Visual fields of mammals are characterized by a non-uniform visual acuity. They contain the best-vision zones of high resolution, whereas visual acuity is markedly lower in other parts of the visual field. To a large extent, the visual acuity depends on the concentration (a number per unit of area) of the retinal ganglion cells: the cell concentration is maximal in the best-vision zones. The best-vision zone may have either a form of an elongated horizontal strip (a visual strike) or a local spot (the area centralis in carnivores or the fovea in primates). A combined structure (the spot of the highest cell concentration within the visual streak) is also possible. In several mammal, there are two best-vision zones; this pattern is the most pronounced in cetaceans. Such retinal pattern could appear as a result of differentiation of the visual streak. Apart from the high concentration of ganglion cells, the regularity of ganglion cell distribution helps to provide the high retinal resolution.
A.M. Mass (2025) studied this question.