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87-447: The LGN receives information directly from the ascending retinal ganglion cells via the optic tract and from the reticular activating system . Neurons of the LGN send their axons through the optic radiation , a direct pathway to the primary visual cortex . In addition, the LGN receives many strong feedback connections from the primary visual cortex . In humans as well as other mammals ,

174-430: A contralateral lesion in the primary visual cortex; however, these patients are able to perform certain motor tasks accurately in their blind field, such as grasping. This suggests that neurons travel from the LGN to both the primary visual cortex and higher cortex regions. The output of the LGN serves several functions. Computations are achieved to determine the position of every major element in object space relative to

261-684: A depressed rate of firing. There is wide variability in ganglion cell types across species. In primates, including humans, there are generally three classes of RGCs: The W, X and Y retinal ganglion types arose from studies of the cat. These physiological types are closely related to the respective morphological retinal ganglion types γ {\displaystyle \gamma } , β {\displaystyle \beta } and α {\displaystyle \alpha } . Based on their projections and functions, there are at least five main classes of retinal ganglion cells: P-type retinal ganglion cells project to

348-443: A full representation of visual space. That is, it contains neurons whose receptive fields together represent the entire visual field. Visual information enters the ventral stream through the primary visual cortex and travels through the rest of the areas in sequence. Moving along the stream from V1 to AIT, receptive fields increase their size, latency, and the complexity of their tuning. For example, recent studies have shown that

435-489: A group of fine fibers and a zone of thinly dispersed neurons. Additionally, several studies have suggested further subdivisions of the vLGN in other species. For example, studies indicate that the cytoarchitecture of the vLGN in the cat differs from rodents. Although five subdivisions of the vLGN in the cat have been identified by some, the scheme that divides the vLGN into three regions (medial, intermediate, and lateral) has been more widely accepted. The intergeniculate leaflet

522-413: A high frequency because of their expression of K v 3 potassium channels . Degeneration of axons of the retinal ganglion cells (the optic nerve ) is a hallmark of glaucoma . Retinal ganglion cells (RGCs) are born between embryonic day 11 and post-natal day zero in the mouse and between week 5 and week 18 in utero in human development. In mammals, RGCs are typically added at the beginning in

609-408: A hole is generated in this gradient, thus allowing RGCs to cross. Molecules mediating attraction include NrCAM, which is expressed by growing RGCs and the midline glia and acts along with Sema6D, mediated via the plexin-A1 receptor. VEGF-A is released from the midline directs RGCs to take a contralateral path, mediated by the neuropilin-1 (NRP1) receptor. cAMP seems to be very important in regulating

696-433: A number of spatial disorders including: The ventral stream is associated with object recognition and form representation. Also described as the "what" stream, it has strong connections to the medial temporal lobe (which is associated with long-term memories ), the limbic system (which controls emotions), and the dorsal stream (which deals with object locations and motion). The ventral stream gets its main input from

783-404: A paper by David Milner and Melvyn A. Goodale in 1992, argues that humans possess two distinct visual systems. Recently there seems to be evidence of two distinct auditory systems as well. As visual information exits the occipital lobe , and as sound leaves the phonological network, it follows two main pathways, or "streams". The ventral stream (also known as the "what pathway") leads to

870-419: A role in defining the ipsilateral projection by altering expression of Zic2 and EphB1 receptor production. Once out of the optic chiasm, RGCs will extend dorsocaudally along the ventral diencephalic surface making the optic tract, which will guide them to the superior colliculus and lateral geniculate nucleus in the mammals, or the tectum in lower vertebrates. Sema3d seems to be promote growth, at least in

957-491: A significant role in altering. For example, Foxg1, also called Brain-Factor 1, and Foxd1, also called Brain Factor 2, are winged-helix transcription factors that are expressed in the nasal and temporal optic cups and the optic vesicles begin to evaginate from the neural tube. These factors are also expressed in the ventral diencephalon, with Foxd1 expressed near the chiasm, while Foxg1 is expressed more rostrally. They appear to play

