EasyEyes - Accurate fixation for online vision testing of crowding and beyond

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 since online observers tend to be inexperienced, and online gaze tracking, using the built-in webcam, has a low precision (±4 deg, Papoutsaki et al., 2016). The EasyEyes open-source software reliably measures peripheral thresholds online with accurate fixation achieved in a novel way, without gaze tracking. EasyEyes 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 lab, using gaze-contingent stimulus presentation (Kurzawski et al., 2023; Pelli et al., 2016); second, in the lab, using EasyEyes while independently monitoring gaze; third, online at home, using EasyEyes. We find that crowding thresholds are consistent (no significant differences in mean and variance of thresholds across ways) 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, EasyEyes 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; peripheral testing; remote 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), colored triangles are data acquired for the purpose of this project. Axes are log-log. B. Histograms of log thresholds across methods. Dashed lines indicate the geometric mean. N indicates number of observers, M is a geometric mean and SD is the standard deviation of all measured log crowding distances (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 Spearman’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 degree (60 pix/deg). A. X and Y coordinates as a function of time relative to stimulus onset of one observer. The light blue bar represents the target duration (150ms). 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 has been rotated around a circle (crosshair’s trajectory) to minimize overlap with other observers. The pink trial in panel A corresponds to S12 plotted in panel B. All X and Y positions have been corrected for estimated calibration bias.

Figure 4.

X and Y coordinates (in degrees) of gaze before, during, and after target presentation. A shows 2D histograms of gaze. The circle indicates the eye tracker precision and the red cross the target location. 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.