2 Optical System and Physiology

A. Optical System

The light incident on the eye penetrates the tear film, cornea, aqueous humor, lens, and vitreous body, which constitute the optical system ofthe eye. Although the eye consists of several refractive components, for simplicity it can be compared to a simple lens system (Aa and b): light rays that pass from one medium (of refractive index n1) into another medium (of refractive index n2) are refracted. When the interface is simply convex, all the rays emerging from an object (O) meet again in an image (I) beyond the interface. The focal length, i.e. the distance of the focus (F1/F2) from the central plane of the lens system, is characteristic of a lens system. Rays that originate from a distant point can be regarded as parallel and they meet in the focal plane. Rays that originate from a near point do not arrive parallel and they form an image behind the focal plane.

B. Accommodation

Accommodation signifies the ability of the eye to focus the rays from objects to form a clear image on the retinal plane in relation to the objects’ distance from the eye. Accommodation is based in particular on the ability of the elastic lens to change from a more spherical shape with high converging power (near focus) to a more elliptical shape with low converging power (distant focus). The passive tendency of the lens to adopt a spherical form is counteracted by the pull of the zonular fibers, which relax through contraction of the parasympathetically innervated ciliary muscle and so enable near accommodation to take place. In near focus this is accompanied by a bilateral convergent movement and miosis. The converging power is the reciprocal of the focal distance measured in meters. The unit of converging power is the diopter (D). The eye has a maximum converging power of ca. 58.8 D when accommodated for distant vision, and on maximum near accommodation this increases by about 10–15 D. This increase in converging power is called the accommodation amplitude.

C. Refraction Errors

creases with increasing age. This physiological event is called presbyopia (impairment of vision due to old age, Ca, upper). The near point moves increasingly into the distance. The patient notices a reduction in the ability to read from about the age of 45 years, when the accommodation amplitude falls below 3 D. Correction of presbyopia is through the use of a converging lens (+D).

In myopia (nearsightedness, Ca, middle) the parallel rays coming from infinity intersect in front of the retinal plane. The cause is excessive converging power of the cornea or lens (convergence myopia) or above-average length of the eyeball (axial myopia). The progressive malignant form of myopia must be distinguished from the simple benign form, which usually does not progress after puberty. Correction of myopia is through the use of a diverging lens (–D)

In hyperopia (farsightedness, Ca, lower) a point situated nearby is focused behind the retinal plane. Axial hyperopia, where the eyeball is too short, is distinguished from convergence hyperopia when the converging power of the cornea or lens is too low. Patients with latent hyperopia use their accommodation even with distant vision, which can lead to asthenopic symptoms (general ocular discomfort). Correction of hyperopia is through the use of a converging lens (+D).

The surface of the cornea is often more curved in one plane than in the other. The result is a difference in convergence in the two planes so that points appear linearly distorted. This regular astigmatism (Cb and c) can be corrected by cylindrical lenses. Irregular astigmatism, e.g., as a result of corneal scarring, can be compensated to a certain degree by hard contact lenses.

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A. Visual Acuity

Vision in the eye relates to the overall function of the visual organ. Apart from pure visual acuity, it also includes the visual field, color vision, and dark vision. Visual acuity means the resolving ability of the eye with an optimally correcting lens, i.e. the ability of the retina barely to distinguish two points from one another (resolution threshold). A normal eye can just differentiate two points when the rays emerging from them form an angle at the eye of one minute of arc (1/60 degree). Visual acuity is calculated from the actual distance of the points from the eye divided by the distance at which the normal eye can resolve the points, and in the normal eye it is therefore 1ßq = 1.0. Optotypes projected into the distance (Landolt’s rings, block letters, numbers, E hooks, children’s pictures) or vision test tables for near vision (e.g., Birkhäuser tables) are used to test vision.

