The visual system is the physiological basis of visual perception (the ability to detect and process light). The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optical system (including cornea and lens) and the neural system (including the retina and visual cortex).
The visual system performs a number of complex tasks based on the image forming functionality of the eye, including the formation of monocular images, the neural mechanisms underlying stereopsis and assessment of distances to (depth perception) and between objects, motion perception, pattern recognition, accurate motor coordination under visual guidance, and colour vision. Together, these facilitate higher order tasks, such as object identification. The neuropsychological side of visual information processing is known as visual perception, an abnormality of which is called visual impairment, and a complete absence of which is called blindness. The visual system also has several non-image forming visual functions, independent of visual perception, including the pupillary light reflex and circadian photoentrainment.
This article describes the human visual system, which is representative of mammalian vision, and to a lesser extent the vertebrate visual system.
System overview
Optical Together, the cornea and lens refract light into a small image and shine it on the retina. The retina transduces this image into electrical pulses using rods and cones. The optic nerve then carries these pulses through the optic canal. Upon reaching the optic chiasm the nerve fibers decussate (left becomes right). The fibers then branch and terminate in three places."How the Human Eye Sees." WebMD. Ed. Alan Kozarsky. WebMD, 3 October 2015. Web. 27 March 2016.Than, Ker. "How the Human Eye Works." LiveScience. TechMedia Network, 10 February 2010. Web. 27 March 2016."How the Human Eye Works | Cornea Layers/Role | Light Rays." NKCF. The Gavin Herbert Eye Institute. Web. 27 March 2016.Albertine, Kurt. Barron's Anatomy Flash CardsTillotson, Joanne. McCann, Stephanie. Kaplan's Medical Flashcards. April 2, 2013."Optic Chiasma." Optic Chiasm Function, Anatomy & Definition. Healthline Medical Team, 9 March 2015. Web. 27 March 2016.Jefferey, G., and M. M. Neveu. "Chiasm Formation in Man Is Fundamentally Different from That in the Mouse." Nature.com. Nature Publishing Group, 21 March 2007. Web. 27 March 2016.
Neural Most of the optic nerve fibers end in the lateral geniculate nucleus (LGN). Before the LGN forwards the pulses to V1 of the visual cortex (primary) it gauges the range of objects and tags every major object with a velocity tag. These tags predict object movement.
The LGN also sends some fibers to V2 and V3.Card, J. Patrick, and Robert Y. Moore. "Organization of Lateral Geniculate-hypothalamic Connections in the Rat." Wiley Online Library. 1 June. 1989. Web. 27 March 2016.
V1 performs edge-detection to understand spatial organization (initially, 40 milliseconds in, focusing on even small spatial and color changes. Then, 100 milliseconds in, upon receiving the translated LGN, V2, and V3 info, also begins focusing on global organization). V1 also creates a bottom-up saliency map to guide attention or gaze shift.
V2 both forwards (direct and via pulvinar) pulses to V1 and receives them. Pulvinar is responsible for saccade and visual attention. V2 serves much the same function as V1, however, it also handles illusory contours, determining depth by comparing left and right pulses (2D images), and foreground distinguishment. V2 connects to V1 - V5.
V3 helps process 'global motion' (direction and speed) of objects. V3 connects to V1 (weak), V2, and the inferior temporal cortex.Catani, Marco, and Derek K. Jones. "Brain." Occipito‐temporal Connections in the Human Brain. 23 June 2003. Web. 27 March 2016.
V4 recognizes simple shapes, and gets input from V1 (strong), V2, V3, LGN, and pulvinar. V5's outputs include V4 and its surrounding area, and eye-movement motor cortices (frontal eye-field and lateral intraparietal area).
V5's functionality is similar to that of the other V's, however, it integrates local object motion into global motion on a complex level. V6 works in conjunction with V5 on motion analysis. V5 analyzes self-motion, whereas V6 analyzes motion of objects relative to the background. V6's primary input is V1, with V5 additions. V6 houses the topographical map for vision. V6 outputs to the region directly around it (V6A). V6A has direct connections to arm-moving cortices, including the premotor cortex.