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1044-483: A subject's ability to reproduce speech (typically by repetition), though it has no influence on the subject's ability to comprehend spoken language. This shows that conduction aphasia must reflect not an impairment of the auditory ventral pathway but instead of the auditory dorsal pathway. Buchsbaum et al found that conduction aphasia can be the result of damage, particularly lesions, to the Spt (Sylvian parietal temporal). This

1131-511: Is a relatively small area found dorsal to the vLGN. Earlier studies had referred to the IGL as the internal dorsal division of the vLGN. Several studies have described homologous regions in several species, including humans. The vLGN and IGL appear to be closely related based on similarities in neurochemicals, inputs and outputs, and physiological properties. The vLGN and IGL have been reported to share many neurochemicals that are found concentrated in

1218-461: Is also expressed in a similar pattern, secreted from the cells in the lens. Adhesion molecules, like N-CAM and L1, will promote growth centrally and will also help to properly fasciculate (bundle) the RGC axons together. Shh is expressed in a high central, low peripheral gradient, promoting central-projecting RGC axons extension via Patched-1, the principal receptor for Shh, mediated signaling. RGCs exit

1305-408: Is also good at detecting and analyzing movements. The dorsal stream commences with purely visual functions in the occipital lobe before gradually transferring to spatial awareness at its termination in the parietal lobe. The posterior parietal cortex is essential for "the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of

1392-424: Is integral in the early steps of color processing, where opponent channels are created that compare signals between the different Photoreceptor cell types. The output of P-cells comprises red-green opponent signals. The output of M-cells does not include much color opponency, rather a sum of the red-green signal that evokes luminance . The output of K-cells comprises mostly blue-yellow opponent signals. In rodents,

1479-460: Is only on growth cones coming from the ipsilaterally projecting RGCs. Other factors influencing ipsilateral RGC growth include the Teneurin family, which are transmembrane adhesion proteins that use homophilic interactions to control guidance, and Nogo, which is expressed by midline radial glia. The Nogo receptor is only expressed by VTc RGCs. Finally, other transcription factors seem to play

1566-609: Is shown by the Spt's involvement in acquiring new vocabulary, for while experiments have shown that most conduction aphasiacs can repeat high-frequency, simple words, their ability to repeat low-frequency, complex words is impaired. The Spt is responsible for connecting the motor and auditory systems by making auditory code accessible to the motor cortex. It appears that the motor cortex recreates high-frequency, simple words (like cup ) in order to more quickly and efficiently access them, while low-frequency, complex words (like Sylvian parietal temporal ) require more active, online regulation by

1653-436: Is variable within species. The average volume of each LGN in an adult human is about 118mm 3 {\displaystyle {}^{3}} . (This is the same volume as a 4.9mm-sided cube.) A study of 24 hemispheres from 15 normal individuals with average age 59 years at autopsy found variation from about 91 to 157mm 3 {\displaystyle {}^{3}} . The same study found that in each LGN,

1740-482: The arcuate fasciculus , which is vital for both speech and language comprehension, as the arcuate fasiculus makes up the connection between Broca and Wernicke's areas. Goodale & Milner's innovation was to shift the perspective from an emphasis on input distinctions, such as object location versus properties, to an emphasis on the functional relevance of vision to behaviour, for perception or for action. Contemporary perspectives however, informed by empirical work over

1827-408: The parvocellular (as opposed to magnocellular ) layer of the lateral geniculate nucleus of the thalamus . These neurons project to V1 sublayers 4Cβ, 4A, 3B and 2/3a successively. From there, the ventral pathway goes through V2 and V4 to areas of the inferior temporal lobe : PIT (posterior inferotemporal), CIT (central inferotemporal), and AIT (anterior inferotemporal). Each visual area contains

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1914-457: The parvocellular layers of the lateral geniculate nucleus . These cells are known as midget retinal ganglion cells, based on the small sizes of their dendritic trees and cell bodies. About 80% of all retinal ganglion cells are midget cells in the parvocellular pathway . They receive inputs from relatively few rods and cones. They have slow conduction velocity , and respond to changes in color but respond only weakly to changes in contrast unless

