Morphology of the retina:The retina consists of the following types of cells:Pigment epithelium, photoreceptors, bipolar cell, ganglion cell, horizontal cell and amacrine cell. There are many different synaptic transmitters. The rods and cones, which are next to the choroid, synapse with bipolar cells, and the bipolar cells synapse with ganglion cells.The axons of the ganglion cells converge and leave the eye as the optic nerve. Horizontal cells connect receptor cells to the other receptor cells in the outer plexiform layer. Amacrine cells connect ganglion cells to one another in the inner plexiform layer via processes of varying length and patterns.

Pigment epithelium: It is a single sheet of melanin- containing epithelial cells, it has the following functions:a. Light absorption: most of the light reaching the back of the eye is absorbed by the pigment epithelium so that light scattering does not degrade visual.

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b. Phagocytosis: the membranous disks and other debris sloughed from the photoreceptors are phagocytosed by cells of the pigment epithelium. c. Vitamin A (retinol) storage: the pigment epithelium serves as repository for vitamin A, which is needed for the synthesis of rhodopsin. The eyes convert energy in the visible spectrum into action potentials in the optic nerve. The wave-lengths of visible light range from approximately 397–723 nm. The images of objects in the environment are focused on the retina. The light rays striking the retina generate potentials in the rods and cones. Impulses initiated in the retina are conducted to the cerebral cortex, where they produce the sensation of vision.

Photoreceptors:Rod Cone1. Morphology :

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The major functional segments of either a rod or cone are: (1) the outer segment, (2) the inner segment, (3) the nucleus, and (4) the synaptic body.Both rod and cone contains nucleus , abundant mitochondria and synaptic vesiclesa .Inner segment : thin inner segmentb. Outer segment :In the rods and cones contains large numbers of discs (as many as 1000 discs) in each rod or cone.rod like appearancethe disks are separated from cell membraneRod outer segment being constantly renewed by formation of new disks at the inner edge of the segment and Phagocytosis of old disks and from the outer tip by cells of the pigment epithelium.2. Distribution :mainly are extrafovealabout 120 million in eye3. Photosensitive pigment :It is called rhodopsin or visual purple.rhodopsin consist ofretinene1:Synthesized from vitamin A; light-absorbing; bonded to membrane protein opsinRetinene is into two forms all-trans retinal (active) and 11-cis retinal (inactive) Opsin called scotopsin:rods contain a single type of opsin with peak sensitivity to light at wavelength of 498 nmscotopsin+11-cis retinal= rhodopsinThis why there is only one type of rodScotopsin is a 348 amino acid and crosses the disc membrane 7 times

a. Inner segment :Thick inner segmentb. Outer segment :

Cone like appearancethe saccules are formed in the outer segment by infoldings of the cell membranecone renewal is more diffuse process and appears to occur at multiple sites in the outer segments

2. Distribution :mainly are intrafovealabout 6 million in eye3. Photosensitive pigment :It is call conopsinConopsin consist ofretinene1:Synthesized from vitamin A; light-absorbing; bonded to membrane protein opsinRetinene is into two forms all-trans retinal (active) and 11-cis retinal (inactive)opsin is called photopsin:opsin moiety contains different aa sequences that determine wavelength of light absorb.It is three types which gives 3 types of cons:a) cyan-opsin: maximally sensitive to blue (420)Short wave cone (Cyanolabe or S cone)b) iod-opsin maximally sensitive to green (534nm)Middle wave cone (chlorolabe or M cone)c) porphyr-opsin: maximally sensitive to red (564nm)Long wave cone (erythrolabe or L-cone)

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4. Functions :I. The rods , which are more sensitive to light than cones , are responsible for night vision (scoptopic vision ) because :A. rods can absorb more light than cones.B. rods contain more rhodopsin in their outer segment.C. rod s can detect light entering the eye from any direction

II. Rods produce a greater response for each photon of light absorbed. The scotopic visual apparatus is incapable of resolving the details and boundaries of objects (low visual acuity) or determining their color. They detect white, black and shades of gray

III. Rods remain polarized for a longer time than cones. Therefore, the response produced by several photons of light can be added together to create a larger response in rods than in cones.6. Dark adaptation:Cones adapt first

I. The cones are responsible for daylight vision;vision in bright light (photopic vision)Cones can respond to light over a large range of intensities

II. Cones achieve high visual acuity becausea) they are concentrated in the fovea (where the images are formed) and have less sensitivity by 200 times than rodsb) They responding only to light directly along their (visual axis)c) they do not respond to scattered lightd) Their response to light is brisk. the cones produce an image of high quality and sharp imagesIII. The cones have three different photo-pigments. Color vision is achieved by combining the information contained in cones, which absorb light in the red ,green or blue range of the visual spectrum6. Light adaptation:Rods adapt first

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Rhodopsin-retinal visual cycle and excitation of the rode:Note:Rode and rhodopsin have been studied more deeply than cons so we believe what happen in rod could also happens in consRhodopsin and its decomposition by light energy:

A light photon interacts with the retinal in a photoreceptor cell.

The retinal undergoes isomerization, changing from the 11-cis to all-trans configuration

Retinal no longer fits into the opsin binding site.

Opsin therefore undergoes a conformational change to metarhodopsin II.

Metarhodopsin II is unstable and splits, yielding opsin and all-trans retinal (bleaching) is a rapid process (up to 5 minutes) while regeneration of rhodopsin is slow process takes minutes (up to 30

minutes)

All-trans retinal is transported to the pigment epithelial cells to be reduced to all-trans retinol, the precursor to 11-cis retinal. This is then transported back to the rods.

