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2. vision pathway 1

The visual pathway begins with light entering the eyes and being refracted onto the retinas. The retinas contain photoreceptor cells that transduce the image into neural signals via the optic nerves. At the optic chiasm, nerve fibers from the temporal visual fields cross while nasal fibers remain uncrossed. The optic tracts then carry these signals to the lateral geniculate nuclei and on to the primary visual cortex via the optic radiations, where a retinotopic map is preserved. Lesions to different parts of this pathway result in visual field defects contralateral to the lesion.

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Visual Pathway Dr MC Tayade Associate Professor Department of Physiology
2
• The visual system is the part of the central nervous system which gives organisms the ability to process visual detail as sight, as well as enabling the formation of several non-image photo response functions 3
• Together the cornea and lens refract light into a small image and shine it on the retina. • The retina transduces this image into action potential pulses using rods and cones. • The optic nerve then carries these pulses through the optic canal. • Upon reaching the optic chiasm the nerve fibers decussate (left becomes right). • The fibers then branch and terminate at cortex ( Visual area ) 4
• The retina consists of a large number of photoreceptor cells which contain particular protein molecules called opsins. • In humans, two types of opsins are involved in conscious vision: rod opsins and cone opsins. 5
• An opsin absorbs a photon (a particle of light) and transmits a signal to the cell through a signal transduction pathway, resulting in hyper- polarization of the photoreceptor. • Rods and cones differ in function. • Rods are found primarily in the periphery of the retina and are used to see at low levels of light. 6
• There are three types of cones that differ in the wavelengths of light they absorb; they are usually called short or blue, middle or green, and long or red. • Cones are used primarily to distinguish color and other features of the visual world at normal levels of light. 7
• In the retina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which will then conduct action potentials to the brain. • A significant amount of visual processing arises from the patterns of communication between neurons in the retina. 8
• In addition, other neurons in the retina, particularly horizontal and amacrine cells, transmit information laterally (from a neuron in one layer to an adjacent neuron in the same layer), resulting in more complex receptive fields that can be either indifferent to color and sensitive to motion or sensitive to color and indifferent to motion 9
• The retina adapts to change in light through the use of the rods. • In the dark, the chromophore retinal has a bent shape called cis-retinal (referring to a cis conformation in one of the double bonds). • When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. • This is called bleaching because the purified rhodopsin changes from violet to colorless in the light. 10
• At baseline in the dark, the rhodopsin absorbs no light and releases glutamate which inhibits the bipolar cell. • This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. • When there is light present, glutamate secretion ceases thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected. 11
• The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain: • M cells, that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus; • P cells, that are sensitive to color and shape; • K cells, that are sensitive to color and indifferent to shape or depth; • another population that is intrinsically photosensitive; and • a final population that is used for eye movements.12
13 Pathway extends from the ‘front’ to the ‘back’ of the brain. • Precise retinotopic organization • Deficits due to lesions of the pathway give valuable localizing information. The Visual Pathway OT ON OC VISUAL CORTEX RETINA VISUAL FIELD LGN OPTIC RADIATIONS ON = Optic Nerve OC = Optic Chiasm OT = Optic Tract LGN = Lateral Geniculate Nucleus of Thalamus
14 Beginning of the Pathway Pg. 2
15 Cells of the Retina Pg. 2 Rods and Cones (Receptors) Ganglion cells axons form the optic nerve Bipolar cells
16 Object to be seen Peripheral Retina The next slide looks at the retina as if you are looking through the patient’s pupil via your ophthalmoscope. Central Retina (fovea in the macula lutea) Pg. 2
17 Visual Fields & the Visual Pathway Pg. 2 OT ON OC VISUAL CORTEX RETINA VISUAL FIELD LGN OPTIC RADIATIONS ON = Optic Nerve OC = Optic Chiasm OT = Optic Tract LGN = Lateral Geniculate Nucleus of Thalamus The following slides begin with the visual fields and then follow the pathway from the retina to the visual cortex.
