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 main article: optic nerve information flow from the eyes (top), crossing at the optic chiasma, 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) 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 (saccades) 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 (pupillary reflex), to several structures involved in the control of circadian rhythms and sleep such as the suprachiasmatic nucleus (the biological clock), and to the ventrolateral preoptic nucleus (a region involved in sleep regulation). 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. optic chiasm main article: optic chiasm the optic nerves from both eyes meet and cross at the optic chiasm, at the base of the hypothalamus of the brain. at this point, the information coming from both eyes is combined and then splits according to the visual field. the corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively, to be processed. that is, the right side of primary visual cortex deals with the left half of the field of view from both eyes, and similarly for the left brain. a small region in the center of the field of view is processed redundantly by both halves of the brain. optic tract main article: optic tract information from the right visual field (now on the left side of the brain) travels in the left optic tract. information from the left visual field travels in the right optic tract. each optic tract terminates in the lateral geniculate nucleus (lgn) in the thalamus. six layers in the lgn lateral geniculate nucleus main article: lateral geniculate nucleus the lateral geniculate nucleus (lgn) is a sensory relay nucleus in the thalamus of the brain. the lgn consists of six layers in humans and other primates starting from catarrhines, including cercopithecidae and apes. layers 1, 4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal retina (temporal visual field); layers 2, 3, and 5 correspond to information from the ipsilateral (uncrossed) fibers of the temporal retina (nasal visual field). layer one contains m cells, which correspond to the m (magnocellular) cells of the optic nerve of the opposite eye and are concerned with depth or motion. layers four and six of the lgn also connect to the opposite eye, but to the p cells (color and edges) of the optic nerve. by contrast, layers two, three and five of the lgn connect to the m cells and p (parvocellular) cells of the optic nerve for the same side of the brain as its respective lgn. spread out, the six layers of the lgn are the area of a credit card and about three times its thickness. the lgn is rolled up into two ellipsoids about the size and shape of two small birds' eggs. in between the six layers are smaller cells that receive information from the k cells (color) in the retina. the neurons of the lgn then relay the visual image to the primary visual cortex (v1) which is located at the back of the brain (posterior end) in the occipital lobe in and close to the calcarine sulcus. the lgn is not just a simple relay station, but it is also a center for processing; it receives reciprocal input from the cortical and subcortical layers and reciprocal innervation from the visual cortex. scheme of the optic tract with image being decomposed on the way, up to simple cortical cells (simplified). optic radiation main article: optic radiation the optic radiations, one on each side of the brain, carry information from the thalamic lateral geniculate nucleus to layer 4 of the visual cortex. the p layer neurons of the lgn relay to v1 layer 4c β. the m layer neurons relay to v1 layer 4c α. the k layer neurons in the lgn relay to large neurons called blobs in layers 2 and 3 of v1. there is a direct correspondence from an angular position in the visual field of the eye, all the way through the optic tract to a nerve position in v1 (up to v4, i.e. the primary visual areas. after that, the visual pathway is roughly separated into a ventral and dorsal pathway). visual cortex main article: visual cortex visual cortex: v1; v2; v3; v4; v5 (also called mt) the visual cortex is the largest system in the human brain and is responsible for processing the visual image. it lies at the rear of the brain (highlighted in the image), above the cerebellum. the region that receives information directly from the lgn is called the primary visual cortex, (also called v1 and striate cortex). it creates a bottom-up saliency map of the visual field to guide attention or eye gaze to salient visual locations, hence selection of visual input information by attention starts at v1 along the visual pathway. visual information then flows through a cortical hierarchy. these areas include v2, v3, v4 and area v5/mt (the exact connectivity depends on the species of the animal). these secondary visual areas (collectively termed the extrastriate visual cortex) process a wide variety of visual primitives. neurons in v1 and v2 respond selectively to bars of specific orientations, or combinations of bars. these are believed to support edge and corner detection. similarly, basic information about color and motion is processed here. heider, et al. (2002) have found that neurons involving v1, v2, and v3 can detect stereoscopic illusory contours; they found that stereoscopic stimuli subtending up to 8° can activate these neurons. visual cortex is active even during resting state fmri. visual association cortex main article: two-streams hypothesis as visual information passes forward through the visual hierarchy, the complexity of the neural representations increases. whereas a v1 neuron may respond selectively to a line segment of a particular orientation in a particular retinotopic location, neurons in the lateral occipital complex respond selectively to complete object (e.g., a figure drawing), and neurons in visual association cortex may respond selectively to human faces, or to a particular object. along with this increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: the dorsal stream and the ventral stream (the two streams hypothesis, first proposed by ungerleider and mishkin in 1982). the dorsal stream, commonly referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates with regions that control eye movements and hand movements. more recently, this area has been called the "how" stream to emphasize its role in guiding behaviors to spatial locations. the ventral stream, commonly referred to as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli. intraparietal sulcus (red) however, there is still much debate about the degree of specialization within these two pathways, since they are in fact heavily interconnected. horace barlow proposed the efficient coding hypothesis in 1961 as a theoretical model of sensory coding in the brain. limitations in the applicability of this theory in the primary visual cortex (v1) motivated the v1 saliency hypothesis that v1 creates a bottom-up saliency map to guide attention exogenously. with attentional selection as a center stage, vision is seen as composed of encoding, selection, and decoding stages. the default mode network is a network of brain regions that are active when an individual is awake and at rest. the visual system's default mode can be monitored during resting state fmri: fox, et al. (2005) have found that "the human brain is intrinsically organized into dynamic, anticorrelated functional networks'", in which the visual system switches from resting state to attention. in the parietal lobe, the lateral and ventral intraparietal cortex are involved in visual attention and saccadic eye movements. these regions are in the intraparietal sulcus (marked in red in the adjacent image). development infancy