All-trans retinal cannot be synthesized by humans and must be supplied by vitamin A in the diet. Deficiency of all-trans retinal can lead to night blindness (as rods are part of night vision).

The rod receptor potential is hyperpolarizing, not depolarizing; why?Activation of the receptor protein in rods (rhodopsin) by light (1 photon activate 1 rhodopsin)

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1. The opsin activates the regulatory protein transducing (heterotrimeric G protein, which in this case is called transducin or Gt1)

2. transducin to dissociate from its bound GDP, and bind GTP3. the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit.4. The alpha subunit-GTP complex activates phosphor-di-esterase or PDE.5. phosphor-di-esterase breaks down cGMP to 5'-GMP6. This lowers the concentration of cGMP 7. cGMP depended sodium channels to close.8. cGMP depended sodium channels causes hyperpolarization of the cell due to the ongoing efflux of potassium ions.a. Na-K ATPase pushes 3Na outside and 2K inside rodeb. Potassium "leak" channels pushes K to outside again and Na cannot return to rode due to closer of cGMP-dependent Na+ channelsc. Rod loses many positive charge ions ►hyperpolarization

9. Hyperpolarization of the cell causes voltage-gated calcium channels to close.

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10. As the calcium level in the photoreceptor cell drops, the amount of the neurotransmitter glutamate that is released by the cell also drops. This is because calcium is required for the glutamate-containing vesicles to fuse with cell membrane and release their contents.11. A decrease in the amount of glutamate released by the photoreceptors causes depolarization of on center bipolar cells (rod and cone on bipolar cells) and hyperpolarization of cone off-center bipolar cells.At dark the reverse will occur and there is depolarization and increase in neurotransmitter release Thus, under normal dark conditions, when the rod is not excited, there is reduced electro-negativity inside the membrane of the rod, measuring about −40 millivolts rather than the usual −70 to −80 millivolts found in most sensory receptorsDark and light adaptation:Between the limits of maximal dark adaptation and maximal light adaptation, the eye can change its sensitivity to light as much as 500,000 to 1 million times, with the sensitivity automatically adjusting to changes in illumination. Because registration of images by the retina requires detection of both dark and light spots in the image, it is essential that the sensitivity of the retina always be adjusted so that the receptors respond to the lighter areas but not to the darker areasThere are 3 mechanisms underlying light/dark adaptation:1. Optic adaptation (Pupil size) the pupil diameter only ranges from 1 or 2 mm to about 8 mm, for an increase in area (or total light entering the eye) of a factor of 16-64. Changes in pupillary size changes in the amount of light allowed through the pupillary opening2. Receptor adaptation a. Switchover from rods to conesb. Bleaching/regeneration of the photo-pigments3. Neural adaptation (Feedback from the horizontal cells to control the responsiveness of the photoreceptors)Neural adaptation, involving the neurons in the successive stages of the visual chain in the retina itself and in the brain. That is, when light intensity first increases, the signals transmitted by the bipolar cells, horizontal cells, amacrine cells, and ganglion cells are all intense مكثف. However, most of these signals decrease rapidly at different stages of transmission in the neural circuit. Although the degree of adaptation is only a few fold rather than the many thousand fold that occurs during adaptation of the photochemical system, neural adaptation occurs in a fraction of a second, in contrast to the many minutes to hours required for full adaptation by the photo-chemicals.Dark adaptation

الفلم تشاهد ان تستطيع انك تالحظ سوف الخارج من قادما السينما الى تدخل عندماوجه وتشاهد الضالم تعتاد حتى اطول وقت الى تحتاج ولكن مباشرة الشاشة على المعروض

لك المجاور الكرسي الى الجالسDuring dark adaptation First: pupillary size will increases (dilation of pupa); dilation will depend on degree of darkness Second:Receptor adaptation will includes a. Switchover from cones to rode. b. Bleaching/regeneration of the photo-pigments

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الظالم في البقاء زمن يمثل السيني المحور ان حيث متشلبهان البنانيان الرسمان هذان انارقام يمثل اليمين والى حقيقية ارقام يمثل اليسار الى فالرسم الصادي المحور اما

لوغارتمية

Note: we have to put in mind that cones have lower sensitivity to light than rods and that building of photosensitive process is slow process while destruction of it is a fast process

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When we transfer from light to dark the cones are still working so in the first minute the sensitivity is low but will decrease gradually due to destruction of photo-sensitive pigment; and because destruction process is fast this why it will not takes more than 5 minute for cons to adapt (reach Plateau)

مباشرة الشاشة على المعروض الفلم تشاهد ان تستطيع لماذا يفسر هذا When we begins dark adaptation the photo-sensitive pigment in cones begins to destruct (so sensitivity decrease) while in rod will be build up (so sensitivity increase). After (7-10 minutes) the sensitivity will be equalizes this is called (Rod-cone Break Or alpha pointe) Because rod will build up photo-sensitive pigment slowly this is why it takes about 20 to 30 minutes for rod to adapt (reach Plateau). Occurs after 93% of rhodopsin has already regenerated

بجانبك الجالس مالمح تالحظ ان الجل اطول وقت الى تحتاج لماذا يفسر هذاAt the end of dark adaptation, the eye is about 100,000 times (i.e., 5 log units) more sensitive than it was at the beginning of the test, a level called dark adapted sensitivity.Radiologists, aircraft pilots, and others who need maximal visual sensitivity in dim light can avoid having to wait 20 minutes in the dark to become dark-adapted if they wear red goggles when in bright light. Light wavelengths in the red end of the spectrum stimulate the rods to only a slight degree while permitting the cones to function reasonably well. Therefore, a person wearing red glasses can see in bright light during the time it takes for the rods to become dark-adapted.

