![]() A lower vernier threshold represents better vernier acuity. Psychophysical methods for measuring vernier thresholds rely on behavioural responses based on the judgement of the observer (their visual perception) when viewing the relative positions of these objects. Features including the background contrast, luminance, gap size between objects and object sizes can be varied. Vernier offsets can be detected from stimuli including dots, lines, and gratings ( Figure 2), which are usually displayed on a computer screen. Several methods for the measurement of vernier acuity are reported in the literature. This review aims to bring together broad information on vernier acuity from a clinical perspective, with the purpose of providing guidance on its utility in clinical settings and factors to consider for measuring vernier acuity robustly in a standardised way. ![]() Given its psychophysical characteristics, vernier acuity may also be a useful outcome measure in clinical trials of emerging neuro-ophthalmic treatments, and be useful for more defined diagnostic protocols. It has previously been studied in visual disorders such as age-related macular degeneration, glaucoma, and amblyopia. Measurement of vernier acuity can provide a diagnostic tool that tests a unique aspect of human perception. Vernier related visual tasks are generally easy for subjects to understand and perform. More complex measurement can be carried out using visual evoked potentials, which require more sophisticated equipment and analysis. Vernier acuity thresholds can be measured with software that is freely available online with as little equipment as a computer. Vernier acuity is a fascinating and important measure of visual function – particularly cortical visual function – but is infrequently used in clinical practice because it is not well understood. Other examples include stereoscopic acuity (binocular vision), line orientation discrimination, and detection of curvature ( Westheimer, 1981). Vernier acuity is hence regarded as a type of hyperacuity ( Westheimer, 1975), a term that describes visual tasks that have thresholds smaller than the size of a foveal cone (2.5 μm, about 30 s of arc), which limits the classical spatial resolution of the eye. The vernier threshold (smallest detectable offset) for humans is as low as 2 to 5 arcseconds ( Westheimer and McKee, 1977b Westheimer, 1987). This review will discuss the measurement of vernier acuity, provide a current understanding of its neuro-ophthalmic mechanisms, and finally explore its utility through a clinical lens, along with our recommendations for best practice. Current clinical utility includes a home-based vernier acuity tool, preferential hyperacuity perimetry, which is used for screening for choroidal neovascularisation in age-related macular degeneration. Although not routinely measured in clinical practice, vernier acuity is known to be reduced in amblyopia, glaucoma and retinitis pigmentosa, and has been explored as a measure of retinal or neural visual function in the presence of optical media opacities. Vernier acuity can be measured, usually in seconds of arc, by freely available automated online tools as well as via analysis of steady state visual-evoked potentials, which allows measurement in non- or pre-verbal subjects such as infants. Vernier acuity relies heavily on cortical processing and is minimally affected by optical media factors, making it a useful indicator of cortical visual function. It is considered a form of visual hyperacuity due to its detectable thresholds being considerably smaller than the diameter of a foveal cone receptor, which limits the spatial resolution of classical visual acuity. Vernier acuity measures the ability to detect a misalignment or positional offset between visual stimuli, for example between two vertical lines when reading a vernier scale. 4Oxford Eye Hospital, Oxford University Hospitals NHS Foundation Trust, Oxford, United Kingdom.3Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom.2Department of Optometry and Vision Sciences, The University of Melbourne, Melbourne, VIC, Australia.1Centre for Eye Research Australia, Royal Victorian Eye and Ear Hospital, Melbourne, VIC, Australia.
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