2001-574: The thalamus , particularly other relay nuclei , the LGN likely helps the visual system focus its attention on the most important information. That is, if you hear a sound slightly to your left, the auditory system likely "tells" the visual system , through the LGN via its surrounding peri-reticular nucleus, to direct visual attention to that part of space. The LGN is also a station that refines certain receptive fields . Axiomatically determined functional models of LGN cells have been determined by Lindeberg in terms of Laplacian of Gaussian kernels over

2088-516: The ECM, will anchor the Slit morphogen at specific points in the posterior chiasm border. RGCs will begin to express Robo, the receptor for Slit, at this point, thus facilitating the repulsion. RGC axons traveling to the contralateral optic tract need to cross. Shh, expressed along the midline in the ventral diencephalon, provides a repulsive cue to prevent RGCs from crossing the midline ectopically. However,

2175-422: The LGN correspond with the similarly named types of retinal ganglion cells . Retinal P ganglion cells send axons to a parvocellular layer, M ganglion cells send axons to a magnocellular layer, and K ganglion cells send axons to a koniocellular layer. Koniocellular cells are functionally and neurochemically distinct from M and P cells and provide a third channel to the visual cortex. They project their axons between

2262-411: The LGN is normally described as having six distinctive layers. The inner two layers, (1 and 2) are magnocellular layers , while the outer four layers, (3, 4, 5 and 6), are parvocellular layers . An additional set of neurons, known as the koniocellular layers , are found ventral to each of the magnocellular and parvocellular layers. This layering is variable between primate species, and extra leafleting

2349-538: The LGN travels out on the optic radiations , which form part of the retrolenticular portion of the internal capsule . The axons that leave the LGN go to V1 visual cortex . Both the magnocellular layers 1–2 and the parvocellular layers 3–6 send their axons to layer 4 in V1. Within layer 4 of V1, layer 4cβ receives parvocellular input, and layer 4cα receives magnocellular input. However, the koniocellular layers, intercalated between LGN layers 1–6 send their axons primarily to

2436-402: The RGC, netrin-1 becomes repulsive, pushing the axon away from the optic disc. This is mediated through a cAMP-dependent mechanism. Additionally, CSPGs and Eph–ephrin signaling may also be involved. RGCs will grow along glial cell end feet in the optic nerve. These glia will secrete repulsive semaphorin 5a and Slit in a surround fashion, covering the optic nerve which ensures that they remain in

2523-432: The Spt, language acquisition is impaired. The information then moves onto the articulatory network, which is divided into two separate parts. The articulatory network 1, which processes motor syllable programs, is located in the left posterior inferior temporal gyrus and Brodmann's area 44 (pIFG-BA44). The articulatory network 2 is for motor phoneme programs and is located in the left M1-vBA6. Conduction aphasia affects

2610-430: The Spt. This explains why conduction aphasiacs have particular difficulty with low-frequency words which requires a more hands-on process for speech production. "Functionally, conduction aphasia has been characterized as a deficit in the ability to encode phonological information for production," namely because of a disruption in the motor-auditory interface. Conduction aphasia has been more specifically related to damage of

2697-527: The Sylvian fissure at the parietal-temporal boundary). The spt is important for perceiving and reproducing sounds. This is evident because its ability to acquire new vocabulary, be disrupted by lesions and auditory feedback on speech production, articulatory decline in late-onset deafness and the non-phonological residue of Wernicke's aphasia; deficient self-monitoring. It is also important for the basic neuronal mechanisms for phonological short-term memory. Without

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2784-665: The V4 area is responsible for color perception in humans, and the V8 (VO1) area is responsible for shape perception, while the VO2 area, which is located between these regions and the parahippocampal cortex, integrates information about the color and shape of stimuli into a holistic image. All the areas in the ventral stream are influenced by extraretinal factors in addition to the nature of the stimulus in their receptive field. These factors include attention , working memory , and stimulus salience . Thus

2871-558: The absence of rods and cones. They project to, among other areas, the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract for setting and maintaining circadian rhythms . Other retinal ganglion cells projecting to the lateral geniculate nucleus (LGN) include cells making connections with the Edinger-Westphal nucleus (EW), for control of the pupillary light reflex , and giant retinal ganglion cells . Most mature ganglion cells are able to fire action potentials at