الن للضالم التأقلم سرعة على تساعد الحمراء النضارات لبس اناالسود. 1 من الطيفوقريب نهاية في هو االحمرمن. 2 واحد نوع يشمل النه التاقلم من يسرع فقط الحمراء المخارط تحفز

المخاريطLight adaptationThis occurs when we move from the dark into bright light. The bright light momentarily dazzles us and all we see is white light because the sensitivity of the receptors is set to dim light. Rods and cones are both stimulated and large amounts of the photo-pigment are broken down instantaneously, producing a flood of signals resulting in the glare.Adaption occurs in 5 minutes and is due toConstriction of pupaThe sensitivity of the retina decreases dramatically due to decrease of photo-sensitive pigment in rod and cones (as we said that degeneration is faster than regeneration) Retinal neurons undergo rapid adaptation inhibiting rod function and favoring the cone system.Within about one minute the cones are sufficiently excited by the bright light to take over. Visual accuracy and color vision continue to improve over the next ten minutes. During light adaptation retinal sensitivity is losthttps://www.youtube.com/watch?v=zSXT9dDpnDARetinal electrophysiologyThe retina extends anteriorly almost to the ciliary body. The different neuronal cell types in the retina are as follows: 1. The photoreceptors—the rods and cones—which transmit signals to the outer plexiform layer, where they synapse with bipolar cells and horizontal cells

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2. The horizontal cells, which transmit signals horizontally in the outer plexiform layer from the rods and cones to bipolar cells 3. The bipolar cells, which transmit signals vertically from the rods, cones, and horizontal cells to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells

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4. The amacrine cells, which transmit signals in two directions, either directly from bipolar cells to ganglion cells or horizontally within the inner plexiform layer from axons of the bipolar cells to dendrites of the ganglion cells or to other amacrine cells 5. The ganglion cells, which transmit output signals from the retina through the optic nerve into the brainThere are many different synaptic transmitters. a. both the rods and the cones release glutamate at their synapses with the bipolar cells in a graded fashion. More depolarized ►more glutamate. More hyperpolarized ►less glutamate b. amacrine cells that secrete inhibitory transmitters including (GABA), glycine, dopamine, acetylcholine, and indolaminec. The transmitters of the bipolar, horizontal, and interplexiform cells are unclearThe eye is unique in that the receptor potentials of the photoreceptors and the electrical responses of most of the other neural elements in the retina are local, graded potentials, and it is only in the ganglion cells that all-or-none action potentials transmitted over appreciable distances are generated.

The importance of electrotonic conduction is that it allows graded conduction of signal strength. Thus, for the rods and cones, the strength of the hyperpolarizing output signal is directly related to the intensity of illumination; the signal is not all or none, as would be the case for each action potential.The responses of the rods, cones, and horizontal cells are hyperpolarizingThe responses of the bipolar cells are either hyperpolarizing or depolarizingThe responses of the horizontal cells are always inhibitory.The responses of the amacrine cells produce depolarizing potentials and spikes that may act as generator potentials for the propagated spikes produced in the ganglion cells.The cone receptor potential has a sharp onset and offset,The rod receptor potential has a sharp onset and slow offset. The major differences between the peripheral retina (extra-foveal) and the central retina (foveal) are:First: As one approaches the fovea, fewer rods and cones converge on each optic fiber, and the rods and cones also become more slender. These effects progressively increase the acuity of vision in the central retina. In the center (central fovea) a. there are only slender cones (about 35,000 of them) and no rods. b. the number of optic nerve fibers leading from this part of the retina is almost exactly equal to the number of cones. The (cones: bipolar cells: ganglion cells) (1:1:1)This phenomenon explains the high degree of visual acuity in the central retina in comparison with the much poorer acuity peripherally. Second: there are much greater sensitivity of the peripheral retina to weak light, which occurs a. partly because rods are 30 to 300 times more sensitive to light than cones are

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b. as many as 200 rods converge on a single optic nerve fiber in the more peripheral portions of the retina Thus signals from the rods summate to give even more intense stimulation of the peripheral ganglion cells and their optic nerve fibers.Because there are approximately 6 million cones and 120 million rods in each human eye but only 1.2 million nerve fibers in each optic nerve, the overall convergence of receptors through bipolar cells on ganglion cells is about 105:1.

Retinal receptive field:The position of the spot of light was systematically varied across the retinal surface, and while they did this the response of the ganglion cell was continuously monitored.For most positions on the surface of the retina, flashing a spot of light has absolutely no effect on the cell's response (that is, it continues responding at its spontaneous firing rate). Within a particular region, called the receptive field, flashing the spot affects the ganglion cell's response.Retinal receptive field is the region of the visual space that makes a retinal ganglion cell change its firing under the presence of a certain visual stimulus.On-center, Off-center retinal ganglion cell:

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On-center retinal ganglion cell Off-center retinal ganglion cell no stimulus was presented: Spontaneous firing rate of a few spikes/sec

no stimulus was presented: Spontaneous firing rate of a few spikes/sec

Flashing small bright spot in the center sub-region increases the cell's response

Flashing small bright spot in the center sub-region decreases the cell's response

There is little or no response to a large (full field) spot of light that covers both the center and the surround because excitation in the center cancels the inhibition from the surround, called lateral inhibition. This is because and cancel each other

There is little or no response to a large (full field) spot of light that covers both the center and the surround because excitation in the center cancels the inhibition from the surround, called lateral inhibition. This is because and cancel each other

Flashing a bright annulus in the surround sub-region inhibits the cell's response

