This study protocol was approved by the University of Waterloo Office of Research Ethics (44843) and the McMaster Research Ethics Board (1733). All procedures were conducted in accordance with the principles outlined in the Declaration of Helsinki. Written informed consent was obtained prior to their participation. For specifications regarding the test stimulus, please refer to Supplementary File 1.
1. Prepare equipment setup
- Set up the mirror stereoscope as shown in Figure 1.
- Position the monitor at a viewing distance of 99.5 cm.
NOTE: The viewing distance is the sum of three components: the distance from the monitor to one of the large mirrors, the distance between the large and small mirrors, and the distance from one of the small mirrors to the chinrest. At this distance, each pixel on the monitor subtended.

- Change viewing distance based on individual hardware, interested eccentricities, and a desired pixel size.
- Perform gamma calibration for the monitor for luminance, red, green, and blue separately.
- Measure the width of the screen in centimeters.
- Open PsychoPy Coder. Click the Monitor icon at the top for monitor calibration and settings.
- In the pop-up PsychoPy Monitor Center window, click New to create a new monitor. Input correct viewing distance, screen size in pixels, screen width, minimum and maximum luminance, and gamma values for grey-scale, red, green, and blue colors. Click Save.

Figure 1: Experimental setup. Experimental setup showing a monitor, a mirror stereoscope, a chinrest, a response box, a keyboard, a mouse, and a numeric pad for participants. Please click here to view a larger version of this figure.
2. Start the experiment
NOTE: This step provides general instructions for running practice tests and formal tests. Specific instructions for each task are provided in steps 3–10.
- Open BinocularBattery.py in PsychoPy Coder.
- Modify the monitor information in the script, including monitor name, refresh rate, and resolution according to the monitor being used.
- Click the Run experiment button at the top.
- Instruct a participant to look through the mirror stereoscope.
- Ask the participant if they see a cross in the middle, comprised of a vertical line and a horizontal line. If not, ask the participant whether they need the vertical line to move to the left or right.
- Use the left and right arrow keys to move the vertical line towards that direction, until the participant sees the vertical line in the middle of the horizontal line. Press Enter to continue.
- Assign and input a participant ID in the program for the participant. At the Main Menu screen, use the numeric pad to select each task.
- Start by running a practice test.
- Provide instructions for each task as provided below in steps 3–10.
- Complete the practice test and evaluate results.
NOTE: Some tasks do not provide accuracy scoring (i.e., letter ocular dominance, plaid motion integration, and Pulfrich). Refer to the Representative Results section for guidance on interpreting performance for these tasks.
- If practice results are below 80% correct (for accuracy‑based practices) or are unexpected (for tasks without accuracy scoring), press Enter and select the test to restart a practice.
- Proceed to run a formal test when a participant’s results are above 80% correct (for accuracy‑based practices) or are consistent and interpretable (for tasks without accuracy scoring) for the last two practice tests.
NOTE: If a participant’s performance remains below 80% correct or inconsistent across several practice tests despite proper instruction, the experimenter should abort the task, as results may be unreliable.
- Run a formal test.
- Instruct the participant to perform the task.
NOTE: Throughout testing, it is critical to remind the participant to maintain fixation on the fixation cross, if present.
- After completion, evaluate the results of the formal test.
NOTE: If the results appear unusual, a second formal test may be performed to confirm the findings.
- Press Enter to return to the Main Menu and select the test again.
3. Run letter ocular dominance
NOTE: One formal test consists of two 1-up-1-down staircases and the method of constant stimuli.
- Present the letter ocular dominance stimulus as shown in Figure 2A Instruct the participant to fixate the central cross while paying attention to the luminance of two letters presented above and below the fixation cross.
- Instruct the participant to use keys 8 or 5 on a numeric pad to indicate which letter (top or bottom) appeared brighter while fixating the central cross.

