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jspsych-virtual-chinrest

This plugin provides a "virtual chinrest" that can measure the distance between the participant and the screen. It can also standardize the jsPsych page content to a known physical dimension (e.g., ensuring that a 200px wide stimulus is 2.2cm wide on the participant's monitor). This is based on the work of Li, Joo, Yeatman, and Reinecke (2020), and the plugin code is a modified version of their implementation. We recommend citing their work in any paper that makes use of this plugin.

Citation

Li, Q., Joo, S. J., Yeatman, J. D., & Reinecke, K. (2020). Controlling for Participants’ Viewing Distance in Large-Scale, Psychophysical Online Experiments Using a Virtual Chinrest. Scientific Reports, 10(1), 1-11. doi: 10.1038/s41598-019-57204-1

The plugin works in two phases.

Phase 1. To calculate the pixel-to-cm conversion rate for a participant’s display, participants are asked to place a credit card or other item of the same size on the screen and resize an image until it is the same size as the credit card. Since we know the physical dimensions of the card, we can find the conversion rate for the participant's display.

Phase 2. To measure the participant's viewing distance from their screen we use a blind spot task. Participants are asked to focus on a black square on the screen with their right eye closed, while a red dot repeatedly sweeps from right to left. They press the spacebar on their keyboard whenever they perceive that the red dot has disappeared. This part allows the plugin to use the distance between the black square and the red dot when it disappears from eyesight to estimate how far the participant is from the monitor. This estimation assumes that the blind spot is located at 13.5° temporally.

Dependency

This plugin requires the SVG.js library, available at https://svgjs.com. You must include the library in the <head> section of your experiment page.

Parameters

Parameters can be left unspecified if the default value is acceptable.

Parameter Type Default Value Descripton
resize_units string "none" Units to resize the jsPsych content to after the trial is over: "none" "cm" "inch" or "deg". If "none", no resizing will be done to the jsPsych content after the virtual-chinrest trial ends.
pixels_per_unit numeric 100 After the scaling factor is applied, this many pixels will equal one unit of measurement, where the units are indicated by resize_units. This is only used when resizing is done after the trial ends (i.e. the resize_units parameter is not "none").
adjustment_prompt HTML string "Click and drag the lower right corner of the image until it is the same size as a credit card held up to the screen. You can use any card that is the same size as a credit card, like a membership card or driver's license. If you do not have access to a real card you can use a ruler to measure the image width to 3.37 inches or 85.6 mm." This string can contain HTML markup. Any content here will be displayed below the card stimulus during the resizing phase.
adjustment_button_prompt HTML string "Click here when the image is the correct size" Content of the button displayed below the card stimulus during the resizing phase.
item_path string "img/card.png" Path of the item to be presented in the card stimulus during the resizing phase. The default image is available in /examples/img/card.png
item_height_mm numeric 53.98 The known height of the physical item (e.g. credit card) to be measured, in mm.
item_width_mm numeric 85.6 The known width of the physical item (e.g. credit card) to be measured, in mm.
item_init_size numeric 250 The initial size of the card stimulus, in pixels, along its largest dimension.
blindspot_reps numeric 5 How many times to measure the blindspot location. If 0, blindspot will not be detected, and viewing distance and degree data will not be computed.
blindspot_prompt HTML string "Now we will quickly measure how far away you are sitting. Put your left hand on the space bar. Cover your right eye with your right hand. Using your left eye, focus on the black square. Keep your focus on the black square. The red ball will disappear as it moves from right to left. Press the space bar as soon as the ball disappears. Press the space bar when you are ready to begin. This string can contain HTML markup. Any content here will be displayed above the blindspot task.
redo_measurement_button_label HTML string 'No, that is not close. Try again' Text for the button on the viewing distance report page to re-do the viewing distance estimate. If the participant click this button, the blindspot task starts again.
blindspot_done_prompt HTML string "Yes" Text for the button on the viewing distance report page that can be clicked to accept the viewing distance estimate.
blindspot_measurements_prompt HTML string 'Remaining measurements: ' Text accompanying the remaining measurements counter that appears below the blindspot task.
viewing_distance_report HTML string "Based on your responses, you are sitting about <span id='distance-estimate' style='font-weight: bold;'></span> from the screen. Does that seem about right?" Estimated viewing distance data displayed after blindspot task. If "none" is given, viewing distance will not be reported to the participant. The HTML span element with id = distance-estimate returns the distance.

Data Generated

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

Note: The deg data are only returned if viewing distance is estimated with the blindspot method (px2deg, win_height_deg, win_width_deg, item_width_deg).

Name Type Value
rt numeric The response time in milliseconds.
item_height_mm numeric The height in millimeters of the item to be measured.
item_width_mm numeric The width in millimeters of the item to be measured
item_height_deg numeric Final height of the resizable div container, in degrees.
item_width_deg numeric Final width of the resizable div container, in degrees.
item_width_px numeric Final width of the resizable div container, in pixels.
px2deg numeric Pixels to degrees conversion factor.
px2mm numeric Pixels to millimeters conversion factor.
scale_factor numeric Scaling factor that will be applied to the div containing jsPsych content.
win_width_deg numeric The interior width of the window in degrees.
win_height_deg numeric The interior height of the window in degrees.
view_dist_mm numeric Estimated distance to the screen in millimeters.

Example

// two blindspot measurements
// measure px2mm, viewing distance and px2deg
// do not resize the jsPsych content after this trial
// note: pixels_per_unit will be ignored since there is no resizing (resize_units: "none")
let no_resize = {
  type: "virtual-chinrest",
  blindspot_reps: 3,
  resize_units: "none",
  pixels_per_unit: 50,
};

// no blindspot task
// resize to cm (50 pixels per unit)
// measure px2mm, but not viewing distance and px2deg (because blindspot_reps is 0)
// note: you may still choose to estimate viewing distance even if resizing to cm or inches
let cm_resize = {
  type: "virtual-chinrest",
  blindspot_reps: 0,
  resize_units: "cm",
  pixels_per_unit: 50,
};

// three blindspot measurements
// measure px2mm, viewing distance and px2deg
// resize to degrees of visual angle (50 pixels per unit)
// don't report viewing distance to subject
let deg_resize = {
  type: "virtual-chinrest",
  blindspot_reps: 3,
  resize_units: "deg",
  pixels_per_unit: 50,
  viewing_distance_report: "none",
};

// resizing to degrees with no blindspot measurment is not possible
// this trial will throw an error
let error_trial = {
  type: "virtual-chinrest",
  blindspot_reps: 0,
  resize_units: "deg",
  pixels_per_unit: 50,
};