Flashing a bright annulus in the surround sub-region stimulate the cell's response

Electrophysiology explanation of on-center off-center retinal ganglion cell action potential:1. On-center retinal ganglion cellThe receptive field of a bipolar cell consists of two parts: a. Center receptive field:Light on the receptive field center, which provides a direct input from the photoreceptors (hyperpolarization) ►to the bipolar cells (depolarization) ►ganglion cell (depolarization)b. Peripheral receptive field:i. Light on the receptive field periphery: light on photoreceptors ►stimulate horizontal cell (inhibitory) ►Bipolar cell (inhibited) ►ganglion cell (inhibited) ii. Dark on the receptive field periphery the horizontal cell will not stimulate so nothing will happen2. Off-center retinal ganglion cellOff-center cells respond in exactly the same way to dark spots as on-center cells respond to bright spots.

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The mechanism for producing the center of a bipolar cell's receptive field is well known: direct innervation of the photoreceptors above it, either through a metabotropic (ON) or inotropic (OFF) receptor.https://www.youtube.com/watch?v=ZR7LzRAXNSw https://www.youtube.com/watch?v=3gWRN5kcopE https://www.youtube.com/watch?v=fFEeAJjambI https://www.youtube.com/watch?v=9ptnmfpDThk https://www.youtube.com/watch?v=NLQCYflVV3M The combination of ON-center and OFF- center ganglion cells is another example of a parallel pathway. ON-center and OFF- center ganglion cells are physiologically distinct (as just described above). ON-center and OFF- center ganglion cells are anatomically distinct: The ganglion cells dendrites branch out in separate sub-layers of the retina The ganglion cells dendrites branch receive synaptic inputs from different subclasses of bipolar cells. There is complete coverage: ON-center and OFF- center receptive fields each completely cover the visual field. The neural signals originating in the ON- and OFF- center retinal ganglion cells remain segregated in the retina and the LGN, then merge completely in the complex cells in primary visual cortex (V1).

What is the benefit of on-center off-center?The center-surround organization of receptive fields is an application of lateral inhibition seen in sensory system which makes retinal bipolar cells and ganglion cells very sensitive to contrast. For example, the response of a given cell to light will be stronger if the light portion of the visual field is adjacent to a dark portion. As a result, the retinal mechanisms for contrast enhancement make our visual systems very sensitive to edges or borders or shapes and allow us to perceive even weak contrasts.

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Types of retinal ganglion cellsMagnocellular: large (M)or alpha or parasol cells

Parvocellular: small (P) beta cells or midget ganglion cells

receptive field 1. large 1. smallaxons conduct impulse 2. slow 2. fastresponses to stimuli, 3. sustained 3. transientsensitivity to the color 4. sensitive to black and white stimuli 4. sensitive to the colormain function 5. Highly sensitive to low-contrast

stimuli and to rapid movement visual signals and stereopsis االشكال.مجسمة

5. highly sensitive to visual signals that relate to fine details (texture, and shape) and to different colors

Central Neurophysiology of VisionVisual pathways Retinas ►optic nerves ►optic chiasm ►optic tracts ►dorsal lateral geniculate nucleus of the thalamus ► geniculocalcarine fibers pass by way of the optic radiation (also called the geniculocalcarine tract) to the primary visual cortex in the medial occipital lobe.

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Visual fibers pass to several older areas of the brain (1) from the optic tracts to the suprachiasmatic nucleus of the hypothalamus, presumably to control circadian rhythms that synchronize various physiological changes of the body with night and day; (2) into the pretectal nuclei in the midbrain, to elicit reflex movements of the eyes to focus on objects of importance and to activate the pupillary light reflex; (3) into the superior colliculus, to control rapid directional movements of the two eyes (4) into the ventral lateral geniculate nucleus of the thalamus and surrounding basal regions of the brain, presumably to help control some of the body’s behavioral functions.Lateral geniculate body functions:1. It relays visual information (so accurate that there is exact point-to-point transmission) from the optic tract to the visual cortex.Layers II, III, and V (from ventral to dorsal) receive signals from the lateral half of the ipsilateral retina, whereas layers I, IV, and VI receive signals from the medial half of the retina of the opposite eye.2. To “gate” the transmission of signals to the visual cortex—that is, to control how much of the signal is allowed to pass to the cortex. The nucleus receives gating control signals from two major sources: (1) corticofugal fibers (2) reticular areas of the mesencephalon.From the lateral geniculate nucleus, a magnocellular pathway and a parvocellular pathway project tothe visual cortex. a. The magnocellular pathway, from layers 1 and 2, carries signals for detection of movement, depth, and flicker, black and white information, point-to-point transmission is poorb. The parvocellular pathway, from layers 3 to 6, carries signals for color vision, texture, shape, and fine detail (accurate point-to-point spatial information)

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Organization and function of visual cortex:Primary Visual Cortex, visual area I or the striate cortex (Brodmann’s area 17, V1):The retina is fully represented in primary visual cortex where the macula represented by largest area Secondary Visual Areas of the Cortex/ visual association areas(Visual area II, Brodmann’s area 18, V2/ Brodmann’s area 19 V3,V4, V5)

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The layers of primary visual cortex:Like almost all other portions of the cerebral cortex, the primary visual cortex has six distinct layers.The axons from the lateral geniculate nucleus that form the magnocellular pathway end in layer 4, specifically in its deepest part, layer 4Cα. Many of the axons that form the parvocellular pathway end in layer 4Cβ and 4A

The layers of the cortex differ not only in their inputs and their local interconnections but also in the more distant structures to which they project. All layers except, 4A, and 4C send fibers out of the cortex. Layers 2 and 3 and layer 4B project mainly to other cortical regions, whereas the deep layers project down to subcortical structures. Layer 5 projects to the superior colliculus in the midbrain Layer 6 projects mainly back to the lateral geniculate body.

