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.