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Important Note

An updated version of the RDK is now available at

(1) Github:
- The new RDK is integrated into the latest jsPsych 6.0.3 library, which can be found here.
- The updated documentation is here.

(2) Codepen:
- This is the updated raw code

The updated version features:

  • Customizable border around the aperture
  • Multiple independently customizable apertures at different locations on screen
  • Parameter for dots moving in the opposite direction of the coherent dots (oppositeCoherence)
  • Decreased lag time between frames

jspsych-RDK plugin

This plugin displays a Random Dot Kinematogram (RDK) and allows the subject to report the primary direction of motion by pressing a key on the keyboard. The stimulus is displayed until a keyboard response is given. The trial can be ended automatically if the subject has failed to respond within a fixed length of time.

For optimal performance, fullscreen mode should be manually triggered by the user (e.g. F11 key in Chrome for Windows). Usage of the default Fullscreen trigger from the JsPsych API library with this plugin might result in the stimuli being displayed incorrectly.


This table lists the parameters associated with this plugin. Parameters can be left unspecified if the default value is acceptable.

Parameter Type Default Value Descripton
choices array [] The valid keys that the subject can press as a response. Must be an array of chars. If left unspecified, any key is a valid key.
correct_choice array or char undefined The keys that are considered the correct response for that particular trial. Can be a single char or an array of chars (not ASCII values). This needs to be linked with the coherent_direction parameter (See Examples section below for an illustration.) This is used to determine whether the subject chose the correct response and return that data.
trial_duration numeric 500 The amount of time that the stimulus is displayed on the screen in ms. If -1, the stimulus will be displayed until the subject keys in a valid response. (choices parameter must contain valid keys or else the stimuli will run indefinitely).
response_ends_trial boolean true If true, then the subject's response will end the trial. If false, the stimuli will be presented for the full trial_duration (the response will be recorded as long as the subject responds within the trial duration).
number_of_dots numeric 300 Number of dots per set. Equivalent to number of dots per frame.
number_of_sets numeric 1 Number of sets to cycle through. Each frame displays one set of dots. (E.g. If 2 sets of dots, frame 1 will display dots from set 1, frame 2 will display dots from set 2, frame 3 will display sets from set 1, etc.)
coherent_direction numeric 0 The direction of movement for coherent dots in degrees. 0 degrees is in the 3 o'clock direction, and increasing this number moves counterclockwise. (E.g. 12 o'clock is 90, 9 o'clock is 180, etc.) Range is 0 - 360.
coherence numeric 0.5 The proportion of dots that move together in the coherent direction. Range is 0 - 1.
dot_radius numeric 2 The radius of each individual dot in pixels.
dot_life numeric -1 The number of frames that pass before a dot disappears and reappears in a new frame. -1 denotes that the dot life is infinite (i.e., a dot will only disappear and reappear if it moves out of the aperture).
move_distance numeric 1 The number of pixel lengths the dot will move in each frame (analogous to speed of dots).
aperture_width numeric 600 The width of the aperture in pixels. For a square aperture, this will determine both the width and height. For circular aperture, this will determine the diameter.
aperture_height numeric 400 The height of the aperture in pixels. For square and circle apertures, this will be ignored.
dot_color string "white" The color of the dots.
background_color string "gray" The color of the background.
RDK_type numeric 3 The Signal Selection Rule (Same/Different) and Noise Type (Random Position/Walk/Direction):

1 - Same && Random Position
2 - Same && Random Walk
3 - Same && Random Direction
4 - Different && Random Position
5 - Different && Random Walk
6 - Different && Random Direction

(See 'RDK parameter' below for more detailed information)
aperture_type numeric 2 The shape of the aperture.

1 - Circle
2 - Ellipse
3 - Square
4 - Rectangle
reinsert_type numeric 2 The type of reinsertion of a dot that has gone out of bounds

1 - Randomly appear anywhere in the aperture
2 - Appear on the opposite edge of the aperture. For squares and rectangles, a random point on the opposite edge is chosen as the reinsertion point. For circles and ellipses, the exit point is reflected about center to become the reinsertion point.
aperture_center_x numeric window.innerWidth/2 The x-coordinate of the center of the aperture, in pixels.
aperture_center_y numeric window.innerHeight/2 The y-coordinate of the center of the aperture, in pixels.
fixation_cross boolean true Whether or not a fixation cross is presented in the middle of the screen.
fixation_cross_width numeric 20 The width of the fixation cross in pixels.
fixation_cross_height numeric 20 The height of the fixation cross in pixels.
fixation_cross_color string "black" The color of the fixation cross.
fixation_cross_thickness numeric 1 The thickness of the fixation cross in pixels.

RDK type parameter

** See Fig. 1 in Scase, Braddick, and Raymond (1996) for a visual depiction of these different signal selection rules and noise types.

Signal Selection rule:

-Same: Each dot is designated to be either a coherent dot (signal) or incoherent dot (noise) and will remain so throughout all frames in the display. Coherent dots will always move in the direction of coherent motion in all frames. -Different: Each dot can be either a coherent dot (signal) or incoherent dot (noise) and will be designated randomly (weighted based on the coherence level) at each frame. Only the dots that are designated to be coherent dots will move in the direction of coherent motion, but only in that frame. In the next frame, each dot will be designated randomly again on whether it is a coherent or incoherent dot.

Noise Type:

-Random position: The incoherent dots appear in a random location in the aperture in each frame.
-Random walk: The incoherent dots will move in a random direction (designated randomly in each frame) in each frame.
-Random direction: Each incoherent dot has its own alternative direction of motion (designated randomly at the beginning of the trial), and moves in that direction in each frame.

Data Generated

In addition to the default data collected by all plugins, this plugin collects all parameter data described above and additional data described below for each trial.

Name Type Value
rt numeric The response time in ms for the subject to make a response.
key_press numeric The key that the subject pressed. The value corresponds to the Javascript Char Code (Key Code).
correct boolean Whether or not the subject's key press corresponded to those provided in correct_choice.
frame_rate numeric The average frame rate for the trial. 0 denotes that the subject responded before the appearance of the second frame.
number_of_frames numeric The number of frames that was shown in this trial.
frame_rate_array JSON string The array that holds the number of miliseconds for each frame in this trial.


Setting the correct_choice parameter by linking it to the coherent_direction parameter:

var trial_right = {
	coherent_direction: 0,
	correct_choice: "P"

var trial_left = {
	coherent_direction: 180,
	correct_choice: "Q"

Displaying a trial with 2 choices and 1 correct choice

var test_block = {
	type: "jspsych-RDK", 
	timing_post_trial: 0,
	number_of_dots: 200,
	RDK_type: 3,
	choices: ["a", "l"],
	correct_choice: "a",
	coherent_direction: 180,
	trial_duration: 1000