jspsych-same-different plugin

The same-different plugin displays two stimuli sequentially. Stimuli can be images or HTML objects. The subject responds using the keyboard, and indicates whether the stimuli were the same or different. Same does not necessarily mean identical; a category judgment could be made, for example.

Parameters

This table lists the parameters associated with this plugin. 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
stimuli	array	undefined	A pair of stimuli, represented as an array with two entries, one for each stimulus. A stimulus can be either a path to an image file or a string containing valid HTML markup. Stimuli will be shown in the order that they are defined in the array.
is_html	boolean	false	If the elements of the `stimuli` array are strings containing HTML content, then this parameter must be set to true.
answer	string	undefined	Either `'same'` or `'different'`.
same_key	numeric or string	'Q'	The key that subjects should press to indicate that the two stimuli are the same.
different_key	numeric or string	'P'	The key that subjects should press to indicate that the two stimuli are different.
timing_first_stim	numeric	1000	How long to show the first stimulus for in milliseconds. If the value of this parameter is `-1` then the stimulus will be shown until the subject presses any key.
timing_gap	numeric	500	How long to show a blank screen in between the two stimuli.
timing_second_stim	numeric	1000	How long to show the second stimulus for in milliseconds. If the value of this parameter is `-1` then the stimulus will be shown until the subject responds.
prompt	string	""	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).

Data Generated

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

Name	Type	Value
stimulus	string	An JSON-encoded array of length 2 containing either the path to the image file or the string containing the HTML formatted content that the subject saw for each trial.
key_press	numeric	Indicates which key the subject pressed. The value is the numeric key code corresponding to the subject's response.
rt	numeric	The response time in milliseconds for the subject to make a response. The time is measured from when the second stimulus first appears on the screen until the subject's response.
correct	boolean	`true` if the subject's response matched the `answer` for this trial.
answer	string	The correct answer to the trial, either `'same'` or `'different'`.

Additionally, if timing_first_stim is -1, then the following data is also collected:

Name	Type	Value
rt_stim1	numeric	The response time in milliseconds for the subject to continue after the first stimulus. The time is measured from when the first stimulus appears on the screen until the subject's response.
key_press_stim1	numeric	Indicates which key the subject pressed to continue. The value is the numeric key code corresponding to the subject's response.

Examples

Presenting two different emotional expressions

    var block = {
      type: 'same-different',
      stimuli: ['img/happy_face_1.jpg', 'img/sad_face_3.jpg'],
      prompt: "<p>Press S if the faces had the same emotional expression. Press D if the faces had different emotional expressions.</p>",
      same_key: 'S',
      different_key: 'D',
      answer: 'different'
    }

    jsPsych.init({
      timeline: [block],
      on_finish: function() {
        jsPsych.data.displayData();
      }
    });

Presenting the same emotional expression

    var block = {
      type: 'same-different',
      stimuli: ['img/happy_face_1.jpg', 'img/happy_face_3.jpg'],
      prompt: "<p>Press S if the faces had the same emotional expression. Press D if the faces had different emotional expressions.</p>",
      same_key: 'S',
      different_key: 'D',
      answer: 'same'
    }

    jsPsych.init({
      timeline: [block],
      on_finish: function() {
        jsPsych.data.displayData();
      }
    });

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