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Columnar architecture of primary visual cortexVertical neuronal columns in the visual cortex.Primary visual cortex is organized into a two-dimensional array of vertical columns.Vertical columns extend from the cortical surface to underlying white matter ( قطعة صمون كأنها(لوفVertical columns are several million Vertical columns having a thickness of 30 to 50 micrometersVertical columns represents a functional unitVertical columns has perhaps 1000 or more neuronsVertical columns processes a characteristic (contrast, color, orientation, movements etc.) of different part of visual field.

Vertical columns has two dimensions:a. Ocular dominance columns which receive preference from either the contralateral or the ipsilateral eye (or right and left eye); which is important for binocular interaction and depth perceptionBinocular interaction: is interaction of visual signals from the two separate eyes.

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القشرة من اليمنى الجهة في اليمين و اليسار للعين الشبكية تمثيل مناطق يمثل الرسمالزبرى جلد وكأنها الدماغية

The visual signals from the two separate eyes remain separated .These signals remain separated from each other but when they arrive in layer IV of the primary visual cortex; the signals from one eye enter the columns of every other stripe, alternating with signals from the second eye so that they will fuse with each other. The result, when viewed from above, is a vivid pattern of stripes that covers much of the visual cortex. The information observed about degree of register of images from the two eyes also allows a person to distinguish the distance of objects by the mechanism of stereopsis (depth perception المجسمة (الرؤياb. Orientation columns The orientation preferences of neighboring columns differ in a systematic way; as one moves from column to column across the cortex, sequential changes occur in orientation preference of 5–10 degrees. Thus, it seems likely that for each ganglion cell receptive field in the visual field, there is a collection of columns in a small area of visual cortex representing the possible preferred orientations at small intervals throughout the full 360 degrees.Orientation columns are important in perceiving form and movements

Cortical Modules:Cortical module is a basic unit processing visual stimuli in visual cortex Cortical module 2mm X 2mm dimensionsCortical module is a cortical image of point in space

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Cortical modules consists of:a. 2 ocular dominance columnb. 2 hyper-columnsHyper-columns is a set of columns that are responsive to line of all orientation from a particular region in the visual cortex and view by two ocular dominance columns, one from eachc. 16 bolbsBolb: Layers 2 and 3 of the cortex contain clusters of cells about 0.2 mm in diameter that, unlike the neighboring cells, contain a high concentration of the mitochondrial enzyme cytochrome oxidase.Bolbs are arranged in a mosaic in the visual cortex and are concerned with color vision

“Color Blobs” in the Visual Cortex. Color Blobs Interspersed among the primary visual columns and some of the secondary visual areasColor Blobs receive lateral signals from adjacent visual columns and are activated specifically by color signalsColor Blobs are presumably the primary areas for deciphering color.

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Feature Detection Model of Form PerceptionThe feature detector model of form perception is based on the fact that neurons farther and farther into the visual system become more and more selective in what they respond to.Retinal neurons respond to spots of light.Neurons in primary visual cortex respond to bars of light.Neurons in the first stages of the visual association cortex respond only to moving bars of light.Neurons in the later stages of the visual association cortex respond to complex patterns, including hand-shaped and face-shaped patternsThe simple and complex cells have been called feature detectors because they respond to and analyze certain features of the stimulus.Simple cells Simple cells are found mainly in layer IV of the primary visual cortexSimple cells are monocularSimple cells respond to bars of light, lines, or edges, with following criteria:a. Simple cells respond only when they have a particular orientation (orientation selectivity) (e.g., vertical) but not to the orthogonal orientation (e.g., horizontal). When, for example, a bar of light is rotated as little as 10 degrees from the preferred orientation, the firing rate of the simple cell is usually decreased, and if the stimulus is rotated much more, the response disappears.

ادراك عن مسؤلة الخاليا هذه من واحدة كل ولكن الخطوط ادراك عن مسؤلة الخاليا هذهمحددة - بزاوية الخط

b. Simple cells have separate ON and OFF sub-regionsSimple cells which fire high frequency of Action potentials only when a bar of light is at the center of its receptive field and in a particular orientation; the areas of the receptive field which cause the high frequency of Action potential is called the ON sub-region and the areas of the receptive field that do not trigger high frequency of Action potential is called the OFF sub-regionc. Simple cells respond best to elongated bars or edgesd. Simple cells perform length summation (they have bigger responses with increasing bar length up to some limit, at which point the response reaches a plateau)e. Simple cells receptive fields are non-concentric and linear. Thus, a response is elicited by stationary linear stimuliComplex cellsa. Complex cells are orientation selective.

ادراك عن مسؤلة الخاليا هذه من واحدة كل ولكن الخطوط ادراك عن مسؤلة الخاليا هذههي اذا الواحد السطح ضمن تحرك وان الخط تدرك ان تستطيع ولكنها محددة بزاوية الخط

االسطح تدرك ان تستطيعb. Complex cells have spatially homogeneous receptive fields (no separate ON/OFF subregions).Complex cells respond to lines that are oriented in the same direction motion (not position specific). That is, even if a line is displaced moderate distances laterally or vertically in the field, the Complex cells will still be stimulated if the line has the same orientation (angle).

