Objective Visual Acuity Estimation via the Optokinetic Response to High-pass Filtered Stimuli: An Exploratory Study

Aims: Optokinetic-based paradigms used to derive objective visual acuity suffer so far from low predictive power. Arguably, the contribution of the peripheral stimulation and the discrepancy between detection and discrimination threshold can bias the outcomes. In this respect, high-pass filtered stimuli (HPSs) have the advantage of activating only the foveal region and their detection and discrimination thresholds converge. In this exploratory study a novel optokinetic-based induction method (named “ghost oktotype”) that uses HPSs is probed. The aim was to establish whether this type of procedure is worth being extensively investigated.

Study Design: Analytical transversal study.

Place and Duration of Study: Department of Ophthalmology, University of Turin, Italy, between June 2023 and June 2024.

Methodology: After testing for their subjective acuity (expressed as logMAR) at ten different defocusing conditions, four normal and cooperative subjects were repeatedly administered sequences of HPS-Landolt rings arranged on a serial pattern that drifted horizontally. Their angular size was logarithmically reduced and objective acuity was computed for each defocusing condition as a function of the smallest size of the rings that evoked the optokinetic reflex (logMAER). The estimate was compared to the subjective values obtained both at the conventional Landolt-C ETDRS charts and with modified ETDRS charts made up of high-pass filtered Landolt Cs.

Results: The repeatability of the HPS-based optokinetic paradigm was good in the three subjects (ICC=0.77; 0.86; 0.89) and the outcomes were significantly correlated with the conventional ETDRS values (R2=0.96, R2=0.88, R2=0.81: P<0.001). Nevertheless, objective visual acuity was lower (higher logMAER) compared to the subjective, conventional assessment with the ETDRS chart, especially at the highest acuities. This difference decreased when logMAER was compared to the subjective discrimination acuity (logMAR) measured with the HPS-ETDRS charts, and, supposedly, to detection acuity.

Conclusion: According to these preliminary results, high-pass filtered stimuli reduce the mismatch between subjective and objective acuity. Within this frame, the “ghost” oktotype could be a promising tool to assess visual acuity in non-collaborative patients and is worth being extensively investigated. Forthcoming studies could support our impression i.e., that detection and not resolution is the main requisite for the optokinetic response. This peculiarity must be considered when investigating the potential of the optokinetic-based paradigms to derive resolution acuity.