18 Definition: The entire area that can be “seen” by the patient without movement of the head and with the eyes fixed on a single spot. Visual Fields Pg. 3 Mapping of Visual Fields: • Confrontational method (see Dr. Meyerson’s “Neurological Exam” notes) • Perimetry (Manual or Automated) Temporal Field of Left Eye Nasal Field of Left Eye F Normal Monocular Visual Field of Right Eye Normal Monocular Visual Field of Left Eye F Monocular Visual Fields Monocular Visual Fields: • Each eye is tested separately. • The monocular visual field is plotted with the Fovea (F) at the center. • The monocular visual field (colored area -- blue for left; green for right in this example) is not round. • Horizontal and Vertical Meridians correspond to those of the retina and divide the visual field into upper temporal, upper nasal, lower temporal and lower nasal quadrants. • Imagine that this is your visual field, i.e. all that you can see with your left eye and your right eye (tested separately) when you look straight ahead and do not move your head or eyes. Vertical Meridian Horizontal Meridian UpperFieldof LeftEye LowerFieldof LeftEye UTQ LTQ UNQ LNQ
19 Blind Spot • 15° to the temporal side of the visual field of each eye • On the horizontal meridian • Corresponds to the location of the optic nerve head 15° to the nasal side of the retina of each eye. Visual Fields Pg. 3 Temporal Field of Left Eye Nasal Field of Left Eye Normal Monocular Visual Field of Left Eye F F Normal Monocular Visual Field of Right Eye UpperFieldof LeftEye LowerFieldof LeftEye
20 Visual Fields: Binocular Pg. 3 Normal Binocular Visual Field F Right Visual FieldLeft Visual Field Upper Fields Lower Fields Temporal Field of Left Eye Nasal Field of Left Eye Normal Monocular Visual Field of Left Eye F F Normal Monocular Visual Field of Right Eye Understand the difference between the “monocular visual field of the left eye” vs. the “binocular left visual field” and vice versa for the right counterparts. • Binocular field combines the two monocular visual fields with the foveas (F) aligned with one another. (i.e. the ‘pink area’ in the image to the right) • Left Visual Field seen by both the left & right eyes. • Right Visual Field seen by both the left & right eyes. • Monocular crescent for each eye (blue for left eye & green for right eye) is only seen by the nasal retina of the same eye. Monocular Crescent of Right Eye Monocular Crescent of Left Eye
21 Visual Fields: Binocular Binocular vision is dependent upon the extraocular muscles aligning the eyes so that an image falls on “corresponding points” on the retina of each eye. This is essential for the brain to perceive a single image. Diplopia occurs when the images are not aligned to fall on corresponding points of each retina. Pg. 3 Normal Binocular Visual Field F Right Visual FieldLeft Visual Field Upper Fields Lower Fields Temporal Field of Left Eye Nasal Field of Left Eye Normal Monocular Visual Field of Left Eye F F Normal Monocular Visual Field of Right Eye Demonstration of the Binocular Visual Field & Monocular Crescent: • Look straight ahead • Close your right eye • Move your finger to the right until it disappears • Open right eye to see the pencil -- in the right temporal monocular crescent of your visual field.
22 Visual Fields Pg. 4 The image of an object in the visual field is inverted and reversed right to left on the retina. • Temporal field of left eye (red & purple) is seen by the nasal retina of the left eye • Nasal field of the left eye (green & yellow) is seen by the temporal retina of the left eye. • Superior field of the left eye (red & green) is seen by the inferior retina of the left eye. • Inferior field of the left eye (purple & yellow) is seen by the superior retina of the left eye. • Similarly, the image is inverted & reversed for the right eye. Retina of Left Eye Retina of Right Eye NOTE: DOTTED OUTLINE = MONOCULAR FIELD OF LEFT EYE SOLID OUTLINE = MONOCULAR FIELD OF RIGHT EYE Binocular Visual Field Monocular Crescent of Right Eye Monocular Crescent of Left Eye Note: To avoid confusion and abide by convention, central representation, visual deficits, etc. will be described in terms of visual fields and not retinal quadrants.
