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The visual system is the part of the Nervous System which allows organisms to See . It interprets the information from Visible Light to build a representation of the world surrounding the Body . The visual system has the complex task of (re)constructing a three dimensional world from a two dimensional projection of that world. The psychological manifestation of visual information is known as Visual Perception . INTRODUCTION This article mostly describes the visual system of Mammal s, although other "higher" animals have similar visual systems. In this case, the visual system consists of:
Different Species are able to see different parts of the Light Spectrum ; for example, Bee s can see into the Ultraviolet J Bellingham, SE Wilkie, AG Morris, JK Bowmaker and DM Hunt (1997), "Characterisation of the ultraviolet-sensitive opsin gene in the honey bee, Apis mellifera", ''European Journal of Biochemistry'', Vol 243, 775-781 , while Pit Viper s can accurately target prey with their Infrared imaging sensors AB Safer and MS Grace (2004), "Infrared imaging in vipers: differential responses of crotaline and viperine snakes to paired thermal targets". ''Behav Brain Res.'' 154(1):55-61. 2004 Sep 23. '' .
EYE The Eye is a complex biological device. The functioning of a camera is often compared with the workings of the Eye , mostly since both focus light from external objects in the Visual Field 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 . 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. , ''Structure of the Mammal ian Retina, 1900'']] Retina The Retina consists of a large number of Photoreceptor cells which contain a particular Protein Molecule called an Opsin . In humans, there are two types of opsins, Rod Opsins and Cone Opsins . Either opsin absorbs a Photon (a particle of light) and transmits a signal to the Cell through a Signal Transduction Pathway , resulting in hyperpolarization of the photoreceptor. (For more information, see 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. 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 are used primarily to distinguish Color and other features of the visual world at normal levels of light. In the retina, the photoreceptors synapse directly onto Bipolar Cell s, which in turn synapse onto Ganglion Cell s of the outermost layer, who will then conduct Action Potentials to the Brain . A significant amount of visual processing arises from the patterns of communication between Neuron s in the retina. About 130 million photoreceptors 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 Cell s, 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. The final result of all this processing is five different populations of ganglion cells that send 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 recent University Of Pennsylvania study calculated the approximate Bandwidth of human retinas as 8.75 megabits per second, whereas Guinea Pig retinas transfer at 875 kilobits. http://www.newscientist.com/article/dn9633-calculating-the-speed-of-sight Photochemistry See Also: Visual cycle In the Vision System , retinal, technically called ''retinene1'' or "retinaldehyde", is a light-sensitive Retinene molecule found in the Photoreceptor Cell s 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. FIBERS TO THALAMUS Optic nerve s (top), crossing at the Optic Chiasm a, joining left and right eye information in the Optic Tract , and layering left and right visual stimuli in the Lateral Geniculate Nucleus . V1 in red at bottom of image. ( 1543 image from Andreas Vesalius ' ''Fabrica'')]] |
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