EasyEyes - A new method for accurate fixation in online vision testing

Abstract

Online methods allow testing of larger, more diverse populations, with much less effort than in-lab testing. However, many psychophysical measurements, including visual crowding, require accurate eye fixation, which is classically achieved by testing only experienced observers who have learned to fixate reliably, or by using a gaze tracker to restrict testing to moments when fixation is accurate. Alas, both approaches are impractical online as online observers tend to be inexperienced, and online gaze tracking, using the built-in webcam, has a low precision (±4 deg). EasyEyes open-source software reliably measures peripheral thresholds online with accurate fixation achieved in a novel way, without gaze tracking. It tells observers to use the cursor to track a moving crosshair. At a random time during successful tracking, a brief target is presented in the periphery. The observer responds by identifying the target. To evaluate EasyEyes fixation accuracy and thresholds, we tested 12 naive observers in three ways in a counterbalanced order: first, in the laboratory, using gaze-contingent stimulus presentation; second, in the laboratory, using EasyEyes while independently monitoring gaze using EyeLink 1000; third, online at home, using EasyEyes. We find that crowding thresholds are consistent and individual differences are conserved. The small root mean square (RMS) fixation error (0.6 deg) during target presentation eliminates the need for gaze tracking. Thus, this method enables fixation-dependent measurements online, for easy testing of larger and more diverse populations.

Keywords: EasyEyes; crosshair tracking; crowding; eye tracker; fixation; gaze control; online testing.

Figure 1

Log crowding thresholds across methods. (A) Test–retest crowding distances across methods. Gray triangles are thresholds measured by Kurzawski et al. (2023), and colored triangles are newly acquired data. Axes are log–log. (B) Histograms of log thresholds across methods. M is a geometric mean (dashed line) and SD is the standard deviation of all measured log crowding distances. N is the number of observations (12 observers, two meridians, test and retest) for our data and for fraction of data from Kurzawski et al. (50 observers, two meridians, test–retest).

Figure 2

Correlations of crowding distance across methods. (A) The cross-method correlations of the geometric mean crowding distance for each observer. Mean is calculated from 4 thresholds (2 meridians, test–retest). (B) Test–retest Pearson’s correlation across mean crowding distance thresholds, across and within methods.

Figure 3

Cursor tracking task. X and Y positions of crosshair (solid black line), cursor (solid colored line), and gaze (colored points) during an EasyEyes trial. The gray bar corresponds to 1 deg (60 pix/deg). (A) X and Y coordinates as a function of time relative to the stimulus onset of one observer. The light blue bar represents the target duration (150 ms). (B) Shows a single representative trial from each observer (750 ms before target onset). The black circle is the trajectory of the crosshair. Again, thick colored lines indicate cursor, and colored dots indicate gaze position. Each observer’s data have been rotated around a circle (crosshair’s trajectory) to minimize overlap with other observers. The pink trial in (A) corresponds to S12 plotted in (B). All X and Y positions have been corrected for estimated calibration bias.

Figure 4

X and Y coordinates (in deg) of gaze before, during, and after target presentation across participants. (A) Shows 2D histograms of gaze. The circle indicates the eye tracker precision and the red cross is the target location. Total counts differ due to variations in the amount of data presented (500 ms vs. 150 ms) and the differences in the count of the bin with max counts. The 500 ms period is entirely within the 700 ms between target offset and response instructions. (B) Shows gaze position in X and Y coordinates. The red vertical line indicates the target location.

Figure 5

Comparing peeking across methods. The plot shows the percentage of trials in which observers peeked that is their gaze position was more than 1.5 deg away from the crosshair during stimulus presentation. For CriticalSpacing.m lab, peeks are detected by the eye tracker and correspond to rejected trials. For EasyEyes lab, we use gaze data to calculate the percentage of peeks post-hoc.