23 Visual Pathway • Optic Nerve (ON) • = Axons of ganglion cells in the retina of the corresponding eye • Outgrowth of diencephalon, so is a CNS tract & not a ‘true’ cranial nerve. • Myelinated by oligodendrocytes. • Optic Chiasm (OC) • Located just anterior to pituitary • Partial crossing of optic nerve axons in the OC is essential to binocular vision • Axons from temporal fields cross • Axons from nasal fields do not cross • “Wilbrand’s knee” may be artifact Note: Reference point = Visual Fields Pgs. 4 - 5 Retinotopic representation • Central (macular) vision • Peripheral vision Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus Calcarin e sulcus III III Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT
24 • Optic Tract (OT) • Optic nerve fibers from the optic chiasm continue as the optic tract & terminate in the lateral geniculate nucleus of thalamus. • Each tract contains axons that carry input from the contralateral visual field. • Left OT receives from R. visual field • Right OT receives from the L. visual field Pgs. 4 - 5 Note: Reference point = Visual Fields Retinotopic representation • Central (macular) vision • Peripheral vision Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus III III Visual Pathway Post-Chiasmatic portion of the pathway: From optic tract to visual cortex, each side of the brain deals with the contralateral visual field. Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT • Lateral Geniculate Nucleus (LGN) • Primary termination of OT fibers • Each LGN receives input from the contralateral visual field. • OT Projections to pretectum for reflexes
25 Retinotopic representation • Central (macular) vision • Peripheral vision • Geniculocalcarine Tract (= optic radiations) • Axons of LGN neurons travel to primary visual cortex (Area 17) via the geniculocalcarine tract located in the retrolenticular and sublenticular portions of the internal capsule. • Axons from upper visual fields take a looping course into the temporal lobe on the way to visual cortex. (=Meyer’s loop) • Axons from lower visual fields take a more direct route to visual cortex. • Macular fibers are in an intermediate location in the optic radiation. Pgs. 4 - 5 Note: Reference point = Visual Fields Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus Calcarin e sulcus III III Meyer’s loop Optic radiation or geniculocalcarine tract Visual Pathway Post-Chiasmatic portion of the pathway: From optic tract to visual cortex, each side of the brain deals with the contralateral visual field. Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT
26 Visual Pathway Pgs. 4 - 5 Note: Reference point = Visual Fields Retinotopic representation • Central (macular) vision • Peripheral vision Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus Calcarin e sulcus III III Meyer’s loop Optic radiation or geniculocalcarine tract Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT • Primary Visual Cortex (Area 17) • Located on either side of & within the calcarine fissure. • Upper fields project to the lingual gyrus. • Lower fields project to the cuneus. • Macular representation is most caudal in Area 17. • Peripheral field representation is in the rostral 2/3rds of Area 17. • Lesions of Area 17 result in blindness in the contralateral visual field. • Association Visual Cortex (Areas 18 & 19) • Input from Area 17 & elsewhere • Deals with complex aspects of vision • Lesions of result in visual agnosia.
• The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the hypothalamus of the brain. • At this point the information coming from both eyes is combined and then splits according to the visual field. • The corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively, to be processed. That is, the right side of primary visual cortex deals with the left half of the field of view from both eyes, and similarly for the left brain 27
• Information from the right visual field (now on the left side of the brain) travels in the left optic tract. • Information from the left visual field travels in the right optic tract. • Each optic tract terminates in the lateral geniculate nucleus (LGN) in the thalamus. 28
• The lateral geniculate nucleus (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN consists of six layers in humans 29
• Layers 1, 4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal retina (temporal visual field); • layers 2, 3, and 5 correspond to information from the ipsilateral (uncrossed) fibers of the temporal retina (nasal visual field). • Layer one (1) contains M cells which correspond to the M (magnocellular) cells of the optic nerve of the opposite eye and are concerned with depth or motion 30
• The neurons of the LGN then relay the visual image to the primary visual cortex (V1) which is located at the back of the brain (posterior end) in the occipital lobe in and close to the calcarine sulcus. • The LGN is not just a simple relay station but it is also a center for processing; it receives reciprocal input from the cortical and subcortical layers and reciprocal innervation from the visual cortex. 31
• The optic radiations, one on each side of the brain, carry information from the thalamic lateral geniculate nucleus to layer 4 of the visual cortex. • The visual cortex is the largest system in the human brain and is responsible for processing the visual image. 32
• The region that receives information directly from the LGN is called the primary visual cortex, (also called V1 and striate cortex). • Visual information then flows through a cortical hierarchy. • These areas include V2, V3, V4 and area V5/MT (the exact connectivity depends on the species of the animal). 33
• These secondary visual areas (collectively termed the extrastriate visual cortex) process a wide variety of visual primitives. • Neurons in V1 and V2 respond selectively to bars of specific orientations, or combinations of bars. • These are believed to support edge and corner detection. Similarly, basic information about color and motion is processed here 34
• As visual information passes forward through the visual hierarchy, the complexity of the neural representations increases. • Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular retinotopic location, neurons in the lateral occipital complex respond selectively to complete object (e.g., a figure drawing), and neurons in visual association cortex may respond selectively to human faces, or to a particular object. 35
