Retinal summation describes the relationship between different types of cells in the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
:
cone photoreceptor cells,
bipolar cell
A bipolar neuron, or bipolar cell, is a type of neuron that has two extensions (one axon and one dendrite). Many bipolar cells are specialized sensory neurons for the transmission of sense. As such, they are part of the sensory pathways for sme ...
s, and
ganglion cell
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A ganglion cell is a cell found in a ganglion. Examples of ganglion cells include:
* Retinal ganglion cell (RGC) found in the ganglion cell layer of the retina
* Cells that reside in the adrenal medulla, where they are involved in the ...
s. 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 vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...
. 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).
References
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