Central Neurophysiology of Vision

1. CentralNeurophysiology of Vision Dr. Ümmühan İşoğlu-Alkaç alkac@istanbul.edu.tr

2. Visual Pathways from the two retinas - to the visual cortex; The visualnerve signals leave the retinas through the opticnerves. At the optic chiasm, the optic nerve fibers from the nasal halves of the retinas cross to theopposite sides, where they join the fibers from theopposite temporal retinas to form the optic tracts.

3. The fibers of each optic tractthen synapse in the dorsal lateral geniculate nucleus of the thalamus, and Fromthere, geniculocalcarine fibers pass by way of the optic radiation (also called the geniculocalcarine tract) to the primary visual cortex in the calcarine fissure area of the medial occipital lobe.

6. Visual Pathways • from the optictracts to the suprachiasmatic nucleus of the hypothalamus (control circadian rhythms, synchronize various physiologic changes of thebody with night and day; • into the pretectal nuclei in the midbrain, to elicitreflex movements of the eyes to focus on objects of importance and to activate the pupillary light reflex;

7. 3) into the superior colliculus, to control rapid directionalmovements of the two eyes; and 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

9. Dorsal LGN; Function of the Dorsal Lateral Geniculate Nucleus of the Thalamus serves two principal functions: First, it relays visual informationfrom the optic tract to the visual cortex by way of the optic radiation (also called the geniculocalcarine tract). This relay function is so accurate thatthere is exact point-to-point transmission with a high degree of spatial fidelityall the way from the retina to the visual cortex.

10. Dorsal LGN It will be recalled that half the fibers in each optic tractafter passing the opticchiasm are derived from one eye and half from the other eye, representing correspondingpoints on the two retinas. However, the signals from the two eyesare kept apart in the dorsal lateral geniculate nucleus.

11. This nucleus is composedof six nuclear layers. Layers II, III, and V; receive signalsfrom the lateral half of the ipsilateral retina, layers I, IV, and VI receivesignals from the medial half of the retina of the opposite eye. The respectiveretinal areas of the two eyes connect with neurons that are superimposed overone another in the paired layers, and similar paralleltransmission is preserved all the way to the visual cortex.

12. DLGN: 1) Layers I and II; magnocellular layers, rapid pathway, black-and-white information These receive their input almost entirely fromthe large type Y retinal ganglion cells. its point-to-point transmission is poor because there are not many Y ganglion cells, and their dendrites spread widely in the retina.

13. 2) Layers III through VI; parvocellular layers, These neurons receive their input almost entirely fromthe type X retinal ganglion cells that transmit colorand convey accurate point-to-point spatial information

14. Principal visual pathways from the eyes to the visual cortex. (Modifiedfrom Polyak SL: The Retina. Chicago: University of Chicago, 1941.)

15. Visual cortex in the calcarine fissure area of the medial occipital cortex.

16. Primary Visual Cortex Signals fromthe macular area of the retina terminate near theoccipital pole, while signalsfrom the more peripheral retina terminate at or in concentrichalf circles anterior to the pole but still alongthe calcarine fissure on the medial occipital lobe. Theupper portion of the retina is represented superiorlyand the lower portion inferiorly.

17. Secondary Visual Areas of the Cortex. fold outward over thelateral surfaces of the occipital and parietal cortex. Secondary signals are transmittedto these areas for analysis of visual meanings. On all sides of the primary visual cortex isBrodmann’s area 18, which is wherevirtually all signals from the primary visual cortex passnext. Theimportance of all these areas is that various aspectsof the visual image are progressively dissected and analyzed.

18. Transmission of visual signals from the primary visual cortex intosecondary visual areas on the lateral surfaces of the occipital andparietal cortices. Note that the signals representing form, thirddimensionalposition, and motion are transmitted mainly into thesuperior portions of the occipital lobe and posterior portions of the parietal lobe. By contrast, the signals for visual detail and colorare transmitted mainly into the anteroventral portion of the occipitallobe and the ventral portion of the posterior temporal lobe.

19. Layered Structure of the Primary Visual Cortex Like almost all other portions of the cerebral cortex,the primary visual cortex has six distinct layers. Also, as is true for the other sensory systems, the geniculocalcarine fibers terminatemainly in layer IV. The rapidly conducted signals from the Yretinal ganglion cells terminate in layer IVcα, andfrom there they are relayed vertically both outwardtoward the cortical surface and inward toward deeper levels.

