural height is used)
autoplay	Does the video play automatically?
controls	Should the video controls be shown?
start	Time point in seconds to start video (NULL starts at the beginning)
stop	Time point in seconds to stop video (NULL stops at the end)
rate	What rate to play the video (1 = normal, <1 slower, >1 faster)
choices	Labels for the buttons. Each element of the character vector will generate a different button.
button_html	A template of HTML for generating the button elements (see details)
margin_vertical	Vertical margin of the buttons
margin_horizontal	Horizontal margin of the buttons
prompt	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).
trial_duration	How long to wait for a response before ending trial in milliseconds. If NULL, the trial will wait indefinitely. If no response is made before the deadline is reached, the response will be recorded as NULL.
response_ends_trial	If TRUE, then the trial will end when a response is made (or the trial_duration expires). If FALSE, the trial continues until the deadline expires.
post_trial_gap	The gap in milliseconds between the current trial and the next trial. If NULL, there will be no gap.
on_finish	A javascript callback function to execute when the trial finishes
on_load	A javascript callback function to execute when the trial begins, before any loading has occurred
data	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

The trial_video_button_response function belongs to the "stimulus-response" family of trials, all of which display a stimulus of a particular type (image, audio, video or HTML) and collect responses using a particular mechanism (button, keyboard or slider). This one plays a video and records responses generated with a button click.

Stimulus display

TBA

Response mechanism

The response buttons can be customized using HTML formatting, via the button_html argument. This argument allows the user to specify an HTML used to generating the button elements. If this argument is a vector of the same length as choices then the i-th element of button_html will be used to define the i-th response button. If button_html is a single string then the same template will be applied to every button. The templating is defined using a special string "%choice%" that will be replaced by the corresponding element of the choices vector. By default the jsPsych library creates an HTML button of class "jspsych-btn" and the styling is governed by the corresponding CSS.

Other behaviour

Depending on parameter settings, the trial can end when the subject responds (response_ends_trial = TRUE), or after a fixed amount of time (specified using the trial_duration argument) has elapsed. The trial can also be made to end automatically at the end of the video using the trial_ends_after_video argument.

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 rt value is the response time in milliseconds taken for the user to make a response. The time is measured from when the stimulus first appears on the screen until the response.
The button_pressed variable is a numeric value indicating which button was pressed. The first button in the choices array is recorded as value 0, the second is value 1, and so on.
The stimulus variable records a JSON encoding of the sources array.

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.