2958-414: The apical process of the RGC is likely mediated by Slit–Robo signaling. RGCs will grow along glial end feet positioned on the inner surface (side closest to the future vitreous humor). Neural cell adhesion molecule (N-CAM) will mediate this attachment via homophilic interactions between molecules of like isoforms (A or B). Slit signaling also plays a role, preventing RGCs from growing into layers beyond

3045-464: The auditory dorsal pathway is necessary because, "learning to speak is essentially a motor learning task. The primary input to this is sensory, speech in particular. So, there must be a neural mechanism that both codes and maintains instances of speech sounds, and can use these sensory traces to guide the tuning of speech gestures so that the sounds are accurately reproduced." In contrast to the ventral stream's auditory processing, information enters from

3132-403: The body in space". It contains individually functioning lobules. The lateral intraparietal sulcus (LIP) contains neurons that produce enhanced activation when attention is moved onto the stimulus or the animal saccades towards a visual stimulus, and the ventral intraparietal sulcus (VIP) where visual and somatosensory information are integrated. Damage to the posterior parietal cortex causes

3219-403: The brainstem that are not involved in visual perception also project to the LGN, such as the mesencephalic reticular formation, dorsal raphe nucleus, periaqueuctal grey matter, and the locus coeruleus. The LGN also receives some inputs from the optic tectum (known as the superior colliculus in mammals). These non-retinal inputs can be excitatory, inhibitory, or modulatory. Information leaving

3306-642: The cells, including neuropeptide Y, GABA, encephalin, and nitric oxide synthase. The neurochemicals serotonin, acetylcholine, histamine, dopamine, and noradrenaline have been found in the fibers of these nuclei. Both the vLGN and IGL receive input from the retina, locus coreuleus, and raphe. Other connections that have been found to be reciprocal include the superior colliculus, pretectum, and hypothalamus, as well as other thalamic nuclei. Physiological and behavioral studies have shown spectral-sensitive and motion-sensitive responses that vary with species. The vLGN and IGL seem to play an important role in mediating phases of

3393-468: The change is great. They have simple center-surround receptive fields , where the center may be either ON or OFF while the surround is the opposite. M-type retinal ganglion cells project to the magnocellular layers of the lateral geniculate nucleus. These cells are known as parasol retinal ganglion cells, based on the large sizes of their dendritic trees and cell bodies. About 10% of all retinal ganglion cells are parasol cells, and these cells are part of

3480-412: The chiasm, the glial cells supporting them will change from an intrafascicular to radial morphology. A group of diencephalic cells that express the cell surface antigen stage-specific embryonic antigen (SSEA)-1 and CD44 will form an inverted V-shape. They will establish the posterior aspect of the optic chiasm border. Additionally, Slit signaling is important here: Heparin sulfate proteoglycans, proteins in

3567-478: The chiasm. Some VTc RGCs will project contralaterally because they express the transcription factor Islet-2, which is a negative regulator of Zic2 production. Shh plays a key role in keeping RGC axons ipsilateral as well. Shh is expressed by the contralaterally projecting RGCs and midline glial cells. Boc, or Brother of CDO (CAM-related/downregulated by oncogenes), a co-receptor for Shh that influences Shh signaling through Ptch1, seems to mediate this repulsion, as it

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3654-527: The circadian rhythms that are not involved with light, as well as phase shifts that are light-dependent. Retinal ganglion cell A retinal ganglion cell ( RGC ) is a type of neuron located near the inner surface (the ganglion cell layer ) of the retina of the eye . It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells . Retina amacrine cells, particularly narrow field cells, are important for creating functional subunits within

3741-514: The contralateral optic tract or remain in the ipsilateral optic tract. In the mouse, about 5% of RGCs, mostly those coming from the ventral-temporal crescent (VTc) region of the retina, will remain ipsilateral, while the remaining 95% of RGCs will cross. This is largely controlled by the degree of binocular overlap between the two fields of sight in both eyes. Mice do not have a significant overlap, whereas, humans, who do, will have about 50% of RGCs cross and 50% will remain ipsilateral. Once RGCs reach

