PL3020/FM2101/PL2033 Physiology Vision 1

The visual system Lecture 1 Structure of the eye Optics, visual acuity and refractive errors Photoreceptors: transduction and adaptation 2

The visual system Lecture 1 Structure of the eye Optics, visual acuity and refractive errors Photoreceptors: transduction and adaptation Lecture 2 Retinal processing and LGN Primary visual cortex: simple and complex cells, edge and feature detection Secondary visual areas, colour processing, stereopsis etc 3

Gordon Reid Physiology g.reid@ucc.ie 4

The simplest camera 5

Diaphragm: adjust amount of light Lens: more light Diaphragm: adjust amount of light 6

The eye as a camera 7

The eye as a camera cornea + lens (eye)  lens (camera) retina (eye)  film (camera) iris (eye)  diaphragm (camera) 8

Structure of the eye Aqueous humour Lens Cornea Sclera Vitreous humour (proteoglycan gel) Cornea Sclera Lens 9

Structure of the eye Iris Aqueous humour Lens Cornea Sclera Ciliary muscle Suspensory ligaments Vitreous humour 10

Structure of the eye Iris Aqueous humour Lens Cornea Sclera Ciliary muscle Suspensory ligaments Retina Fovea Optic disc Vitreous humour Optic nerve 11

Aqueous humour flow Ciliary body 12

Aqueous humour flow Blocking canal of Schlemm  glaucoma Normal pressure 15 mm Hg Can increase to 60-70 mm Hg 13

Refraction 14

Refractive index Low High Speed of light reduced  light rays bent 15

Bending of light and refractive index Snell’s law: N1 sin a1 = N2 sin a2  if refractive index N becomes larger then angle a becomes smaller  This is why water bends light 16

Lenses, focusing and refractive power Power (diopters) = 1/focal length (metres) e.g. focal length = 0.5 m: power = 2 D 17

Lenses, focusing and refractive power 18

Refracting structures in the eye Four refracting surfaces: Front of cornea +48 D; back of cornea -5 D Front of lens +5 D; back of lens +8 D Four refracting surfaces: Front of cornea +48 D; back of cornea -5 D Front of lens +5 D; back of lens +8 D 19

Widely used model for human optics Reduced eye Widely used model for human optics 20

Visual acuity 21

Limit of visual acuity Limit ~0.5 minutes (0.008°) = 1 mm at 10 m = 2 μm on retina (foveal cone diameter ~1.5 μm) 22

Normal visual acuity “Normal” is considered to be 1 minute (0.017°) 1.75 mm 5 mm 6 m (20 ft) “Normal” is considered to be 1 minute (0.017°) = 1.75 mm at 6 m = 5 μm on retina: If you can resolve 1.75 mm at 6 m you have 6/6 vision (USA: 20/20 vision) 23

6 m (6/6 vision if resolved at 6 m) Landolt C test 1.75 mm 6 m (6/6 vision if resolved at 6 m) 3.5 mm 12 m (6/12 vision if resolved at 6 m) 24

Accommodation 25

You need increased refractive power to focus on something close up Accommodation: You need increased refractive power to focus on something close up 26

The lens becomes more curved Accommodation: The lens becomes more curved 27

1. Suspensory ligaments keep the lens stretched Accommodation: 1. Suspensory ligaments keep the lens stretched 2. Contraction of the ciliary muscle (parasympathetic) allows the lens to relax 28

Range of accommodation (D) = 1/near point (m) If d = 12.5 cm then accommodation = 8 D If d = 25 cm then accommodation = 4 D 29

Decline in accommodation with age 30

Refractive errors 31

“Far-sighted” Eyeball too short Refractive errors Normal Hyperopia “Far-sighted” Eyeball too short Myopia “Short-sighted” Eyeball too long 32

Correcting refractive errors Myopia Use a concave lens (negative) Hyperopia Use a convex lens (positive) 33

Astigmatism Means that refracting power is not homogeneous: e.g. more in vertical (BD) than horizontal (AC) plane 34

Astigmatism Correction: cylindrical lens to increase refractive power of AC 35

Contact lenses can repair more complex defects 36

Depth of field (near and far objects simultaneously in focus) Maximised when pupil is constricted 37

Pupil constriction Depends on antagonistic iris muscles Sympathetic: pupil dilation Parasympathetic: pupil constriction 38

The retina 39

The retina 40

The retina Peripheral LIGHT  Fovea Photoreceptors 41

The retina Peripheral 42

Rods and cones Rod Cone 43

Rods and cones Rod Cone Rod Cone 44

Rods and cones Rods Operate in dim light: saturated in daylight Not involved in colour vision Present only in peripheral retina Cones Operate only in bright light: inoperative at night Involved in colour vision (red, green, blue cones) High density in fovea 45

Spectral sensitivity of rods and cones 46

Spectral sensitivity of cones (2) Three broad groups Inter-individual variation 47

Rod and cone distribution in the retina Peripheral Fovea Many rods Relatively few cones Maximally close packing of cones 48

Rod and cone distribution in the retina (2) Fovea 49

Visual transduction (1) Rhodopsin = opsin + retinal: Opsin Retinal attachment 50

Visual transduction (2) A single “flip” in retinal  conformational change of opsin 51

Visual transduction (3) Opsin is a 7-transmembrane-helix receptor It couples to a G protein (transducin)  Transducin activates phosphodiesterase (PDE)  PDE breaks down cGMP 52

Visual transduction (4) PDE breaks down cGMP  Closure of cGMP-gated ion channels  Hyperpolarisation 53

Visual transduction (5): Amplification Single photon flashes Two photons 54

Spectral sensitivity of rods and cones 55

...depends on sequence differences in opsins 56

Dark adaptation After exposure to bright light, subjects were given dim red or green spots as stimuli Threshold measured (i.e. dimmest spot that subject could see) Red spot Green spot Red light stimulates only cones: green both cones and rods Cones adapt faster but rods are more sensitive 57

Retinal processing of light-induced signals What next? Retinal processing of light-induced signals 58