jsPsych/docs/markdown_docs/plugins/jspsych-xab.md

jspsych-xab plugin

The XAB plugin displays either an image or HTML object stimulus (X). After a short gap, the plugin displays two additional stimuli (A and B). The subject selects which of the two stimuli matches X using the keyboard.

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	Array of arrays. Each interior array represents the stimuli for a single trial. Each interior array can be two or three elements. If it is two elements, then the plugin will show the first element as X and as the target during the A/B portion (the second element will be the foil). If it is three elements, then the first is X the second is the target (A) and the third is the foil (B). This is useful if X and A are not identical, but A is still the correct choice (e.g. a categorization experiment where the goal is to pick the item that is in the same category). Stimuli can be paths to images, or html strings.
is_html	boolean	false	If the elements of the stimuli array are strings containing HTML content, then this parameter must be set to true.
left_key	numeric or string	'Q'	Which key the subject should press to indicate that the target is on the left side.
right_key	numeric or string	'P'	Which key the subject should press to indicate that the target is on the right side.
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).
timing_x	numeric	1000	How long to show the X stimulus for in milliseconds.
timing_xab_gap	numeric	1000	How long to show a blank screen in between X and AB in milliseconds.
timing_ab	numeric	-1	How long to show A and B in milliseconds. If the value of this parameter is -1, then the stimuli will remain on the screen until a response is given.

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_x	string	Either the path to the image file or the string containing the HTML formatted content that was the X stimulus on this trial.
stimulus_a	string	Either the path to the image file or the string containing the HTML formatted content that was the A stimulus on this trial.
stimulus_b	string	Either the path to the image file or the string containing the HTML formatted content that was the B stimulus on this 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 A and B stimuli first appear on the screen until the subject's response.
correct	boolean	True if the subject picks the correct answer.

Examples

Doing an exact match task

var block = {
	type: 'xab',
	stimuli: [['img/happy_face.png', 'img/sad_face.png']],
	prompt: "Press Q if the face you just saw is on the left. Press P if the face you just saw is on the right."
}

Matching based on a feature

var block = {
	type: 'xab',
	stimuli: [['img/happy_joe_face.png', 'img/sad_joe_face.png', 'img/sad_fred_face.png']],
	prompt: "Press Q if the person you just saw is on the left. Press P if the person you just saw is on the right."
}