Figure 2: Letter ocular dominance task and results. (A) Test stimulus. (B) Results from a practice test using the method of constant stimuli. (C) Results from two staircases. Black and blue lines represent contrasts at each step of the two staircases. Colored bars represent thresholds obtained from respective staircases. (D) Psychometric function from the method of constant stimuli. Dots indicate percentages of trials where the participant perceived a letter presented to the left eye as brighter at each contrast tested. The red curve represents a fitted psychometric curve. The intersection of the dotted lines depicts the estimated point of subjective equality (PSE). Please click here to view a larger version of this figure.
4. Run plaid motion integration
NOTE: Use the initial demonstration to familiarize the participant, then press Enter to start a practice trial.
- Present the moving grating stimulus as shown in Figure 3A. Instruct the participant to look at the moving gratings, which can move in 1 of 5 directions, as indicated on the screen, or in both left and right directions simultaneously (split).
- Ask the participant to verbally report the perceived motion direction and any change in the perceived motion while viewing the stimulus.
- Press and hold the corresponding number (1–5 for each direction as labeled or 0 for split motion) until the participant reports a different number.

Figure 3: Plaid motion integration task and results. (A) Test stimulus. (B) Representative results. Colored bars represent the percentage of time when the participant reported each perceived motion direction as labeled, including split when both left and right directions are seen simultaneously. Please click here to view a larger version of this figure.
5. Run fine peripheral stereopsis
- Present the peripheral and central stereopsis stimuli as shown in Figure 4A,B. Use the Stationary task to familiarize the participant.
- Instruct the participant to fixate the cross, wherever it is presented on the screen.
- NOTE: Watch the participant’s eye movement to ensure that their gaze follows the fixation cross.
- Ask the participant whether the black square appears behind or in front of the white frame or appears on the same plane.
- Press 1 if the black square has depth, or 2 if not.
- Use the Regular task to practice with the 2-interval forced-choice task.
- Inform the participant that the stimulus will flash twice, and the depth will appear in one of the two flashes.
- Instruct the participant to report in which of the two flashes they see the depth.
- Press 1 or 2 based on the participant’s choice.
- Run a formal test with the same instructions as the Regular practice.

Figure 4: Fine peripheral stereopsis task and results. (A) Test stimulus for the peripheral task. (B) Test stimulus for the central task. (C) Representative results. Red and blue curves represent staircase data for crossed and uncrossed disparities, respectively, at each location (0° and 10° eccentricity). Colored bars on the right side of each graph indicate thresholds obtained from respective staircases. Please click here to view a larger version of this figure.
6. Run coarse peripheral stereopsis
NOTE: Press the space bar to start a trial.
- Present the coarse stereopsis stimulus as shown in Figure 5A. Instruct the participant to maintain fixation on the cross.
- When the stars have disappeared, ask the participant the following questions: did you see depth?’ if yes, ‘were the star(s) appearing behind or in front of the moons?’ and ‘how many stars were presented?’
- Use the up and down arrow keys to select a response and use the left and right keys to navigate between questions.

Figure 5: Coarse peripheral stereopsis task and results. (A) Test stimulus. (B) Representative results. Red, blue, and green colors represent data from crossed, uncrossed, and catch trials, respectively. Left bars represent the percentage of trials in which depth was reported. Middle bars represent the proportion of correct depth direction responses from trials where depth was reported. The right bars show the percentage of trials in which two stars were reported. Please click here to view a larger version of this figure.
7. Run dynamic local stereopsis
NOTE: This test contains four practice trials designed to provide step-by-step guidance to help readers become familiar with the task.
- Present the dynamic stereopsis stimulus and task interface as shown in Figure 6A,B. Start by running practice Task 1 (single location, single interval).
- Instruct the participants to look at the center, while paying attention to the target location as specified.
NOTE: To help the participant understand the stimulus, allow them to look directly at the target location only during practice.
- Ask the participant whether the square moved closer to or farther away from them.
- Press 1 if the participant saw motion in depth, or 2 if not.
NOTE: Horizontal motion does not count as motion in depth.
- For patients with better vision in the periphery than in the center, advise the patient to use their peripheral vision to look at the target location.
- If the participant is able to see motion-in-depth, proceed to the next practice task.
- Run practice Task 2 (single location, two-interval forced-choice).
- Inform the participant that the stimulus will now flash twice.
- Ask the participant which of the two flashes contains motion in depth at the target location as specified.
- Press 1 or 2 based on the participant’s choice.
- If the participant fails to see motion-in-depth reliably, reinstruct them and allow them to look directly at the target location.
- If the participant is able to see motion-in-depth, proceed to the next practice task.
- Run practice Task 3 (two locations, two-interval forced-choice).
- Provide the same instructions as Task 2, except that the target may now be at either of two locations.
- Run practice Task 4 (Full Practice).
- Provide the same instructions as Task 3, except that the target may now be at any of the four peripheral locations.
- When ready, run Dynamic Periphery and Dynamic Center for a formal test.