20. The visual signals from the medium-sized optic nerve fibers, retinalX ganglion cells --- terminate in layer IVa and IVcβ --- surface of the cortex and to deeper layers. X ganglion pathways that transmit the accurate pointto-point type of vision as well as color vision.

21. Six layers of the primary visual cortex. left side; magnocellular layers of LGN) and transmit rapidly changingblack-and-white visual signals. right side; parvocellular layers (layers III through VI) of the LGN;they transmit signals that depict accurate spatial detail as well ascolor.

22. Interaction of Visual Signals from the Two Separate Eyes In fact, layer IV; deciphers whether the respective areas of the two visual images from the two separate eyes are “inregister” with each other—that is, whether correspondingpoints from the two retinas fit with each other. Inturn, the deciphered information is used to adjust thedirectional gaze of the separate eyes so that they willfuse with each other (be brought into “register”).

23. Theinformation observed about degree of register ofimages from the two eyes also allows a person to distinguishthe distance of objects by the mechanism ofstereopsis.

24. Two Major Pathways for Analysis ofVisual Information • The Fast “Position” and “Motion” Pathway; 2) The Accurate Color Pathway

25. 1. Analysis of Third-Dimensional Position, Gross Form, and Motion of Objects. primary visualcortex ----- posteriormidtemporal area and occipitoparietalcortex anterior border of the parietalcortex ----- that analyze threedimensionalaspects of somatosensory signals.

26. posteriorsomatic association areas: *Thesignals transmitted in this position-form-motion *pathway are mainly from the large Y optic nerve fibersof the retinal Y ganglion cells, *transmitting rapidsignals but depicting only black and white with *nocolor.

27. 2. Analysis of Visual Detail and Color. principal pathway for analysis of visualdetail, color,recognizing letters,reading, determining the texture of surfaces, determiningdetailed colors of objects, and decipheringfrom all this information what the object is and what it means: Retina X gang– LGN (parvocelluler)---primary visual cortex ---- secondary visual areas----inferior, ventral, andmedial regions of the occipital and temporal cortex

30. Fields of Vision; Perimetry The area seen to the nasal side is calledthe nasal field of vision, and the area seen to the lateral side is called the temporal field of vision. a blind spot caused by lack of rods and cones in the retina over the optic disc is found about 15degrees lateral to the central point of vision.

31. Abnormalities in the Fields of Vision. Blindspots are found in portions of the field of vision otherthan the optic disc area. Such blind spots are called scotomata; damage to theoptic nerve resulting from glaucoma, fromallergic reactions in theretina, or from toxic conditions such aslead poisoningor excessive use of tobacco. Retinitis pigmentosa. Portionsof the retina degenerate, and excessive melaninpigment deposits in the degenerated areas. Retinitis pigmentosausually causes blindness in the peripheral fieldof vision first and then gradually encroaches on thecentralareas.

32. Perimetry card for the left eye.

33. Neural Pathways for Control of Eye Movements. 3, 4, 6 cranial nerves and their connections with theperipheral nerves to the ocular muscles. Each of the three sets of muscles to eacheye is reciprocally innervated so that one muscle of thepair relaxes while the other contracts

34. Effect of Lesions in the Optic Pathway on the Fields ofVision. Destruction of an entire optic nerve causesblindness of the affected eye. Destruction of the optic chiasm prevents the crossing of impulses from the nasal half of each retina to the opposite optic tract. Therefore, the nasal half of eachretina is blinded, which means that the person is blindin the temporal field of vision for each eye because theimage of the field of vision is inverted on the retina bythe optical system of the eye; this condition is calledbitemporal hemianopsia.

35. Such lesions frequently resultfrom tumors of the pituitary gland pressing upwardfrom the sella turcica on the bottom of the optic chiasm. Interruption of an optic tract denervates the correspondinghalf of each retina on the same side as thelesion; as a result, neither eye can see objects to theopposite side of the head. This condition is known ashomonymous hemianopsia.