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Complex cells probably receive input from the simple cells.

c. Complex cells are nearly all binocular.d. Complex cells perform length summation.Hyper-complex cell (currently called an end-stopped cell)

a. Hyper Complex cells respond to lines that are oriented in the same direction motion (not position specific) Length (property of end-stopping) which means if the bar is too short or too long, it will not be able to excite hyper-complex cellb. hypercomplex cells can provide a means for corners and curves in the environment by identifying the ends of a given stimulusHypercomplex cells were originally characterized as the superordinate class of visual processing cells above simple and complex cell because a. Hyper-complex cells respond to complex stimuli such as hand and face

b. if complex cell respond to line with particular orientation such as , then Hyper complex respond to combination of features such as (A)

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OTHER CORTICAL AREASCONCERNED WITH VISIONOther cortical areas concerned with visionAs mentioned above, the primary visual cortex (V1) projects tomany other parts of the occipital lobes and other parts of thebrain. These are often identified by number (V2, V3, etc) or byletters (LO, MT, etc). The distribution of some of these in thehuman brain is shown in Figure 12–19, and their putative functionsare listed in Table 12–1. Studies of these areas have beencarried out in monkeys trained to do various tasks and then fittedwith implanted microelectrodes. In addition, the availabilityof PET and functional magnetic resonance imaging (fMRI)scanning has made it possible to conduct sophisticated experimentson visual cognition and other cortical visual functions innormal, conscious humans. The visual projections from V1 canbe divided roughly into a dorsal or parietal pathway, concernedprimarily with motion, and a ventral or temporal pathway,concerned with shape and recognition of forms and faces.In addition, connections to the sensory areas are important. For

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example, in the occipital cortex, visual responses to an object arebetter if the object is felt at the same time. There are many otherrelevant connections to other systems.It is apparent from the preceding paragraphs that parallel processingof visual information occurs along multiple paths. Insome as yet unknown way, all the information is eventually pulledtogether into what we experience as a conscious visual image.

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http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/V1/lgn-V1.html

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In primary visual cortex there is Simple cells which are found in layer 4. Simple and complex cells are stacked on top of one another in primary visual cortex in specific way with in each orientation columns. Hyper-complex cells are found only in higher visual processing area

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vertical columns are concerned with orientation (orientation columns). Each is about 1 mm in diameter. “Ocular dominance columns” “vertical columns” “orientation columns”

However,the orientation preferences of neighboring columns differ in a

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systematic way; as one moves from column to column acrossthe cortex, sequential changes occur in orientation preferenceof 5–10 degrees. Thus, it seems likely that for each ganglion cellreceptive field in the visual field, there is a collection of columnsin a small area of visual cortex representing the possible preferredorientations at small intervals throughout the full 360degrees. The simple and complex cells have been called featuredetectors because they respond to and analyze certain featuresof the stimulus. Feature detectors are also found in the corticalareas for other sensory modalities.

In each orientation column (Fig. 7.19), there are two types of cell. Simple cells are found in layers 4 and 6 and respond to stationary bars of a certain orientation from a single visual field; that is, they are monocular. In the other layers of each orientation column, mainly 2, 3 and 5, there are complex cells, which are not direction sensitive but respond preferentially to bars of light of the same orientation as the simple cells, but moving across the receptive field, parallel to the preferred orientation. However, these cells are binocular, as they respond to input from both eyes, although they show a preference for the visual field of the simple cells. Moving across the cortex, each successive column has a preferred orientation, which changes by about 15°. Across a distance of about 1 mm, there are columns for each possible orientation for a single eye, from a given part of the visual field. This set of orientation columns is called an ocular dominance column. Parallel to this are a similar set of columns representing input from the same part of the visual field of the other eye. The area of the cortex consisting of two ocular dominance columns, one from each eye, which is about 1 mm2, is called a hypercolumn and represents all of the input from a part of the visual field to the primary visual cortex.Arranged within the ocular dominance columns are columns of complex cells, which are wavelength-sensitive. These colour-sensitive cells are arranged in cylindrical regions called blobs; they receive convergent input from the parvocellular and koniocellular LGN layers. They can be detected using staining methods, as they contain high levels of the mitochondrial enzyme cytochrome oxidase. The areas between the blobs are called the interblob regions. The receptive fields of most of the blob neurones are circular, and show varying types of colour opponency. The most complex of these are the double opponent cells, which signal colour contrast. There are four types, depending on the preferred stimulus. For example, one type has a maximal ‘on’ response to a red spot on a green background, and an ‘off’ response to a green spot on a red background. The other red–green type is the reverse, and a similar combination occurs for blue and yellow.

Simple Cells[edit]Simple cells are found in layers 4 and 6 and respond to stationary bars of a certain orientation from a single visual field; that is, they are monocular.Following their initial finding, Hubel and Wiesel discovered the presence of a variety of visual processing cells, each with unique receptive field properties. At the lowest and simplest level of the hierarchy are the aforementioned centre-surround cells of the retinal ganglion and LGN. Next, within the visual cortex, are simple cells.[4] Simple cells exist within the primary visual cortex (Brodmann Area 17). These cells are found specifically in layer IV, at which most outgoing projections from the LGN terminate.[4] The receptive fields of simple cells are non-concentric and linear, in which excitatory and inhibitory regions exist adjacent to one another. Thus, a response is elicited by stationary linear stimuli. Furthermore, the regions exhibit mutual cancellation (antagonism) and produce stronger responses as the stimuli fill more space (spatial summation). A discerning feature

