A duplex retina is a
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 ...
consisting of both
rod cells
Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in per ...
and
cone cells
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones ...
, which are the
photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
s for two parallel but mostly separate
visual system
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
s. The rods enable the
scotopic visual system, which is active in dim light. The cones enable the
photopic visual system, which is active in bright light. While one is active, the other is generally inactive; either the rods are photobleached, or ''oversaturated'', in bright light, or the cones are not sensitive enough to hyperpolarize, or instigate the
phototrasduction cascade, in dim light. However, at mesopic (twilight) conditions, both visual systems are active. In this region of overlap, both systems are active and combine to contribute to
mesopic vision
Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions. Mesopic levels range approximately from 0.01 to 3.0 cd/m2 in luminance. Most nigh ...
.
Advantages
Like all sensors, photoreceptors are limited in
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
, i.e. the ratio between the lowest and highest signal they can detect. Having two photoreceptors of differing sensitivities can together cover more dynamic range of light. Human rods can detect 7 orders of magnitude between their minimum threshold and saturation and cones can detect 11 orders of magnitude between their minimum threshold and point of damage. However, together, considering their overlap, a human duplex retina can detect 14 orders of magnitude.
For any visual system, there is a tradeoff between sensitivity and spatial/temporal acuity. A duplex retina uses two visual systems, one of which trades acuity for sensitivity (scotopic), and the other which trades sensitivity for high spatial and temporal acuity (photopic), which gives the best of both worlds.
[
]
Simplex retina
Most vertebrates exhibit duplex retinas, including all major classes: mammals, birds, reptiles, bony fish, etc. However, some sub-clades will have evolved from the common vertebrate ancestor to lose one of the visual systems and develop a ''simplex'' retina, often called a pure-rod or pure-cone retina. Vertebrates that have lost their cone cells and exhibit a pure-rod retina include:
* Almost 90% of deep-sea fish species are rod monochromats.
* Xenarthra
Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Ex ...
(armadillos, anteaters, and tree sloths)
Many vertebrates have lost their rod cells and exhibit a pure-cone retinas, which include:
* most diurnal lizards
* some colubrid snakes [
* ground squirrels were long considered to lack rod cells, but only part of the population lacks functional scotopic vision despite possessing rod cells.]
While most humans possess duplex retinas, some conditions lead to a failure of one of the visual systems. A human lacking cone cells and therefore a photopic system is called an achromat or rod monochromat and experiences day blindness and monochromacy
Monochromacy (from Greek ''mono'', meaning "one" and ''chromo'', meaning "color") is the ability of organisms or machines to perceive only light intensity, without respect to spectral composition (color). Organisms with monochromacy are called ...
. A human lacking rod cells and therefore a scotopic system has nyctalopia
Nyctalopia (; ), also called night-blindness, is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition ( ...
or ''night blindness''.
See also
* Mesopic vision
Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions. Mesopic levels range approximately from 0.01 to 3.0 cd/m2 in luminance. Most nigh ...
* Purkinje effect
The Purkinje effect (; sometimes called the Purkinje shift, often incorrectly pronounced ) is the tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of da ...
References
Eye
{{Eye-stub