36. Eye movement and its muscles..

37. Eye Movements and Their Muscular Control The eye movementsare controlled by three pairs of muscles, • the medial and lateral recti, sidetoside 2) thesuperior and inferior recti, and Upwardordownward 3) the superior andinferior obliques. Rotateeyeballs

38. Neural pathways for control ofconjugate movement of the eyes.

39. Fusion of the Visual Images from the Two Eyes The visual cortex plays an important role in fusion. Itwas pointed out earlier in the chapter that correspondingpoints of the two retinas transmit visual signals todifferent neuronal layers of the lateral geniculatebody, and these signals in turn are relayed to parallelneurons in thevisualcortex.

40. Interactions occur between these cortical neurons to cause interferenceexcitation in specific neurons when the two visualimages are not “in register”—that is, are not precisely“fused.” This excitation presumably provides thesignal that is transmitted to the oculomotor apparatusto cause convergence or divergence or rotation of theeyes so that fusion can be re-established. Once the correspondingpoints of the two retinas are in register,excitation of the specific “interference” neurons in the visual cortex disappears.

41. Interactionsoccurbetween these cortical neurons to cause interimages are not “in register”—that is, are not precisely“fused.” This excitation presumably provides thesignal that is transmitted to the oculomotor apparatusto cause convergence or divergence or rotation of theeyes so that fusion can be re-established. Once the correspondingpoints of the two retinas are in register,excitation of the specific “interference” neurons in thevisualcortexdisappears.

42. Neural Mechanism of Stereopsis for Judging; Distances of Visual Objects because the twoeyes are more than 2 inches apart, the images on thetwo retinas are not exactly the same. the righteye sees a little more of the right-hand side of theobject, and the left eye a little more of the left-handside, and the closer the object, the greater the disparity.

43. Therefore, even when the two eyes are fused witheach other, it is still impossible for all correspondingpoints in the two visual images to be exactly in registerat the same time. Furthermore, the nearer theobject is to the eyes, the less the degree of register.Thisdegree of nonregister provides the neural mechanismfor stereopsis, an important mechanism for judgingthe distances of visual objects up to about 200 feet (60 meters).

44. The neuronal cellular mechanism for stereopsis isbased on the fact that some of the fiber pathways fromthe retinas to the visual cortex stray 1 to 2 degrees oneach side of the central pathway.Therefore, some opticpathways from the two eyes are exactly in register forobjects 2 meters away; still another set of pathways isin register for objects 25 meters away. Thus, the distanceis determined by which set or sets of pathwaysare excited by nonregister or register. This phenomenonis called depth perception, which is another name for stereopsis.

45. Stereopsinin Sinirsel Mekanizması • Retinadan görme korteksine giden yolların merkezi yolun iki tarafında 1 ila 2 derece ayrılmasına bağlıdır. • İki gözden gelen bazı optik yollar 2, diğer yollar 25 metre uzaktaki cisimlere uyum sağlar. • Mesafe, hangi grup yolların başka hangi grup yollarla etkileşimde olduğunu belirler; stereopsis, derinlik algılaması

46. Autonomic Control of Accommodation and Pupillary Aperture Autonomic Nerves to the Eyes. The eye is innervated byboth parasympathetic and sympathetic nerve fibers; The parasympathetic preganglionicfibers arise in the Edinger-Westphal nucleus--- in the third nerve to the ciliary ganglion These nerves excite 1)the ciliary muscle that controls focusing of the eye lensand (2) the sphincter of the iris that constricts the pupil.

47. The sympathetic innervation; first thoracic segment of the spinal cord --- sympathetic chain and pass upward tothe superior cervical ganglion --- postganglionic sympathetic neurons sympathetic fibersinnervate the radial fibers of the iris (which open thepupil) as well as several extraocular muscles of the eye (Horner’s syndrome).

48. Light reflex.

49. Control of Pupillary Diameter Stimulation of the parasympathetic nerves also excitesthe pupillary sphincter muscle, thereby decreasing thepupillary aperture; this is called miosis. Conversely,stimulation of the sympathetic nerves excites theradial fibers of the iris and causes pupillary dilation, called mydriasis.

50. Pupillary Light Reflex. When light is shone into the eyes,the pupils constrict, a reaction called the pupillary light reflex. When light impinges on the retina, a few of the resultingimpulses pass from the optic nerves to the pretectalnuclei ---Edinger-Westphal nucleus --- parasympathetic nerves to constrict the sphincter of theiris. Conversely, in darkness, the reflex becomes inhibited,which results in dilation of the pupil.