Predicting Letter Acuity Using Vernier-Induced OKN and Consumer-Grade Cameras

Optokinetic nystagmus (OKN) is used for objective visual acuity assessment, but precise quantification remains challenging and requires specialised eye trackers. This study uses OKN induced by vernier acuity instead of grating stimuli, recorded with and without eye trackers, to establish an optimised method for acuity measurement and examine its correlation with subjective letter acuity.

Thirty-nine adults completed uncorrected monocular testing (letter acuity: -0.10 to 1.00logMAR). Vertically displaced stimuli containing two levels of vernier offsets (0.38 and 9.89 arcmin) moving horizontally were presented. Eye movements were recorded simultaneously using a research-grade eye tracker and a consumer-grade USB camera. Using an automated algorithm, the OKN responses were quantified at each vernier size level, and the reduction between the two levels was registered as a relative OKN index for predicting letter acuity.

Subjective letter acuity showed strong correlations with relative OKN indexes from the eye tracker (r = 0.82, p < 0.001) and USB camera (r = 0.76, p < 0.001) recordings, and their letter acuity predictions had mean absolute errors of 0.15 and 0.16 logMAR, respectively. The OKN index demonstrated high accuracy in detecting participants with acuity worse than 0.50 logMAR, achieving receiver operating characteristic (ROC) area under the curve (AUC) values of 0.97 and 0.95 using the eye tracker and camera, respectively.

This study provides proof-of-concept for applying OKN-based vernier acuity measurement in adults with or without eye trackers, although extension to a paediatric cohorts requires future investigation.