The inferior temporal gyrus recognizes complex shapes, objects, and faces or, in conjunction with the hippocampus, creates new memories.Moser, May-Britt, and Edvard I. Moser. "Functional Differentiation in the Hippocampus." Wiley Online Library. 1998. Web. 27 March 2016. The pretectal area is seven unique nuclei. Anterior, posterior and medial pretectal nuclei inhibit pain (indirectly), aid in REM, and aid the accommodation reflex, respectively. The Edinger-Westphal nucleus moderates pupil dilation and aids (since it provides parasympathetic fibers) in convergence of the eyes and lens adjustment.Reiner, Anton, and Harvey J. Karten. "Parasympathetic Ocular Control — Functional Subdivisions and Circuitry of the Avian Nucleus of Edinger-Westphal."Science Direct. 1983. Web. 27 March 2016. Nuclei of the optic tract are involved in smooth pursuit eye movement and the accommodation reflex, as well as REM.
The suprachiasmatic nucleus is the region of the hypothalamus that halts production of melatonin (indirectly) at first light.
Structure
The eye, especially the retina The optic nerve The optic chiasma The optic tract The lateral geniculate body The optic radiation The visual cortex The visual association cortex.
These are components of the visual pathway, also called the optic pathway, that can be divided into anterior and posterior visual pathways. The anterior visual pathway refers to structures involved in vision before the lateral geniculate nucleus. The posterior visual pathway refers to structures after this point.
Eye
Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil (controlled by the iris) and is further refracted by the lens. The cornea and lens act together as a compound lens to project an inverted image onto the retina.
Retina
The retina consists of many photoreceptor cells which contain particular protein molecules called opsins. In humans, two types of opsins are involved in conscious vision: rod opsins and cone opsins. (A third type, melanopsin in some retinal ganglion cells (RGC), part of the body clock mechanism, is probably not involved in conscious vision, as these RGC do not project to the lateral geniculate nucleus but to the pretectal olivary nucleus.) An opsin absorbs a photon (a particle of light) and transmits a signal to the cell through a signal transduction pathway, resulting in hyper-polarization of the photoreceptor.
Rods and cones differ in function. Rods are found primarily in the periphery of the retina and are used to see at low levels of light. Each human eye contains 120 million rods. Cones are found primarily in the center (or fovea) of the retina. There are three types of cones that differ in the wavelengths of light they absorb; they are usually called short or blue, middle or green, and long or red. Cones mediate day vision and can distinguish color and other features of the visual world at medium and high light levels. Cones are larger and much less numerous than rods (there are 6-7 million of them in each human eye).
In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which then conduct action potentials to the brain. A significant amount of visual processing arises from the patterns of communication between neurons in the retina. About 130 million photo-receptors absorb light, yet roughly 1.2 million axons of ganglion cells transmit information from the retina to the brain. The processing in the retina includes the formation of center-surround receptive fields of bipolar and ganglion cells in the retina, as well as convergence and divergence from photoreceptor to bipolar cell. In addition, other neurons in the retina, particularly horizontal and amacrine cells, transmit information laterally (from a neuron in one layer to an adjacent neuron in the same layer), resulting in more complex receptive fields that can be either indifferent to color and sensitive to motion or sensitive to color and indifferent to motion.
Mechanism of generating visual signals The retina adapts to change in light through the use of the rods. In the dark, the chromophore retinal has a bent shape called cis-retinal (referring to a cis conformation in one of the double bonds). When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. This is called bleaching because the purified rhodopsin changes from violet to colorless in the light. At baseline in the dark, the rhodopsin absorbs no light and releases glutamate, which inhibits the bipolar cell. This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. When there is light present, glutamate secretion ceases, thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected.Saladin, Kenneth D. Anatomy & Physiology: The Unity of Form and Function. 5th ed. New York: McGraw-Hill, 2010.
The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain: M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus; P cells, with smaller center-surround receptive fields that are sensitive to color and shape; K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth; another population that is intrinsically photosensitive; and a final population that is used for eye movements.
A 2006 University of Pennsylvania study calculated the approximate bandwidth of human retinas to be about 8,960 kilobits per second, whereas guinea pig retinas transfer at about 875 kilobits.
In 2007 Zaidi and co-researchers on both sides of the Atlantic studying patients without rods and cones, discovered that the novel photoreceptive ganglion cell in humans also has a role in conscious and unconscious visual perception. The peak spectral sensitivity was 481 nm. This shows that there are two pathways for vision in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photo-receptive ganglion cells which act as rudimentary visual brightness detectors.
Photochemistry
The functioning of a camera is often compared with the workings of the eye, mostly since both focus light from external objects in the field of view onto a light-sensitive medium. In the case of the camera, this medium is film or an electronic sensor; in the case of the eye, it is an array of visual receptors. With this simple geometrical similarity, based on the laws of optics, the eye functions as a transducer, as does a CCD camera.