Visual system


The visual system is the physiological basis of visual perception. 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 and the neural system.
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 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. The fibers then branch and terminate in three places.

Neural

Most of the optic nerve fibers end in the lateral geniculate nucleus. Before the LGN forwards the pulses to V1 of the visual cortex 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.
V1 performs edge-detection to understand spatial organization. V1 also creates a bottom-up saliency map to guide attention or gaze shift.
V2 both forwards 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, and foreground distinguishment. V2 connects to V1 - V5.
V3 helps process 'global motion' of objects. V3 connects to V1, V2, and the inferior temporal cortex.
V4 recognizes simple shapes, and gets input from V1, V2, V3, LGN, and pulvinar. V5's outputs include V4 and its surrounding area, and eye-movement motor cortices.
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 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. The pretectal area is seven unique nuclei. Anterior, posterior and medial pretectal nuclei inhibit pain, aid in REM, and aid the accommodation reflex, respectively. The Edinger-Westphal nucleus moderates pupil dilation and aids in convergence of the eyes and lens adjustment. 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 at first light.

Structure

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 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.
File:Cajal Retina.jpg|thumb|left|S. Ramón y Cajal, ''Structure of the Mammalian Retina, 1900''

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. An opsin absorbs a photon 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 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.
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, 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. 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.
The final result of all this processing is five different populations of ganglion cells that send visual information to the brain:
  1. M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus;
  2. P cells, with smaller center-surround receptive fields that are sensitive to color and shape;
  3. K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
  4. another population that is intrinsically photosensitive; and
  5. 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 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.
In the visual system, retinal, technically called retinene1 or "retinaldehyde", is a light-sensitive molecule found in the rods and cones of the retina. Retinal is the fundamental structure involved in the transduction of light into visual signals, i.e. nerve impulses in the ocular system of the central nervous system. In the presence of light, the retinal molecule changes configuration and as a result, a nerve impulse is generated.

Optic nerve

The information about the image via the eye is transmitted to the brain along the optic nerve. Different populations of ganglion cells in the retina send information to the brain through the optic nerve. About 90% of the axons in the optic nerve go to the lateral geniculate nucleus in the thalamus. These axons originate from the M, P, and K ganglion cells in the retina, see above. This parallel processing is important for reconstructing the visual world; each type of information will go through a different route to perception. Another population sends information to the superior colliculus in the midbrain, which assists in controlling eye movements as well as other motor responses.
A final population of photosensitive ganglion cells, containing melanopsin for photosensitivity, sends information via the retinohypothalamic tract to the pretectum, to several structures involved in the control of circadian rhythms and sleep such as the suprachiasmatic nucleus, and to the ventrolateral preoptic nucleus. A recently discovered role for photoreceptive ganglion cells is that they mediate conscious and unconscious vision – acting as rudimentary visual brightness detectors as shown in rodless coneless eyes.