Special sensesThe Eye:

I. Optics of vision

II. Function of retina

III. Central neurophysiology of vision

Dr Z. Akbari

In the clinicIn the clinicIn the clinicIn the clinic• 65-year-old man presents to his primary care doctor

complaining of difficulties with his vision, particularly atnight The blurred vision is primarily in the right and leftnight. The blurred vision is primarily in the right and leftperipheral fields. He has myopia and wears correctivelenses.The physical examination reveals visual acuity ofp y y20/100 bilaterally with visual field deficits on the right andleft periphery.

◆ Where is the cranial lesion that results in bitemporal hemianopia?◆ What type of lens is necessary to correct myopia?◆ Why does a deficiency of vitamin A result in night bli d ?blindness?

Dr Z. Akbari

ObjectivesObjectivesjj

Understand the peripheral processing of visual information.

Know about the central processing of visual informationinformation.

D ib th l t t Describe the oculomotor system.

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Physical principle of opticsPhysical principle of opticsChapter Chapter 4949

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Optics of the EyeOptics of the Eye- Eye as camera

The major structures of the eyeThe major structures of the eyeThe major structures of the eyeThe major structures of the eye

ooThe cornea:The cornea:ooThe cornea:The cornea:ooThe iris: The iris: oo Pupil:Pupil:oo Pupil:Pupil:oo PupillaryPupillary Diameter: Diameter:

oo Light accommodationLight accommodationoo Light accommodationLight accommodationoo Depth of focusDepth of focus

ooThe lens:The lens:ooThe lens:The lens:oo AccommodationAccommodation

ooFluid compartments of the eyeFluid compartments of the eyeooFluid compartments of the eyeFluid compartments of the eyeooRetinaRetinaooChroidChroid

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ooChroidChroid

Mechanism of accommodation

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Accommodation is controlled bycontrolled by parasympathetic nerves

PupillaryPupillary reflexreflex

light signals from eyelight signals from eye

optic nerve (N.II)optic nerve (N.II)

pretectumpretectum (midbrain)(midbrain)

EdingerEdinger--westphalwestphal (Preganglionic n)(Preganglionic n)

OculumotorOculumotor nerve (N.III)nerve (N.III)

CiliaryCiliary ganglion (Postganglionic n)ganglion (Postganglionic n)

SMC of SMC of PupillaryPupillary sphinctersphincter

Effect of small Effect of small (top) and large (bottom) (top) and large (bottom) pupillarypupillaryapertures on apertures on ""depth of focusdepth of focus””

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Determination of object distance from eye by:

Sizes of retinal image of known object- Sizes of retinal image of known object

- Moving parallax

- Stereopsis - binocular vision ( < 6m)

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The perceived size of an object depends on other objects in The perceived size of an object depends on other objects in p j p jp j p jthe visual fieldthe visual field

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Fluid system of the eye (intraocular fluid)

Aqueous humor- Aqueous humor

- vitreous humor

- Formation of aqueous humorFormation of aqueous humor

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Outflow of aqueous humor

The The endend

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Visual processing by the Visual processing by the p g yp g yretinaretinaChapter 50

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Dr Z. Akbari

Dr Z. Akbari

Functional Anatomy Functional Anatomy of the retinaof the retinaof the retinaof the retina

Prevents the scattering of lightPhagocytose bits of cell g ymem.Provide nutrients Reconvert metabolizedReconvert metabolized photopigment

Blood supply of the retina - central retinal artery and the choroidthe choroid

Foveal region of the retina is important in acute vision

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Rod and cone density along the horizontal meridian through the human retina

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PeripheralPeripheral & central

tiretina

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Photoreceptors: Rods & ConesRods & Cones

Structure, function

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Difference between rods and conesDifference between rods and cones

•• RodsRods•• High sensitivity to light (forHigh sensitivity to light (for

•• ConesCones lower sensitivity (for day vision)lower sensitivity (for day vision)High sensitivity to light (for High sensitivity to light (for

night vision)night vision)•• More photopigment, capture More photopigment, capture

lower sensitivity (for day vision)lower sensitivity (for day vision)

less photopigmentless photopigmentmore lightmore light•• High amplification, single High amplification, single

photon detectionphoton detection

less photopigmentless photopigment

lower amplificationlower amplificationphoton detection photon detection •• Slow responseSlow response

lower amplificationlower amplification

Fast responseFast response

Rod systemRod system::

pp

Cone systemCone systemRod systemRod system::•• AchromaticAchromatic•• Low acuity: not present in Low acuity: not present in

ChromaticChromatic High acuity:High acuity: concentratedconcentrated i f di d ti li f di d ti l

y py pcentral fovea, highly central fovea, highly convergent retinal pathwayconvergent retinal pathway

in fovea, dispersed retinal in fovea, dispersed retinal pathwayspathwaysDr Z. Akbari

Tricolor mechanism of color detectionSpectral sensitivity of Cons :- Spectral sensitivity of Cons :

RedRed--sensitive pigment is sensitive pigment is 560 560 nmnm(L cones); ;

GreenGreen--sensitive pigment, is sensitive pigment, is 530 530 nm (M cones);nm (M cones);

BlueBlue--sensitive pigment, is aboutsensitive pigment, is about 420420 nm (S cones).nm (S cones).BlueBlue sensitive pigment, is about sensitive pigment, is about 420 420 nm (S cones). nm (S cones).

