Visual acuity (VA) may be thought of as how well the eye is able to see fine detail, which describes acuteness, sharpness, or clearness of the vision. It measures the spatial resolving power of the visual system, particularly with its ability to distinguish letters or numbers at a given distance. Clinically it describes the minimal angle in which two visual objects can still be perceived separately.
1–3 In human beings, VA does not only depend on optical factors that are related to the eye such as the cornea or the lense, but also involves higher cognitive functions. Hence, degraded VA can be associated with various neurological disorders such as optic neuritis,
4 commonly caused by multiple sclerosis, and stroke
5 that causes damage to optic fibers transmitting visual information from the eyes to the occipital lobe. Clinically, VA is measured by the widely used Snellen chart or other optotype charts such as Landolt C, E charts, or logMAR chart.
1–3 In recent years, gratings with alternating black and white stripes have also been used to analyze vision,
2,6,7 Measuring VA with gratings is simpler compared with using an eye chart because subjects only have to detect the grating without having to identify any symbol. Gratings can be characterized by their spatial frequency, contrast and type, such as sine- or square-wave grating patterns. Hereupon, VA is estimated by the highest spatial frequency, defined as threshold frequency, at which the grating can still be perceived. Usually humans have a threshold frequency of 30 to 40 cyc/deg.
3 Visual acuity assessment with gratings can be especially practical in infants or patients who cannot verbally respond. Infants show a behavior called preferential looking, which refers to the preference in children of rather looking at a grating than a plain surface.
8,9 The examiner observes the eye movements of the infant while showing it gratings with different spatial frequencies next to a blank space. This test, however, is rather time consuming. In order to simplify the method, the optokinetic reflex (OKR) can instead be used to assess VA in infants in a similar manner.
9,10