Tests of Macular Function

Abstract

The human macula with its central foveola is the most powerful human organ for spatial resolution and object recognition; this ability is based on the high density of photoreceptors (on average more than 200,000 cones/mm²). As shown in ■ Fig. 7.1, visual acuity for a single optotype with optimal conditions for refraction is closely linked with cone density (Curcio et al. 1990). In order to allow for comfortable reading, the visual field has to span a visual angle of at least 4° horizontally and 2° vertically. To read newsprint without magnifying aids a visual acuity of 0.4 and a cone density of about 20,000/mm² are necessary, as also shown in ■ Fig. 7.1. If cones are lost, for example as the result of an acquired or hereditary retinal disease, the following functions — related to the role of cones in the retinal center and their distribution — are affected:

• Visual acuity, because of decreased cone density

• Reading performance, when scotomata occur in the minimal field required for reading (see above)

• Visual field, because of the local loss of small cone populations

• Increment threshold loss for central vision, because of reduced spatial integration of bright areas

• Color vision discrimination, because of the thinned mosaic of short, middle, and long wavelength sensitive cones

• Contrast vision, because of disintegration of receptive fields

These functional losses determine the kind of subjective and objective function testing that are optimally suited for diseases of the macula.