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This plugin can be used to draw a stimulus on a HTML canvas element and record a keyboard response. The canvas stimulus can be useful for displaying dynamic, parametrically-defined graphics, and for controlling the positioning of multiple graphical elements (shapes, text, images). The stimulus can be displayed until a response is given, or for a pre-determined amount of time. The trial can be ended automatically if the subject has failed to respond within a fixed length of time.


In addition to the parameters available in all plugins, this plugin accepts the following parameters. Parameters with a default value of undefined must be specified. Other parameters can be left unspecified if the default value is acceptable.

Parameter Type Default Value Description
stimulus function undefined The function to draw on the canvas. This function automatically takes a canvas element as its only argument, e.g. function(c) {...} or function drawStim(c) {...}, where c refers to the canvas element. Note that the stimulus function will still generally need to set the correct context itself, using a line like let ctx = c.getContext("2d").
canvas_size array [500, 500] Array that defines the size of the canvas element in pixels. First value is height, second value is width.
choices array of strings jsPsych.ALL_KEYS This array contains the key(s) that the subject is allowed to press in order to respond to the stimulus. Keys should be specified as characters (e.g., 'a', 'q', ' ', 'Enter', 'ArrowDown') - see this page and this page (event.key column) for more examples. Any key presses that are not listed in the array will be ignored. The default value of jsPsych.ALL_KEYS means that all keys will be accepted as valid responses. Specifying jsPsych.NO_KEYS will mean that no responses are allowed.
prompt string null This string can contain HTML markup. Any content here will be displayed below the stimulus. The intention is that it can be used to provide a reminder about the action the subject is supposed to take (e.g., which key to press).
stimulus_duration numeric null How long to display the stimulus in milliseconds. The visibility CSS property of the stimulus will be set to hidden after this time has elapsed. If this is null, then the stimulus will remain visible until the trial ends.
trial_duration numeric null How long to wait for the subject to make a response before ending the trial in milliseconds. If the subject fails to make a response before this timer is reached, the subject's response will be recorded as null for the trial and the trial will end. If the value of this parameter is null, then the trial will wait for a response indefinitely.
response_ends_trial boolean true If true, then the trial will end whenever the subject makes a response (assuming they make their response before the cutoff specified by the trial_duration parameter). If false, then the trial will continue until the value for trial_duration is reached. You can use this parameter to force the subject to view a stimulus for a fixed amount of time, even if they respond before the time is complete.

Data Generated

In addition to the default data collected by all plugins, this plugin collects the following data for each trial.

Name Type Value
response string Indicates which key the subject pressed.
rt numeric The response time in milliseconds for the subject to make a response. The time is measured from when the stimulus first appears on the screen until the subject's response.

Note: the canvas stimulus is not included in the trial data because it is a function. Any stimulus information that should be saved in the trial data can be added via the data parameter.


Draw rectangle and wait for response

function drawRect(c){
    var ctx = c.getContext('2d');
    ctx.rect(30, 30, 200, 50);

var trial = {
    type: 'canvas-keyboard-response',
    stimulus: drawRect,
    choices: ['e','i'],
    prompt: '<p>Is this a circle or a rectangle? Press "e" for circle and "i" for rectangle.</p>',
    data: {shape: 'rectangle'}

Draw circle, no response allowed

function drawCirc(c){
    var ctx = c.getContext('2d');
    ctx.arc(100, 75, 50, 0, 2 * Math.PI);

var trial = {
    type: 'canvas-keyboard-response',
    stimulus: drawCirc,
    choices: jsPsych.NO_KEYS,
    trial_duration: 1000,
    data: {shape: 'circle', radius: 50}