• Along with this increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: the dorsal stream and the ventral stream • The dorsal stream, commonly referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates with regions that control eye movements and hand movements. • The ventral stream, commonly referred as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli. 36
• Visual Cycle 37
• The processing of visual information begins in the retina with the detection of light by photoreceptor cells. • In humans, two specialized types of photoreceptors detect light under different conditions. • Rod photoreceptors are highly sensitive and mediate vision in dim light, while cone photoreceptors function in bright light • To detect light, both rods and cones exploit the unique properties of 11-cis retinal, a photosensitive derivative of vitamin A. 38
The 11-cis retinal in photoreceptors is covalently bound to an opsin signaling protein to form a visual pigment molecule. 39
11-cis Retinal is the light-sensitive component of rod and cone photoreceptors. In the first step of vision, photoreceptors are activated when light induces the isomerization of 11-cis retinal to all- trans retinal. 40
To generate a cellular response to light, the 11-cis retinal in photoreceptors is linked to an opsin protein capable of activating signaling pathways. Together, the 11-cis retinal and opsin protein are known as a visual pigment 41
• The classical visual cycle involves the cycling of retinoids between the rod outer segments (OS) and the RPE. • The visual cycle begins in the outer segment with all-trans retinal's release from the opsin. • After reduction to all-trans retinol, the photoproducts cross the sub-retinal space and enter the RPE. Here, 11-cis retinal is regenerated in three enzymatic steps and returned to the photoreceptors. 42
43
• The retinal pigmented epithelium (RPE) is a single layer of polarized epithelial cells which plays many important roles for visual function. • One of such roles is production of visual chromophore, 11-cis-retinal through the visual cycle. • The visual cycle consists of biochemical processes for regenerating chromophore by a collective action of the RPE and photoreceptor. • Photoreceptors harbor the G protein-coupled receptors, opsin which enables to receive light when it bounds to 11-cis-retinal. 44
• With absorption of a photon of light, 11-cis- retinal photoisomerizes to all-trans-retinal. All- trans-retinal reduces to all-trans-retinol in the photoreceptor and further recycles back to 11- cis-retinal in the RPE. • Acyltransferases and isomerohydrolase(s) along with retinol dehydrogenases sequentially convert all-trans-retinol to 11-cis-retinal in the RPE. • Dysfunctions of any retinoid cycle enzymes in the RPE can cause retinal diseases. 45
Visual Field Defects 46
Strabismus: A condition in which the visual axes of the eyes are not parallel and the eyes appear to be looking in different directions. • In divergent strabismus, the visual axes diverge. • In convergent strabismus , the visual axes converge. 47
• Diplopia : is the simultaneous perception of two images of a single object • Amblyopia : is a disorder of sight in which the brain fails to process inputs from one eye and over time favors the other eye. 48
• scotoma : is a break or interruption in the visual field. • A positive scotoma is one where the person actually perceives a patch blocking part of his/her vision. • A negative scotoma is one where the person is not aware of the blind spot normally, but which can be detected on visual field testing 49
• Anopia : the inability to see. • Altitudinal visual field defect is a condition in which there is defect in the superior or inferior portion of the visual field that respects the horizontal midline. • A congruous visual field defect is identical between the two eyes, whereas an incongruous defect differs in appearance between the eyes. 50
• Hemianopia, is a visual field loss on the left or right side of the vertical midline. It can affect one eye but usually affects both eyes. • Homonymous hemianopsia (or homonymous hemianopia) is hemianopic visual field loss on the same side of both eyes. • Heteronymous defects are attributable to bilateral disorders of the eyes and/or the optic nerves, or chiasmal lesions. The variety of heteronymous defects is immense and hinders a unified approach to simulation. 51
52
53 Lesions of the Visual Pathway 1. Normal visual fields 2. Blindness of the right eye 3. Blindness of right eye + contralateral left upper quadrantanopia 4. Bitemporal heteronymous hemianopsia 5. Left homonymous hemianopsia 6. Left upper homonymous quadrantanopsia 7. Left homonymous hemianopsia with macular sparing RightLeft Definitions Strabismus Diplopia Amblyopia Scotoma Quadrantanopsia - # 3, 6 Hemianopsia - # 4, 5, 7 Heteronymous Defects - # 3, 4 Homonymous Defects - # 5, 6, 7 Congruous Defects - # 5, 6, 7 Incongruous Defects - # 3 Altitudinal Defects - # 6 Masked area = area of visual loss Pg. 6 Aka “field cuts” Fields, not retinal quadrants
54 Lesions of the Visual Pathway 1. Normal visual fields 2. Blindness of the right eye 3. Blindness of right eye + contralateral left upper quadrantanopia 4. Bitemporal heteronymous hemianopsia 5. Left homonymous hemianopsia 6. Left upper homonymous quadrantanopsia 7. Left homonymous hemianopsia with macular sparing RightLeft Pg. 6

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2. vision pathway 1

  • 1.
    Visual Pathway Dr MCTayade Associate Professor Department of Physiology
  • 2.
  • 3.
    • The visualsystem is the part of the central nervous system which gives organisms the ability to process visual detail as sight, as well as enabling the formation of several non-image photo response functions 3
  • 4.
    • Together thecornea and lens refract light into a small image and shine it on the retina. • The retina transduces this image into action potential pulses using rods and cones. • The optic nerve then carries these pulses through the optic canal. • Upon reaching the optic chiasm the nerve fibers decussate (left becomes right). • The fibers then branch and terminate at cortex ( Visual area ) 4
  • 5.