3828-402: The cytochrome-oxidase rich blobs of layers 2 and 3 in V1. Axons from layer 6 of visual cortex send information back to the LGN. Studies involving blindsight have suggested that projections from the LGN travel not only to the primary visual cortex but also to higher cortical areas V2 and V3. Patients with blindsight are phenomenally blind in certain areas of the visual field corresponding to

3915-417: The dLGN comes from the retina. It is laminated and shows retinotopic organization. The ventrolateral geniculate nucleus has been found to be relatively large in several species such as lizards, rodents, cows, cats, and primates. An initial cytoarchitectural scheme, which has been confirmed in several studies, suggests that the vLGN is divided into two parts. The external and internal divisions are separated by

4002-399: The defining property of having a long axon that extends into the brain. These axons form the optic nerve , optic chiasm , and optic tract . A small percentage of retinal ganglion cells contribute little or nothing to vision, but are themselves photosensitive; their axons form the retinohypothalamic tract and contribute to circadian rhythms and pupillary light reflex , the resizing of

4089-442: The dissociation was not as strong as first thought. A 2009 review of the accumulated evidence for the model concluded that whilst the spirit of the model has been vindicated the independence of the two streams has been overemphasised. Goodale & Milner themselves have proposed the analogy of tele-assistance, one of the most efficient schemes devised for the remote control of robots working in hostile environments. In this account,

4176-441: The dorsal 'action' stream transforms incoming visual information to the requisite egocentric (head-centered) coordinate system for skilled motor planning . The model also posits that visual perception encodes spatial properties of objects, such as size and location, relative to other objects in the visual field; in other words, it utilizes relative metrics and scene-based frames of reference. Visual action planning and coordination, on

4263-622: The dorsal central aspect of the optic cup , or eye primordium. Then RC growth sweeps out ventrally and peripherally from there in a wave-like pattern. This process depends on a host of factors, ranging from signaling factors like FGF3 and FGF8 to proper inhibition of the Notch signaling pathway. Most importantly, the bHLH (basic helix-loop-helix)-domain containing transcription factor Atoh7 and its downstream effectors, such as Brn3b and Isl-1, work to promote RGC survival and differentiation . The "differentiation wave" that drives RGC development across

4350-408: The dorsal stream is viewed as a semi-autonomous function that operates under guidance of executive functions which themselves are informed by ventral stream processing. Thus the emerging perspective within neuropsychology and neurophysiology is that, whilst a two-systems framework was a necessary advance to stimulate study of the highly complex and differentiated functions of the two neural pathways;

4437-416: The execution of the grasping act. Norman proposed a similar dual-process model of vision, and described eight main differences between the two systems consistent with other two-system models. The dorsal stream is proposed to be involved in the guidance of actions and recognizing where objects are in space. The dorsal stream projects from the primary visual cortex to the posterior parietal cortex . It

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4524-532: The existence of two visual systems for localisation and identification in 1969. Ingle described two independent visual systems in frogs in 1973. Ettlinger reviewed the existing neuropsychological evidence of a distinction in 1990. Moreover, Trevarthen had offered an account of two separate mechanisms of vision in monkeys back in 1968. In 1982, Ungerleider and Mishkin distinguished the dorsal and ventral streams, as processing spatial and visual features respectively, from their lesion studies of monkeys – proposing

4611-411: The focus of some criticism of the model due to the perceived over-reliance on findings from a single case. Goodale and Milner amassed an array of anatomical, neuropsychological, electrophysiological, and behavioural evidence for their model. According to their data, the ventral 'perceptual' stream computes a detailed map of the world from visual input, which can then be used for cognitive operations, and

4698-486: The ganglion cell layer and making it so that ganglion cells can observe a small dot moving a small distance. Retinal ganglion cells collectively transmit image-forming and non-image forming visual information from the retina in the form of action potential to several regions in the thalamus , hypothalamus , and mesencephalon , or midbrain . Retinal ganglion cells vary significantly in terms of their size, connections, and responses to visual stimulation but they all share

