Topography of Ganglion Cells in the Mammal’s Retina: Ecological and Evolutionary Aspects

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.