Retinal summation describes the relationship between different types of cells in the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

bipolar cell A bipolar neuron, or bipolar cell, is a type of neuron characterized by having both an axon and a dendrite extending from the soma (cell body) in opposite directions. These neurons are predominantly found in the retina and olfactory system. The em ...

s, and ganglion cells. With high retinal summation, a large number of photoreceptor cells converge on a smaller number of bipolar cells in transferring their signals to ganglion cells. Zero summation occurs when each cone photoreceptor cell contacts a single ganglion cell via a single bipolar cell. High summation increases sensitivity to light at the expense of

visual acuity Visual acuity (VA) commonly refers to the clarity of visual perception, vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye ...

. Low retinal summation results in high visual acuity, with individual photoreceptor cells sending their own signals. High retinal summation yields high sensitivity to low light levels, where the signal is summed before reaching the brain—presumably advantageous when the signals reaching individual photoreceptor cells are weak. High retinal summation is an adaptation to low light levels, and low retinal summation to high light levels (thus sharpening the images).

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