    • The retinaconsists of a large number of photoreceptor cells which contain particular protein molecules called opsins. • In humans, two types of opsins are involved in conscious vision: rod opsins and cone opsins. 5
  • 6.
    • An opsinabsorbs a photon (a particle of light) and transmits a signal to the cell through a signal transduction pathway, resulting in hyper- polarization of the photoreceptor. • Rods and cones differ in function. • Rods are found primarily in the periphery of the retina and are used to see at low levels of light. 6
  • 7.
    • There arethree types of cones that differ in the wavelengths of light they absorb; they are usually called short or blue, middle or green, and long or red. • Cones are used primarily to distinguish color and other features of the visual world at normal levels of light. 7
  • 8.
    • In theretina, the photoreceptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which will then conduct action potentials to the brain. • A significant amount of visual processing arises from the patterns of communication between neurons in the retina. 8
  • 9.
    • In addition,other neurons in the retina, particularly horizontal and amacrine cells, transmit information laterally (from a neuron in one layer to an adjacent neuron in the same layer), resulting in more complex receptive fields that can be either indifferent to color and sensitive to motion or sensitive to color and indifferent to motion 9
  • 10.
    • The retinaadapts to change in light through the use of the rods. • In the dark, the chromophore retinal has a bent shape called cis-retinal (referring to a cis conformation in one of the double bonds). • When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. • This is called bleaching because the purified rhodopsin changes from violet to colorless in the light. 10
  • 11.
    • At baselinein the dark, the rhodopsin absorbs no light and releases glutamate which inhibits the bipolar cell. • This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. • When there is light present, glutamate secretion ceases thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected. 11
  • 12.
    • The finalresult of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain: • M cells, that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus; • P cells, that are sensitive to color and shape; • K cells, that are sensitive to color and indifferent to shape or depth; • another population that is intrinsically photosensitive; and • a final population that is used for eye movements.12
  • 13.
    13 Pathway extends fromthe ‘front’ to the ‘back’ of the brain. • Precise retinotopic organization • Deficits due to lesions of the pathway give valuable localizing information. The Visual Pathway OT ON OC VISUAL CORTEX RETINA VISUAL FIELD LGN OPTIC RADIATIONS ON = Optic Nerve OC = Optic Chiasm OT = Optic Tract LGN = Lateral Geniculate Nucleus of Thalamus
  • 14.
    14 Beginning of thePathway Pg. 2
  • 15.
    15 Cells of the Retina Pg. 2 Rodsand Cones (Receptors) Ganglion cells axons form the optic nerve Bipolar cells
  • 16.
    16 Object to beseen Peripheral Retina The next slide looks at the retina as if you are looking through the patient’s pupil via your ophthalmoscope. Central Retina (fovea in the macula lutea) Pg. 2
  • 17.
    17 Visual Fields &the Visual Pathway Pg. 2 OT ON OC VISUAL CORTEX RETINA VISUAL FIELD LGN OPTIC RADIATIONS ON = Optic Nerve OC = Optic Chiasm OT = Optic Tract LGN = Lateral Geniculate Nucleus of Thalamus The following slides begin with the visual fields and then follow the pathway from the retina to the visual cortex.
  • 18.
    18 Definition: The entirearea that can be “seen” by the patient without movement of the head and with the eyes fixed on a single spot. Visual Fields Pg. 3 Mapping of Visual Fields: • Confrontational method (see Dr. Meyerson’s “Neurological Exam” notes) • Perimetry (Manual or Automated) Temporal Field of Left Eye Nasal Field of Left Eye F Normal Monocular Visual Field of Right Eye Normal Monocular Visual Field of Left Eye F Monocular Visual Fields Monocular Visual Fields: • Each eye is tested separately. • The monocular visual field is plotted with the Fovea (F) at the center. • The monocular visual field (colored area -- blue for left; green for right in this example) is not round. • Horizontal and Vertical Meridians correspond to those of the retina and divide the visual field into upper temporal, upper nasal, lower temporal and lower nasal quadrants. • Imagine that this is your visual field, i.e. all that you can see with your left eye and your right eye (tested separately) when you look straight ahead and do not move your head or eyes. Vertical Meridian Horizontal Meridian UpperFieldof LeftEye LowerFieldof LeftEye UTQ LTQ UNQ LNQ
  • 19.
    19 Blind Spot • 15°to the temporal side of the visual field of each eye • On the horizontal meridian • Corresponds to the location of the optic nerve head 15° to the nasal side of the retina of each eye. Visual Fields Pg. 3 Temporal Field of Left Eye Nasal Field of Left Eye Normal Monocular Visual Field of Left Eye F F Normal Monocular Visual Field of Right Eye UpperFieldof LeftEye LowerFieldof LeftEye
  • 20.