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of simple cells is that their responses display orientation and positional selectivity. This means that a simple cell fires at an optimal orientation. Elicited responses get progressively weaker as a stimulus's orientation shifts sub-optimally and ceases to fire when at 90˚ from the optimal orientation. Positional selectivity simply refers to the cell's receptiveness to the position of the stimulus within part or all of the excitatory/inhibitory regions. Accordingly, simple cell receptive fields exist in a variety of different geometries and sizes for all possible orientation and positions in the visual field. It is presumed that multiple concentric LGN receptive fields converge in a line to develop a single simple receptive field.[4][5]

Simple cells are sensitive to the orientation of a visual stimulus. A simple cell will fire weakly or not at all if both excitatory and inhibitory regions are activated (a), but will fire optimally if the stimulus is oriented within the excitatory region only (b). Orientation selectivity is produced by multiple centre-surround receptive fields aligned at a certain angle (c). A complex cell responds to moving stimuli and is sensitive to direction as well as orientation (d).Complex Cells[edit]In the other layers of each orientation column, mainly 2, 3 and 5, there are complex cells, which are not direction sensitive but respond preferentially to bars of light of the same orientation as the simple cells, but moving across the receptive field, parallel to the preferred orientation. However, these cells are binocular, as they respond to input from both eyes, although they show a preference for the visual field of the simple cells.Beyond simple cells are complex cells, which are the most common type in the primary visual cortex (but are also found in Brodmann area 18). Akin to simple cells, complex cell receptive fields are orientation selective. However, unlike simple cells, complex cells do not respond to stationary stimuli. To produce a sustained response, the stimulus must be moving across the receptive field. The motion selectivity of complex cells means that a response is elicited over a vast range of stimulus positions. A substantial number of complex cells also display directional selectivity, such that movement in only one direction produces an optimal response. The cortical architecture of complex cells consists of converging adjacent simple cells with receptive fields that display the same orientation selectivity. To account for the motion selectivity of complex cells, Hubel and Wiesel postulated that the system of simple cells only elicits a brief response to stationary stimuli (i.e. the response adapts). Accordingly, successive stimulations that proceed across the complex receptive field are required to elicit a sustained response; thereby, producing motion selectivity. [4]

Although the above definitions, established by Hubel and Wiesel, are the most widely accepted, some of their contemporaries had initially distinguished the classes along different criteria. In sum, Hubel and Wiesel identified simple cells by discernibly separate excitatory and inhibitory regions that responded to stationary stimuli. Contrastingly, Peter Bishop used other criteria and included moving stimuli within the definition of simple cells.[1]

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The rods and cones make synaptic contacts with the dendrites of bipolar and horizontal cells. The signals from rods and cones are transmitted to the bipolar and horizontal cells via chemical synapses. As mentioned earlier, vision during normal daylight depends on cones, while night vision involves rods.Processing of Signals from the Photoreceptors by Different Retinal CellsBipolar, Horizontal, and Ganglion CellsThe cell bodies of bipolar neurons are located in the inner nuclear layer of the retina. These cells constitute the main link in the transmission of visual signals from rods and cones to ganglion cells. The receptive field of a bipolar cell is a circular area of the retina that, when stimulated by a light stimulus, changes the membrane potential of the bipolar cell. The receptive field of a bipolar cell consists of two parts: the receptive field center, which provides a direct input from the photoreceptors to the bipolar cells, and the receptive field surround, which provides an indirect input from the photoreceptors to the bipolar cells via horizontal cells (Fig. 16-4A). The changes in membrane potential of bipolar cells to a light stimulus upon the receptive field center and surround are opposite.The mechanism of membrane potential changes in the bipolar cells in response to light can be summarized as follows. There are two populations of bipolar cells: "on"-center bipolar cells and "off"-center bipolar cells. When stimulated, bipolar cells exhibit graded potentials rather than action

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potentials.Each photoreceptor cell (e.g., a cone; Fig. 16-4B, 1) synapses on an on-center (Fig. 16-4B, 2) and an off-center bipolar cell (Fig. 16-4B, 3). Each on-center bipolar cell, in turn, synapses with an on-center ganglion cell (Fig. 16-4B, 4), and each off-center bipolar cell synapses with an off-center ganglion cell (Fig. 16-4B, 5).

FIGURE 16-4 Receptive fields of photoreceptors and their connections. (A) The receptive field center provides a direct input from the photoreceptors to the bipolar cell, and the receptive field surround provides indirect input from the photoreceptor to the bipolar cells via horizontal cells. (B) 1: Photoreceptor cell; 2: on-center bipolar cell; 3: off-center bipolar cell; 4: on-center ganglion cell; 5: off-center ganglion cell.

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FIGURE 16-5 Responses of retinal bipolar and ganglion cells to darkness and illumination in the receptive field center. (A) Changes in the electrical activity of the photoreceptor and on-center and off-center bipolar and ganglion cells when the photoreceptor receptive field center is in the dark. (B) Changes in the electrical activity of the photoreceptor and on-center and off-center bipolar and ganglion cells when the photoreceptor receptive field center is illuminated.