- Interpretation of color in the nervous system

P ti f hit li ht- Perception of white light

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Color blindness

R d l bli dRed-green color blindness21

Protanope: loss of red cone 2Protanope: loss of red cone 2

Deuteranope: loss of green 4

cone

Photopigments

Rods: Rhodopsin (scotopsin+ retinal)Rods: Rhodopsin (scotopsin+ retinal)

Cones: photopsin (3 type) + retinal

Melanopsin?Melanopsin?

Phototransduction :

Three stage cascade of biochemical events in photoreceptorThree stage cascade of biochemical events in photoreceptor

stage 1: Light activates pigment molecules in the photoreceptors

stage 2: Activation of pigment molecules reduces the cytoplasmicstage 2: Activation of pigment molecules reduces the cytoplasmic

concentration of cGMP

stage 3: cGMP closes cGMP-gated ion channels

Hyperpolarization of photoreceptorype po a at o o p oto ecepto

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Dr Z. Akbari

stage 1: Light activates pigment molecules in the photoreceptors

(Rhodopsin= scotopsin + retinal)

D i i f Rh d i- Decomposition of Rhodopsin

by light energy

- Re-formation of Rhodopsin

- Role of Vit A

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stage 2: Activation of pigment molecules reduces the cytoplasmic concentration of cGMPconcentration of cGMP

1. Photon activates Rhodopsinp

2. Rhodopsin activates Transdusin

3. Transducin activates Phosphodiesterase

4. PDE hydrolyzes cGMP

5. Rhodopsin kinase inactivates Rhodopsin

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stage 3: cGMP closes cGMP-gated ion channels Hyperpolarization of photoreceptor

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Automatic regulation of retinal sensitivity

Light and Dark adaptationLight and Dark adaptationBuildup of rhodopsin store

Cone is faster than rods

Cone store is limited

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Neural function of the retinaNeural circuitry of the retina- Neural circuitry of the retina

- Different cell types

- Visual pathway from Cones and Rods to the ganglion cell

- Neurotransmitter of retinal neurons

- Electrotonic conduction

Dr Z. Akbari

Dr Z. Akbari

Function of retinal interneurons1 Horizontal cells:1. Horizontal cells:

Lateral inhibition to enhance visual contrast

2. Bipolar cells: Depolarizing & Hyperpolarizing

a. Lateral inhibition b. Transmitting +ve & -ve signals

3. Amacrine cells:

Begin analysis of visual signalBegin analysis of visual signal

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Retinal Ganglion cells

- Different pattern in connectivity of photoreceptor to ganglion cells in

peripheral and central retina

- Types of ganglion cell- Types of ganglion cell

-Large ganglion cells (arge ganglion cells (magnomagno, or , or M cellsM cells): ):

movement & movement & stereopsisstereopsis

--Small ganglion cells (Small ganglion cells (parvoparvo, or , or P cellsP cells):):g g (g g (pp ))

color, texture, and shape. color, texture, and shape.

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Excitation of ganglion cells- Spontaneous, continuous action potential in the ganglion cells

- Receptive field of ganglion cell has center-surround organization

Two type of ganglion cells:Two type of ganglion cells:onon--centercenteroffoff--centercenteroffoff--centercenter

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Dr Z. Akbari

Dr Z. Akbari

Role of lateral inhibition:

Transmission of signal depicting contrast in visual scene

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Transmission of color signal by the ganglion cells *

The endThe end

Central neurophysiology of vision

chapter 51

Dr Z. Akbari

VisualVisual

fieldfieldfieldfield

Central visual pathwayCentral visual pathway O tiO ti Optic nerveOptic nerve

Optic chiasmOptic chiasm

Optic tractOptic tract

II MidbrainMidbrainI.I. MidbrainMidbrain

Superior colliculusSuperior colliculus

PretectumPretectum

IIII LGNLGNII.II. LGNLGN

Optic radiationOptic radiation

Visual cortexVisual cortex

Dr Z. Akbari

Dr Z. Akbari

In The ClinicIn The ClinicInterruption of the visual pathway at any level will cause a defect in

In The ClinicIn The Clinicthe appropriate part of the visual field . For example, a tiny lesion inthe retina would result in a blind spot (scotoma) in that eye, whereasa similar lesion in the striate cortex would produce correspondinga similar lesion in the striate cortex would produce correspondingscotomas in both eyes. Interruption of the optic nerve on one sideproduces blindness in that eye. Damage to the optic nerve fibers ash i h i hi l i l f i i i b h lthey cross in the optic chiasm results in loss of vision in both temporalfields of vision; this condition is known as bitemporal hemianopsiaand occurs because the crossing fibers originate from ganglion cellsand occurs because the crossing fibers originate from ganglion cellsin the nasal halves of each retina. A lesion of the entire optic tract,LGN, visual radiation, or visual cortex on one side causesh h i i hi h i l f i i i th tihomonymous hemianopsia, which is loss of vision in the entirecontralateral visual field. Partial lesions result in partial visual fielddefects. For example, a lesion in the lingual gyrus causes an upperp , g gy pphomonymous quadrantanopsia, which in this case is loss of visionin the contralateral, upper visual field.