    20 Visual Fields: Binocular Pg. 3 NormalBinocular Visual Field F Right Visual FieldLeft Visual Field Upper Fields Lower Fields Temporal Field of Left Eye Nasal Field of Left Eye Normal Monocular Visual Field of Left Eye F F Normal Monocular Visual Field of Right Eye Understand the difference between the “monocular visual field of the left eye” vs. the “binocular left visual field” and vice versa for the right counterparts. • Binocular field combines the two monocular visual fields with the foveas (F) aligned with one another. (i.e. the ‘pink area’ in the image to the right) • Left Visual Field seen by both the left & right eyes. • Right Visual Field seen by both the left & right eyes. • Monocular crescent for each eye (blue for left eye & green for right eye) is only seen by the nasal retina of the same eye. Monocular Crescent of Right Eye Monocular Crescent of Left Eye
  • 21.
    21 Visual Fields: Binocular Binocular visionis dependent upon the extraocular muscles aligning the eyes so that an image falls on “corresponding points” on the retina of each eye. This is essential for the brain to perceive a single image. Diplopia occurs when the images are not aligned to fall on corresponding points of each retina. Pg. 3 Normal Binocular Visual Field F Right Visual FieldLeft Visual Field Upper Fields Lower Fields Temporal Field of Left Eye Nasal Field of Left Eye Normal Monocular Visual Field of Left Eye F F Normal Monocular Visual Field of Right Eye Demonstration of the Binocular Visual Field & Monocular Crescent: • Look straight ahead • Close your right eye • Move your finger to the right until it disappears • Open right eye to see the pencil -- in the right temporal monocular crescent of your visual field.
  • 22.
    22 Visual Fields Pg.4 The image of an object in the visual field is inverted and reversed right to left on the retina. • Temporal field of left eye (red & purple) is seen by the nasal retina of the left eye • Nasal field of the left eye (green & yellow) is seen by the temporal retina of the left eye. • Superior field of the left eye (red & green) is seen by the inferior retina of the left eye. • Inferior field of the left eye (purple & yellow) is seen by the superior retina of the left eye. • Similarly, the image is inverted & reversed for the right eye. Retina of Left Eye Retina of Right Eye NOTE: DOTTED OUTLINE = MONOCULAR FIELD OF LEFT EYE SOLID OUTLINE = MONOCULAR FIELD OF RIGHT EYE Binocular Visual Field Monocular Crescent of Right Eye Monocular Crescent of Left Eye Note: To avoid confusion and abide by convention, central representation, visual deficits, etc. will be described in terms of visual fields and not retinal quadrants.
  • 23.
    23 Visual Pathway • OpticNerve (ON) • = Axons of ganglion cells in the retina of the corresponding eye • Outgrowth of diencephalon, so is a CNS tract & not a ‘true’ cranial nerve. • Myelinated by oligodendrocytes. • Optic Chiasm (OC) • Located just anterior to pituitary • Partial crossing of optic nerve axons in the OC is essential to binocular vision • Axons from temporal fields cross • Axons from nasal fields do not cross • “Wilbrand’s knee” may be artifact Note: Reference point = Visual Fields Pgs. 4 - 5 Retinotopic representation • Central (macular) vision • Peripheral vision Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus Calcarin e sulcus III III Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT
  • 24.
    24 • Optic Tract(OT) • Optic nerve fibers from the optic chiasm continue as the optic tract & terminate in the lateral geniculate nucleus of thalamus. • Each tract contains axons that carry input from the contralateral visual field. • Left OT receives from R. visual field • Right OT receives from the L. visual field Pgs. 4 - 5 Note: Reference point = Visual Fields Retinotopic representation • Central (macular) vision • Peripheral vision Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus III III Visual Pathway Post-Chiasmatic portion of the pathway: From optic tract to visual cortex, each side of the brain deals with the contralateral visual field. Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT • Lateral Geniculate Nucleus (LGN) • Primary termination of OT fibers • Each LGN receives input from the contralateral visual field. • OT Projections to pretectum for reflexes
  • 25.
    25 Retinotopic representation • Central(macular) vision • Peripheral vision • Geniculocalcarine Tract (= optic radiations) • Axons of LGN neurons travel to primary visual cortex (Area 17) via the geniculocalcarine tract located in the retrolenticular and sublenticular portions of the internal capsule. • Axons from upper visual fields take a looping course into the temporal lobe on the way to visual cortex. (=Meyer’s loop) • Axons from lower visual fields take a more direct route to visual cortex. • Macular fibers are in an intermediate location in the optic radiation. Pgs. 4 - 5 Note: Reference point = Visual Fields Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus Calcarin e sulcus III III Meyer’s loop Optic radiation or geniculocalcarine tract Visual Pathway Post-Chiasmatic portion of the pathway: From optic tract to visual cortex, each side of the brain deals with the contralateral visual field. Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT
  • 26.