4785-607: The idea that skilled actions such as grasping are not affected by pictorial illusions. Moreover, recent neuropsychological research has questioned the validity of the dissociation of the two streams that has provided the cornerstone of evidence for the model. The dissociation between visual agnosia and optic ataxia has been challenged by several researchers as not as strong as originally portrayed; Hesse and colleagues demonstrated dorsal stream impairments in patient DF; Himmelbach and colleagues reassessed DF's abilities and applied more rigorous statistical analysis demonstrating that

4872-479: The koniocellular pathway. They receive inputs from intermediate numbers of rods and cones. They may be involved in color vision. They have very large receptive fields that only have centers (no surrounds) and are always ON to the blue cone and OFF to both the red and green cone. Photosensitive ganglion cells , including but not limited to the giant retinal ganglion cells, contain their own photopigment , melanopsin , which makes them respond directly to light even in

4959-498: The lateral geniculate nucleus contains the dorsal lateral geniculate nucleus (dLGN), the ventral lateral geniculate nucleus (vLGN), and the region in between called the intergeniculate leaflet (IGL). These are distinct subcortical nuclei with differences in function. The dorsolateral geniculate nucleus is the main division of the lateral geniculate body. In the mouse, the area of the dLGN is about 0.48mm 2 {\displaystyle {}^{2}} . The majority of input to

5046-585: The layers of the lateral geniculate nucleus where M and P cells project. Their role in visual perception is presently unclear; however, the koniocellular system has been linked with the integration of somatosensory system-proprioceptive information with visual perception, and it may also be involved in color perception. The parvo- and magnocellular fibers were previously thought to dominate the Ungerleider–Mishkin ventral stream and dorsal stream , respectively. However, new evidence has accumulated showing that

5133-495: The left LGN receives visual information from the right visual field. Within one LGN, the visual information is divided among the various layers as follows: This description applies to the LGN of many primates, but not all. The sequence of layers receiving information from the ipsilateral and contralateral (opposite side of the head) eyes is different in the tarsier . Some neuroscientists suggested that "this apparent difference distinguishes tarsiers from all other primates, reinforcing

5220-400: The magnocellular layers comprised about 28mm 3 {\displaystyle {}^{3}} in total, and the parvocellular layers comprised about 90mm 3 {\displaystyle {}^{3}} in total. *Size describes the cell body and dendritic tree, though also can describe the receptive field The magnocellular, parvocellular, and koniocellular layers of

5307-655: The magnocellular pathway. They receive inputs from relatively many rods and cones. They have fast conduction velocity, and can respond to low-contrast stimuli, but are not very sensitive to changes in color. They have much larger receptive fields which are nonetheless also center-surround. BiK-type retinal ganglion cells project to the koniocellular layers of the lateral geniculate nucleus. K-type retinal ganglion cells have been identified only relatively recently. Koniocellular means "cells as small as dust"; their small size made them hard to find. About 10% of all retinal ganglion cells are bistratified cells, and these cells go through

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5394-416: The nickname of the dorsal stream to be updated to the "how" pathway. The dorsal stream is interconnected with the parallel ventral stream (the "what" stream) which runs downward from V1 into the temporal lobe . The dorsal stream is involved in spatial awareness and guidance of actions (e.g., reaching). In this it has two distinct functional characteristics—it contains a detailed map of the visual field, and

5481-465: The optic fiber layer. Axons from the RGCs will grow and extend towards the optic disc , where they exit the eye. Once differentiated, they are bordered by an inhibitory peripheral region and a central attractive region, thus promoting extension of the axon towards the optic disc. CSPGs exist along the retinal neuroepithelium (surface over which the RGCs lie) in a peripheral high–central low gradient. Slit

5568-401: The optic nerve. Vax1, a transcription factor, is expressed by the ventral diencephalon and glial cells in the region where the chiasm is formed, and it may also be secreted to control chiasm formation. When RGCs approach the optic chiasm, the point at which the two optic nerves meet, at the ventral diencephalon around embryonic days 10–11 in the mouse, they have to make the decision to cross to

