The Neuro-Circular Optotype (NCO): optimizing visual acuity assessment via isotropic spatial-frequency tuning and cortical sparsity

Purpose: To introduce and evaluate the Neuro-Circular Optotype (NCO), a filled-disk visual acuity stimulus with a single calibrated radial gap designed to align more closely with early visual coding principles than conventional optotypes. Methods: Thirty healthy adults (mean age \((24.5 \pm 3.2)\) years) completed adaptive forced-choice acuity testing with Tumbling E, Landolt C, and NCO. NCO and Landolt C were tested in an eight-alternative forced-choice format, whereas Tumbling E was tested in a four-alternative forced-choice format. Thresholds were estimated using a 3-down--1-up staircase and defined as the mean of the final six reversals. Additional experiments assessed recognition accuracy under reduced contrast and Gaussian blur. A computational model consisting of a difference-of-Gaussians retinal/LGN stage and an oriented V1 filter bank was used to compare sparsity and orientation selectivity across optotypes. Results: NCO yielded better mean acuity thresholds (\((-0.08 \pm 0.05)\) \logmar) than Landolt C (\((-0.01 \pm 0.06)\) \logmar) and Tumbling E (\((0.04 \pm 0.08)\) \logmar). Compared with Tumbling E, the NCO advantage was significant (\((p<0.001)\), Cohen's \((d=1.2)\)); compared with Landolt C, the difference also remained significant (\((p<0.05)\), \((d=0.6)\)). Under degraded viewing, NCO maintained the highest accuracy, including \((75.8%)\) at 20% contrast and \((71.2%)\) with Gaussian blur (\((\sigma=2)\) pixels). Tumbling E showed orientation-dependent anisotropy, whereas NCO did not. The computational model suggested that NCO evoked sparser and more orientation-concentrated responses than the comparison optotypes. Conclusion: A filled circular optotype with isotropically distributed gap orientations may provide a more robust and neurophysiologically aligned probe of visual resolution than legacy optotypes. Further validation in clinical populations and against standard chart-based outcomes is warranted.