    26 Visual Pathway Pgs. 4- 5 Note: Reference point = Visual Fields Retinotopic representation • Central (macular) vision • Peripheral vision Left visual field Right visual field Right retinaLeft retina Left LGN Tempora l Nasal Tempora l Nasal lateral lateralmedial medial LVFLVF UVFUVF E.W. Right visual cortex midbrain Right LGN Left visual cortex Left temporal retina Right temporal retina Nasal retina Ciliary ganglion pretectal nuclei cuneus lingual gyrus Calcarin e sulcus III III Meyer’s loop Optic radiation or geniculocalcarine tract Upper field Lower field VISUAL FIELDS: Hatched = binocular Stippled = monocular Central area = macula ON OC OT • Primary Visual Cortex (Area 17) • Located on either side of & within the calcarine fissure. • Upper fields project to the lingual gyrus. • Lower fields project to the cuneus. • Macular representation is most caudal in Area 17. • Peripheral field representation is in the rostral 2/3rds of Area 17. • Lesions of Area 17 result in blindness in the contralateral visual field. • Association Visual Cortex (Areas 18 & 19) • Input from Area 17 & elsewhere • Deals with complex aspects of vision • Lesions of result in visual agnosia.
  • 27.
    • The opticnerves from both eyes meet and cross at the optic chiasm, at the base of the hypothalamus of the brain. • At this point the information coming from both eyes is combined and then splits according to the visual field. • The corresponding halves of the field of view (right and left) are sent to the left and right halves of the brain, respectively, to be processed. That is, the right side of primary visual cortex deals with the left half of the field of view from both eyes, and similarly for the left brain 27
  • 28.
    • Information fromthe right visual field (now on the left side of the brain) travels in the left optic tract. • Information from the left visual field travels in the right optic tract. • Each optic tract terminates in the lateral geniculate nucleus (LGN) in the thalamus. 28
  • 29.
    • The lateralgeniculate nucleus (LGN) is a sensory relay nucleus in the thalamus of the brain. The LGN consists of six layers in humans 29
  • 30.
    • Layers 1,4, and 6 correspond to information from the contralateral (crossed) fibers of the nasal retina (temporal visual field); • layers 2, 3, and 5 correspond to information from the ipsilateral (uncrossed) fibers of the temporal retina (nasal visual field). • Layer one (1) contains M cells which correspond to the M (magnocellular) cells of the optic nerve of the opposite eye and are concerned with depth or motion 30
  • 31.
    • The neuronsof the LGN then relay the visual image to the primary visual cortex (V1) which is located at the back of the brain (posterior end) in the occipital lobe in and close to the calcarine sulcus. • The LGN is not just a simple relay station but it is also a center for processing; it receives reciprocal input from the cortical and subcortical layers and reciprocal innervation from the visual cortex. 31
  • 32.
    • The opticradiations, one on each side of the brain, carry information from the thalamic lateral geniculate nucleus to layer 4 of the visual cortex. • The visual cortex is the largest system in the human brain and is responsible for processing the visual image. 32
  • 33.
    • The regionthat receives information directly from the LGN is called the primary visual cortex, (also called V1 and striate cortex). • Visual information then flows through a cortical hierarchy. • These areas include V2, V3, V4 and area V5/MT (the exact connectivity depends on the species of the animal). 33
  • 34.
    • These secondaryvisual areas (collectively termed the extrastriate visual cortex) process a wide variety of visual primitives. • Neurons in V1 and V2 respond selectively to bars of specific orientations, or combinations of bars. • These are believed to support edge and corner detection. Similarly, basic information about color and motion is processed here 34
  • 35.
    • As visualinformation passes forward through the visual hierarchy, the complexity of the neural representations increases. • Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular retinotopic location, neurons in the lateral occipital complex respond selectively to complete object (e.g., a figure drawing), and neurons in visual association cortex may respond selectively to human faces, or to a particular object. 35
  • 36.
    • Along withthis increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: the dorsal stream and the ventral stream • The dorsal stream, commonly referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates with regions that control eye movements and hand movements. • The ventral stream, commonly referred as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli. 36
  • 37.
  • 38.
    • The processingof visual information begins in the retina with the detection of light by photoreceptor cells. • In humans, two specialized types of photoreceptors detect light under different conditions. • Rod photoreceptors are highly sensitive and mediate vision in dim light, while cone photoreceptors function in bright light • To detect light, both rods and cones exploit the unique properties of 11-cis retinal, a photosensitive derivative of vitamin A. 38
  • 39.