5655-458: The original where vs what distinction. Though this framework was superseded by that of Milner & Goodale, it remains influential. One hugely influential source of information that has informed the model has been experimental work exploring the extant abilities of visual agnosic patient D.F. The first, and most influential report, came from Goodale and colleagues in 1991 and work is still being published on her two decades later. This has been

5742-404: The other hand, uses absolute metrics determined via egocentric frames of reference, computing the actual properties of objects relative to the observer. Thus, grasping movements directed towards objects embedded in size-contrast-ambiguous scenes have been shown to escape the effects of these illusions, as different frames of references and metrics are involved in the perception of the illusion versus

5829-469: The past two decades, offer a more complex account than a simple separation of function into two-streams. Recent experimental work for instance has challenged these findings, and has suggested that the apparent dissociation between the effects of illusions on perception and action is due to differences in attention, task demands, and other confounds. There are other empirical findings, however, that cannot be so easily dismissed which provide strong support for

5916-418: The pigment epithelium, undergo a terminal cell division and differentiation, and then migrate backwards towards the inner limiting membrane in a process called somal translocation . The kinetics of RGC somal translocation and underlying mechanisms are best understood in the zebrafish . The RGC will then extend an axon in the retinal ganglion cell layer, which is directed by laminin contact. The retraction of

6003-412: The primary auditory cortex into the posterior superior temporal gyrus and posterior superior temporal sulcus. From there the information moves to the beginning of the dorsal pathway, which is located at the boundary of the temporal and parietal lobes near the Sylvian fissure. The first step of the dorsal pathway begins in the sensorimotor interface, located in the left Sylvian parietal temporal (Spt) (within

6090-461: The primary auditory cortex. In this pathway, phonemes are processed posteriorly to syllables and environmental sounds. The information then joins the visual ventral stream at the middle temporal gyrus and temporal pole. Here the auditory objects are converted into audio-visual concepts. The function of the auditory dorsal pathway is to map the auditory sensory representations onto articulatory motor representations. Hickok & Poeppel claim that

6177-434: The principal plane. Through subsequent motion of the eyes, a larger stereoscopic mapping of the visual field is achieved. It has been shown that while the retina accomplishes spatial decorrelation through center surround inhibition, the LGN accomplishes temporal decorrelation. This spatial–temporal decorrelation makes for much more efficient coding. However, there is almost certainly much more going on. Like other areas of

6264-660: The production of NRP1 protein, thus regulating the growth cones response to the VEGF-A gradient in the chiasm. The only component in mice projecting ipsilaterally are RGCs from the ventral-temporal crescent in the retina, and only because they express the Zic2 transcription factor. Zic2 will promote the expression of the tyrosine kinase receptor EphB1, which, through forward signaling (see review by Xu et al. ) will bind to ligand ephrin B2 expressed by midline glia and be repelled to turn away from

6351-423: The proximal optic tract, and cytoskeletal re-arrangements at the level of the growth cone appear to be significant. In most mammals, the axons of retinal ganglion cells are not myelinated where they pass through the retina. However, the parts of axons that are beyond the retina, are myelinated. This myelination pattern is functionally explained by the relatively high opacity of myelin—myelinated axons passing over

6438-406: The pupil. There are about 0.7 to 1.5 million retinal ganglion cells in the human retina. With about 4.6 million cone cells and 92 million rod cells , or 96.6 million photoreceptors per retina, on average each retinal ganglion cell receives inputs from about 100 rods and cones. However, these numbers vary greatly among individuals and as a function of retinal location. In the fovea (center of

6525-424: The reality is more likely to involve considerable interaction between vision-for-action and vision-for-perception. Robert McIntosh and Thomas Schenk summarize this position as follows: We should view the model not as a formal hypothesis, but as a set of heuristics to guide experiment and theory. The differing informational requirements of visual recognition and action guidance still offer a compelling explanation for

6612-407: The retina is also regulated in particular of the bHLH factors Neurog2 and Ascl1 and FGF/Shh signaling, deriving from the periphery. Early progenitor RGCs will typically extend processes connecting to the inner and outer limiting membranes of the retina with the outer layer adjacent to the retinal pigment epithelium and inner adjacent to the future vitreous humor. The cell soma will pull towards