Figure 6: Dynamic local stereopsis task and results. (A) Menu for step-by-step practice. (B) Test stimulus. (C) Representative results. Red and blue curves represent staircase data for crossed and uncrossed disparities, respectively, at each retinal location (0° and 5° eccentricity). Colored bars on the right of each graph indicate thresholds obtained from respective staircases. Please click here to view a larger version of this figure.
8. Run the Pulfrich task
NOTE: Luminance on one half of the screen is reduced (Figure 7A) to mimic neutral density (ND) filters.
- Present the Pulfrich task stimulus as shown in Figure 7A. Confirm whether the participant can see depth and whether the moons are moving in front of or behind the stars.
- Ask the participant to use the / or * key to move the stationary moon at the bottom of the screen forward or backward, until it appears to be at the same depth as the moving moons.
- Optional: ask the participant to indicate the amount to depth perceived using the response box (Figure 7B).
- Present the response box to the participant, facing the participant, with the center of the box approximately 40 cm away from their eyes.
- Ask the participant to move the slider on both sides of the box until the moons inside the box appear to be at the same depth as the moving moons on the computer screen.
- Type the reading on the slider onto the computer screen. Press + to move on.
NOTE: The physical response box is described in Supplementary File 2.

Figure 7: Pulfrich task and results. (A) Test stimulus. All moons are in motion except the one at the bottom. (B) Inside view of the response box, showing stars affixed to the ceiling and moons on a rail. (C) Results from the disparity-matching task. Medians are indicated by black dots and orange lines. Boxplots represent the range (minimum and maximum) and the interquartile range. Note that because only three trials were tested per filter strength, the medians and extremes reflect all data for each condition. Blue lines represent linear regressions performed using median data across all filter strengths. (D) Results from the physical depth-matching task. Medians are indicated by black dots and orange lines. Boxplots represent the range (minimum and maximum) and the interquartile range. Note that because only three trials were tested per filter strength, the medians and extremes reflect all data for each condition. Blue lines represent linear regressions performed using median data across all filter strengths. Please click here to view a larger version of this figure.
9. Run motion parallax
- Present the motion parallax stimuli as shown in Figure 8A,B. Instruct the participant to fixate on the central cross.
- Press Enter to start a trial.
- Instruct the participant to report the square that appeared elevated in depth during the rotation, while fixating the central cross.
- Press the corresponding number for that square.

Figure 8: Motion parallax task and results. (A) Test stimulus for the “parallax” task. (B) Test stimulus for the “parallax + disparity” task. Note that the bottom left square in this example (number “4”) contains a binocular disparity cue. (C) Representative results from the “parallax” task. Red and blue lines represent raw data from two staircases. Colored bars represent thresholds obtained from respective staircases. (D) Representative results from the “parallax + disparity” task. Red and blue lines represent raw data from two staircases. Colored bars represent thresholds obtained from respective staircases. Please click here to view a larger version of this figure.
10. Run da Vinci stereopsis
- Present the dichoptic stimulus configuration as shown in Figure 9A. Instruct the participant to compare the depth of the two bars relative to the large box and report which bar appears to be in front of the box.
- Press 8 or 5 for the top or bottom bar, or press Enter if they all appear on the same plane.
- During practice, if the participant sees depth in a catch trial, reinstruct them to ensure that they understand the task.

Figure 9: Da Vinci stereopsis task and results (A) Illustration of the test stimulus, showing two pairs of dichoptic stimuli. (B) Perceptual outcomes after fusion. By crossing the eyes, fusing the left and middle stimuli produces the illusion shown in (a), and fusing the middle and right stimuli produces the illusion shown in (b). Note that during each trial, only one stimulus pair is shown. (C) Representative results. Blue, red, and gray bars indicate the percentage of trials in which participants reported correct depth, incorrect depth, or no depth. Note that all responses were correct in this example; hence, no red bar is presented. Please click here to view a larger version of this figure.