The capacity of a visual system to resolve fine spatial detail depends on several factors, some associated with the stimulus object, others with the eye, and still others with the central visual pathways. Sensitivity to spatial detail is commonly called acuity, but it is important to remember that this word is applied to several different performance measures. One reads, for example, of minimum separable acuity, grating acuity, vernier acuity, and stereoscopic acuity. The neural mechanisms that mediate these discriminative capacities are not necessarily all the same, despite the fact that the same word, “acuity,” is used for them. It should also be kept in mind that these measurements usually reflect performance at the extreme limit of some functional capacity, rather like determinations of the absolute threshold for detection of light. Thus, acuity measurements of all kinds can give misleading impressions of the sensory tasks in which the nervous system is routinely engaged.

Minimum Separable Acuity and Minimum Angle of Resolution

When two dots or short line segments are made to approach each other in the visual field, a separation is reached at which the subject reports the presence of only one object. The critical angular spacing of the stimuli when they are just resolved is called the minimum angle of resolution (MAR) (Figure 9-1), a measure analogous to the two-point discrimination threshold in somatic sensation. The MAR is affected by many factors, including the brightness of the stimuli, the state of retinal adaptation, and the position of the stimuli on the retina.