6699-460: The retina would absorb some of the light before it reaches the photoreceptor layer, reducing the quality of vision. There are human eye diseases where this does, in fact, happen. In some vertebrates, such as the chicken, the ganglion cell axons are myelinated inside the retina. Ventral stream The two-streams hypothesis is a model of the neural processing of vision as well as hearing . The hypothesis, given its initial characterisation in

6786-418: The retina), a single ganglion cell will communicate with as few as five photoreceptors. In the extreme periphery (edge of the retina), a single ganglion cell will receive information from many thousands of photoreceptors. Retinal ganglion cells spontaneously fire action potentials at a base rate while at rest. Excitation of retinal ganglion cells results in an increased firing rate while inhibition results in

6873-540: The retinal ganglion cell layer through the optic disc, which requires a 45° turn. This requires complex interactions with optic disc glial cells which will express local gradients of Netrin-1, a morphogen that will interact with the Deleted in Colorectal Cancer (DCC) receptor on growth cones of the RGC axon. This morphogen initially attracts RGC axons, but then, through an internal change in the growth cone of

6960-698: The spatial domain in combination with temporal derivatives of either non-causal or time-causal scale-space kernels over the temporal domain. It has been shown that this theory both leads to predictions about receptive fields with good qualitative agreement with the biological receptive field measurements performed by DeAngelis et al. and guarantees good theoretical properties of the mathematical receptive field model, including covariance and invariance properties under natural image transformations. Specifically according to this theory, non-lagged LGN cells correspond to first-order temporal derivatives, whereas lagged LGN cells correspond to second-order temporal derivatives. The LGN

7047-497: The temporal lobe, which is involved with object and visual identification and recognition . The dorsal stream (or, "where pathway") leads to the parietal lobe, which is involved with processing the object's spatial location relative to the viewer and with speech repetition. Several researchers had proposed similar ideas previously. The authors themselves credit the inspiration of work on blindsight by Weiskrantz , and previous neuroscientific vision research. Schneider first proposed

7134-473: The two streams appear to feed on a more even mixture of different types of nerve fibers. The other major retino–cortical visual pathway is the tectopulvinar pathway , routing primarily through the superior colliculus and thalamic pulvinar nucleus onto posterior parietal cortex and visual area MT . Both the LGN in the right hemisphere and the LGN in the left hemisphere receive input from each eye. However, each LGN only receives information from one half of

7221-508: The two strongest pathways linking the eye to the brain are those projecting to the dorsal part of the LGN in the thalamus, and to the superior colliculus . Both the left and right hemispheres of the brain have a lateral geniculate nucleus, named after its resemblance to a bent knee ( genu is Latin for "knee"). In humans as well as in many other primates , the LGN has layers of magnocellular cells and parvocellular cells that are interleaved with layers of koniocellular cells. In humans

7308-411: The ventral stream does not merely provide a description of the elements in the visual world—it also plays a crucial role in judging the significance of these elements. Damage to the ventral stream can cause inability to recognize faces or interpret facial expression. Along with the visual ventral pathway being important for visual processing, there is also a ventral auditory pathway emerging from

7395-485: The view that they arose in an early, independent line of primate evolution". The LGN receives input from the retina and many other brain structures, especially visual cortex. The principal neurons in the LGN receive strong inputs from the retina. However, the retina only accounts for a small percentage of LGN input. As much as 95% of input in the LGN comes from the visual cortex, superior colliculus, pretectum, thalamic reticular nuclei, and local LGN interneurons. Regions in

7482-401: The visual field. Retinal ganglion cells (RGCs) from the inner halves of each retina (the nasal sides) decussate (cross to the other side of the brain) through the optic chiasma ( khiasma means "cross-shaped"). RGCs from the outer half of each retina (the temporal sides) remain on the same side of the brain. Therefore, the right LGN receives visual information from the left visual field, and

7569-423: Was initially termed the "where" pathway since it was thought that the dorsal stream processes information regarding the spatial properties of an object. However, later research conducted on a famous neuropsychological patient, Patient D.F., revealed that the dorsal stream is responsible for processing the visual information needed to construct the representations of objects one wishes to manipulate. Those findings led

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