Function of dorsal lateral geniculate nucleus (LGNLGN)1.Main terminus of retinal output fiber

2. Relay nucleus

3. As a Gate

4 Diff t l f LGN i i t f ifi li ll4. Different layer of LGN receives input from specific ganglion cells

M Pathway (Rapidly conducting – Color blind)

P Pathway (Moderate velocity – Color contrast)

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Organization and function of visual cortex -Primary and secondary visual cortexPrimary and secondary visual cortex

- Two major Pathways for analysis of visual information

1. The “Fast” & “Position” Pathway: 3D position, Gross form & Motion

2. The “accurate Color” pathway: Detail & Color

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Each half of the visual Each half of the visual field is represented in field is represented in the the contralateralcontralateral primary primary visual cortexvisual cortexvisual cortexvisual cortex

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Layered structure of the primary visual cortexC- Columnar organization

- Color Blob

- Interaction of visual signal from

the separate eyesthe separate eyes

Pathways from the retina project Pathways from the retina project to visual cortexto visual cortex

Magnocellular system (Where Pathway)

Parvocellular system (What Pathway)Parvocellular system (What Pathway)

Parvocellular blob system (Color Pathway)Parvocellular-blob system (Color Pathway)

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Magnocellular system (Where Pathway)

M-type ganglion cells of retina Magnocellular layers of LGN Layer 4Ca of V1 Layer 4B & 6 of V1 Thi k t i f V2 V3 V5 (MT) V5 (MST) &Thick strips of V2 V3 V5 (MT) V5a (MST) & parietal cortex

• Specialized for motion and spatial relationship• Contributes to StereopsisContributes to Stereopsis• Cells respond rapidly and transiently (rapidly adapting)• Cells are insensitive to color • Have limited capability for depth perception • Poor delectability of stationary objects

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Parvocellular system (What Pathway)

P-type ganglion cells of retina Parvocellular layers of LGN Layer 4Cβ of V1 Interblobs of layer 2 and 3 of V1 Pale strips of V2 V4 Inferotemporal cortex (IT)

• Perception of objects P ti f d th• Perception of depth

• Cells are sensitive to orientation of edges • Cells code high resolution spatial information • Cells are slowly adapting

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Parvocellular-blob system (Color Pathway)P t li ll f ti P ll l l f P-type ganglion cells of retina Parvocellular layers of LGN Blobs in layer 2 and 3 of V1 Thin strips of V2

V4 I f t l t (IT)V4 Inferotemporal cortex (IT)

Dr Z. Akbari

Analysis of visual image

- Analysis of contrastAnalysis of contrast

- Simple cells: Detection of orientation of lines and borders,

C l ll D t ti f i li i t ti- Complex cells : Detection of moving line orientation

- Detection of color

Dr Z. Akbari

Dr Z. Akbari

Control of the eye movements

- Muscular control of eye movements

Neural pathways controlling eye movement

Movement of the eyesMovement of the eyesO t ki ti tO t ki ti t h ld i d i t i d h dh ld i d i t i d h d--Optokinetic movement: Optokinetic movement: hold image during sustained head hold image during sustained head

rotation , driven by visual stimulationrotation , driven by visual stimulation

-- VestibuloVestibulo--ocular movement: ocular movement: hold image still on the retina during hold image still on the retina during fast and brief head movement driven by vestibular signalsfast and brief head movement driven by vestibular signalsfast and brief head movement, driven by vestibular signalsfast and brief head movement, driven by vestibular signals

Saccadic movementSaccadic movement:: sudden jerky movement gaze shifts fromsudden jerky movement gaze shifts from-- Saccadic movementSaccadic movement: : sudden jerky movement gaze shifts from sudden jerky movement gaze shifts from one object to another (Saccadic movement during reading )one object to another (Saccadic movement during reading )

-- Pursuit movementPursuit movement: : tracking movement as eyes follow moving tracking movement as eyes follow moving objectobjectobject object

-- Vergence movement:Vergence movement: move the eyes in opposite directionsmove the eyes in opposite directionsVergence movement: Vergence movement: move the eyes in opposite directions move the eyes in opposite directions …..the image is positioned on both fovea…..the image is positioned on both foveaDr Z. Akbari

Superior Superior colliculicolliculi

Receive fiber from: M fiber of retina & cortex

3 maps: visual space, somatic, sound in space

+ Motor map: Regulate eye movement+ Motor map: Regulate eye movement

Integrates visual and motor information into oculomotor signals to the brain

Responsible for turning the eyes and head toward a visual disturbance

The endThe endDr Z. Akbari

Dr Z. Akbari