    The 11-cis retinalin photoreceptors is covalently bound to an opsin signaling protein to form a visual pigment molecule. 39
  • 40.
    11-cis Retinal isthe light-sensitive component of rod and cone photoreceptors. In the first step of vision, photoreceptors are activated when light induces the isomerization of 11-cis retinal to all- trans retinal. 40
  • 41.
    To generate acellular response to light, the 11-cis retinal in photoreceptors is linked to an opsin protein capable of activating signaling pathways. Together, the 11-cis retinal and opsin protein are known as a visual pigment 41
  • 42.
    • The classicalvisual cycle involves the cycling of retinoids between the rod outer segments (OS) and the RPE. • The visual cycle begins in the outer segment with all-trans retinal's release from the opsin. • After reduction to all-trans retinol, the photoproducts cross the sub-retinal space and enter the RPE. Here, 11-cis retinal is regenerated in three enzymatic steps and returned to the photoreceptors. 42
  • 43.
  • 44.
    • The retinalpigmented epithelium (RPE) is a single layer of polarized epithelial cells which plays many important roles for visual function. • One of such roles is production of visual chromophore, 11-cis-retinal through the visual cycle. • The visual cycle consists of biochemical processes for regenerating chromophore by a collective action of the RPE and photoreceptor. • Photoreceptors harbor the G protein-coupled receptors, opsin which enables to receive light when it bounds to 11-cis-retinal. 44
  • 45.
    • With absorptionof a photon of light, 11-cis- retinal photoisomerizes to all-trans-retinal. All- trans-retinal reduces to all-trans-retinol in the photoreceptor and further recycles back to 11- cis-retinal in the RPE. • Acyltransferases and isomerohydrolase(s) along with retinol dehydrogenases sequentially convert all-trans-retinol to 11-cis-retinal in the RPE. • Dysfunctions of any retinoid cycle enzymes in the RPE can cause retinal diseases. 45
  • 46.
  • 47.
    Strabismus: A conditionin which the visual axes of the eyes are not parallel and the eyes appear to be looking in different directions. • In divergent strabismus, the visual axes diverge. • In convergent strabismus , the visual axes converge. 47
  • 48.
    • Diplopia :is the simultaneous perception of two images of a single object • Amblyopia : is a disorder of sight in which the brain fails to process inputs from one eye and over time favors the other eye. 48
  • 49.
    • scotoma :is a break or interruption in the visual field. • A positive scotoma is one where the person actually perceives a patch blocking part of his/her vision. • A negative scotoma is one where the person is not aware of the blind spot normally, but which can be detected on visual field testing 49
  • 50.
    • Anopia :the inability to see. • Altitudinal visual field defect is a condition in which there is defect in the superior or inferior portion of the visual field that respects the horizontal midline. • A congruous visual field defect is identical between the two eyes, whereas an incongruous defect differs in appearance between the eyes. 50
  • 51.
    • Hemianopia, isa visual field loss on the left or right side of the vertical midline. It can affect one eye but usually affects both eyes. • Homonymous hemianopsia (or homonymous hemianopia) is hemianopic visual field loss on the same side of both eyes. • Heteronymous defects are attributable to bilateral disorders of the eyes and/or the optic nerves, or chiasmal lesions. The variety of heteronymous defects is immense and hinders a unified approach to simulation. 51
  • 52.
  • 53.
    53 Lesions of theVisual Pathway 1. Normal visual fields 2. Blindness of the right eye 3. Blindness of right eye + contralateral left upper quadrantanopia 4. Bitemporal heteronymous hemianopsia 5. Left homonymous hemianopsia 6. Left upper homonymous quadrantanopsia 7. Left homonymous hemianopsia with macular sparing RightLeft Definitions Strabismus Diplopia Amblyopia Scotoma Quadrantanopsia - # 3, 6 Hemianopsia - # 4, 5, 7 Heteronymous Defects - # 3, 4 Homonymous Defects - # 5, 6, 7 Congruous Defects - # 5, 6, 7 Incongruous Defects - # 3 Altitudinal Defects - # 6 Masked area = area of visual loss Pg. 6 Aka “field cuts” Fields, not retinal quadrants
  • 54.
    54 Lesions of theVisual Pathway 1. Normal visual fields 2. Blindness of the right eye 3. Blindness of right eye + contralateral left upper quadrantanopia 4. Bitemporal heteronymous hemianopsia 5. Left homonymous hemianopsia 6. Left upper homonymous quadrantanopsia 7. Left homonymous hemianopsia with macular sparing RightLeft Pg. 6