How does a vernier acuity test measure visual alignment perception?

In a vernier acuity test, visual alignment perception is measured by asking a viewer to discern the alignment of two segments of a broken line.1

This task serves as a classic example of hyperacuity because the degree of displacement a person can perceive between the two lines is smaller than the diameter of a single photoreceptor in the human eye. Because this level of performance exceeds the physical resolution limits of individual photoreceptors, it is believed to be associated with altered activity in the visual cortex of the brain. Through perceptual learning, individuals can improve their ability to detect these minute misalignments with practice, reflecting the brain's capacity to refine its neural processing of sensory information.1