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FIGURE 16-6 Responses of retinal bipolar and ganglion cells to darkness and illumination in the receptive field surround. (A) Changes in the electrical activity of the photoreceptor and on-center and off-center bipolar and ganglion cells when the photoreceptor receptive field surround is in the dark. (B) Changes in the electrical activity of the photoreceptor and on-center and off-center bipolar and ganglion cells when the photoreceptor receptive field surround is illuminated. See text for details. GABA = gamma aminobutyric acid.When the receptive field center is in dark (Fig. 16-5A, 1), the photoreceptors are depolarized (Fig. 16-5A, 2), and they release glutamate constantly (Fig. 16-5A, 3). Glutamate released from the photoreceptor terminals stimulates metabotropic glutamate receptors on the on-center bipolar cells, K+ (potassium) channels are opened, there is an efflux of K+, the on-center bipolar cell is hyperpolarized, and the release of its transmitter (probably glutamate) is decreased (Fig. 16-5A, 4). On the other hand, glutamate released from the photoreceptor terminals stimulates ionotropic glutamate receptors on the off-center bipolar cells, Na+ channels are opened, Na+ flows into the cell, the off-center bipolar cell is depolarized, and the release of its transmitter (probably glutamate) is increased (Fig. 16-5A, 5).Hyperpolarization of on-center bipolar cells (Fig. 16-5A, 4) results in a decrease in the release of their transmitter, which, in turn, results in a decrease in the firing of the corresponding on-center ganglion cells (Fig. 16-5A, 6). Depolarization of off-center bipolar cells (Fig. 16-5A-5) results in an increase in the release of their transmitter which, in turn, results in an increase in the firing of the corresponding off-center ganglion cells (Fig. 16-5A, 7).When the photoreceptor in the receptive field center receives a light stimulus (Fig. 16-5B, 1), it is hyperpolarized (Fig. 16-5B, 2), and glutamate release from its terminals is decreased (Fig. 16-5B, 3). The reduction in the release of glutamate from the photoreceptor terminals causes depolarization of the on-center bipolar cell and an increase in its transmitter release (Fig. 16-5B, 4), whereas the off-center bipolar cell is hyperpolarized, and there is a decrease in its transmitter release (Fig. 16-5B, 5). Depolarization of on-center bipolar cells (Fig. 16-5B, 4) results in an increase in the release of their transmitter, which, in turn, results in an increase in the firing of the corresponding on-center

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ganglion cells (Fig. 16-5B, 6). Hyperpolarization of off-center bipolar cells (Fig. 16-5B, 5) results in a decrease in the release of their transmitter, which, in turn, results in a decrease in the firing of the corresponding off-center ganglion cells (Fig. 16-5B, 7).Bipolar and ganglion cells elicit opposite responses when light is received at the receptive field surround. The photoreceptors located in the receptive field surround of the bipolar cells are connected to photoreceptors in the receptive field center by interneurons called horizontal cells. During darkness (Fig. 16-6A, 1), the photoreceptors in the receptive field surround are depolarized, and there is an increase in the release of their transmitter (glutamate) (Fig. 16-6A, 2). Glutamate released from the photoreceptors activates horizontal cells (Fig. 16-6A, 3) that, in turn, release an inhibitory transmitter (probably gamma aminobutyric acid [GABA]) (Fig. 16-6A, 4) at their synapses with photoreceptors located in the receptive field center and cause their hyperpolarization and decrease in the glutamate release from their terminals (Fig. 16-6A, 5). This phenomenon is called lateral inhibition. Decrease in glutamate release from the terminals of photoreceptors in the receptive field center results in depolarization of on-center bipolar cells and an increase in the release of their transmitter (Fig. 16-6A, 6), whereas the off-center bipolar cells are hyperpolarized, and there is a decrease in the release of their transmitter (Fig. 16-6A, 7). As mentioned earlier, depolarization of on-center bipolar cells (Fig. 16-6A, 6) causes an increase in the firing of corresponding ganglion cells (Fig. 16-6A, 8), while hyperpolarization of the off-center bipolar cells (Fig. 16-6A, 7) causes a decrease in the firing of corresponding ganglion cells (Fig. 16-6A, 9).When light is received at the receptive field surround (Fig. 16-6B, 1), the photoreceptor in this field is hyperpolarized (Fig. 16-6B, 2). There is a decrease in the release of glutamate from their terminals (Fig. 16-6B, 3), activity of horizontal cells is decreased (Fig. 16-6B, 4), GABA release at the terminals of horizontal cells is decreased, and lateral inhibition exerted by them on the photoreceptors in the receptive field center is decreased. Thus, the photorecep-tors located in the receptive field center are released from lateral inhibition from horizontal cells. Consequently, the photoreceptors in the receptive field center are depolarized (Fig. 16-6B, 5), and there is an increase in the glutamate release from their terminals (Fig. 16-6B, 6). Subsequently, the on-center bipolar cells are hyperpolarized, and there is a decrease in the release of their transmitter (Fig. 16-6B, 7). The off-center bipolar cells are depolarized, and there is an increase in the release of their transmitter (Fig. 16-6B, 8). Again, hyperpolarization of on-center bipolar cells (Fig. 16-6B, 7) causes a decrease in the firing of corresponding ganglion cells (Fig. 16-6B, 9), whereas depolarization of off-center bipolar cells (Fig. 16-6B, 8) causes an increase in the firing of corresponding ganglion cells (Fig. 16-6B, 10).

The foveal portion of the retina, cons are predominate (fast cone system); which shows three neurons in the direct pathway: (1) cones, (2) bipolar cells, and (3) ganglion cells.The extrafoveal portions of the retina, rods predominate, and there is a good deal of convergence. Flat bipolar cells make synaptic contact with several cones Rod bipolar cells make synaptic contact with several rods.

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https://www.youtube.com/watch?v=bOcwrbTR4U8 https://www.youtube.com/watch?v=bjL-Z7fW2FM https://www.youtube.com/watch?v=pfFTGiQS69M

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The ganglion cells transmit signals to the brain by action potentials. Each ganglion cell has a receptive field that can be complex. For example, there are on-center, off-surround ganglion cells which fire most strongly when there is light in the center of the field but no light in the surround. There are also off-center, on-surround ganglion cells that fire most strongly when the opposite is true. The axons of the ganglion cells form the optic nerve.

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https://www.slideshare.net/bimal0107/physiology-of-retina

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