B. Receptors

Rods and cones constitute the retina’s receptors. While the fovea centralis consists exclusively of cones, which are responsible for color vision in good lighting (photopic vision), their density diminishes rapidly toward the periphery. The rods are responsible for vision in poor light (scotopic vision); their greatest density is around the fovea centralis but they are also distributed over the entire retina (Ba). The photoreceptors are absent in the region of the optic disc. Under the effect of light, the rhodopsin located in the outer limbs of the rods and cones is bleached, leading to the conversion of light energy into electrical impulses through a change in the conformation of the pigment part from 11-cis- to all-trans-retinal (Bb). Rhodopsin is a chromoprotein and is a component of visual purple. It is composed of the protein opsin and the vitamin A derivatives 11-cis- and all-trans-retinal. In the dark, regeneration of the rhodopsin occurs with expenditure of energy. Absorption of light is required for the bleaching of the rhodopsin. As the rhodopsin contained in the rods absorbs light from the entire (visible) wavelength spectrum, different wavelengths (colors) cannot be distinguished by the rods. In contrast, the three visual pigments of the cones each absorb only light of a certain wavelength region, which allows for color vision.

C. Visual Field

The term visual field describes the area that is perceived at the same time when the eye is not moving. A distinction is made between monocular and binocular visual fields. The outer boundaries depend on adaptation and on the size, brightness, and color of the object and on whether the object is mobile or static. The boundaries are usually 60° nasally, 70° above, 80° below, and 90° temporally. The visual field is measured by perimetry (C). Two forms are distinguished:

●Kinetic perimetry: this records the site where a stimulus of defined brightness is first perceived as it is brought into the visual field.

●Static perimetry: measurement of the minimum brightness that a stimulus must have in order to be identified at a certain site with defined background brightness.

Defects in the visual field are called scotomas and can be symptoms of many eye diseases. The blind spot due to the absence of receptors in the optic disc is a physiological “scotoma.” In the binocular visual field, each eye’s blind spot is compensated by the other side. Temporally located objects are projected on the nasal half of the retina and vice versa. Objects in the upper visual field are imaged on the lower half of the retina, and objects from the lower region in the upper half of the retina.

B. Receptors

bPhotocycle of bacteriorhodopsin (BR): numbers correspond to the wavelength of the absorption

maximum in nanometers

Visual Acuity/Receptors/Visual Field

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2 Optical System and Physiology

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A. Adaptation

Adaptation (A) signifies the adjustment of the eye to different light levels. This is a complex process, which comprises a change in pupil size, a change between rod and cone vision, and a change in the sensitivity of the retina. According to the duplicity theory of vision, daytime and color vision (photopic vision) is a function of the cone apparatus, while vision in dim light and night vision (scotopic vision) are provided by the rod apparatus. Light adaptation means the transition to photopic vision and is based on pupil constriction and the transition from rod to cone vision with the breakdown of rhodopsin. The first phase of the transition (alpha adaptation) occurs in about 0.05 seconds, while the second phase (beta adaptation) lasts 6–7 minutes. The transition to scotopic vision (dark adaptation) takes place much more slowly and is complete after about 30 minutes. The first phase comprises cone adaptation and ends after about 7–8 minutes with the Kohlrausch notch, the transition to cone adaptation with the regeneration of rhodopsin. Dark adaptation is associated with pupil dilatation, a loss of color vision, a reduction in visual acuity, and a physiological central scotoma.

B. Color Vision

Color vision is a function of the cones. The wavelength spectrum of light perceived by the eye is between 400 nm and about 700 nm. According to the Young-Helmholtz three-color theory (Bb), three types of cones are distinguished: those that absorb blue-violet light, those that absorb green light, and those that absorb yellow-red light (the trichromatic system, Ba). According to the laws of color mixing, all other colors (including white) can be mixed from light of these three colors. The rhodopsin of the rods absorbs light from the entire visible wavelength spectrum, which is why it is not possible to distinguish colors with scotopic vision.

C. Central Processing

The stimulus of incident light leads to hyperpolarization of the primary membrane potential in the receptors of the retina. The magnitude of the potential increases with increase in the stimulus strength. When it crosses the

threshold, this secondary receptor potential leads to action potentials in the corresponding ganglion cell, the frequency of which is proportional to the magnitude of the receptor potential. Through cross-connections within the retina (horizontal cells and amacrine cells), receptive fields occur that exert stimulating and inhibiting influences on the action potentials. Such a receptive field consists of a round center and a concentrically ordered periphery. If the light impulse falls on the center, the frequency of the action potentials rises. Illumination of the periphery leads to a fall in the action potential frequency. If the light stimulus is absent, excitation occurs in the peripheral part of the receptive field. Such a receptive field is called an ON field in contrast to an OFF field with the opposite reaction. The function of the receptive fields is to contrast the sensory stimulus.

D. Pupillary Reflex

The pupillary reflex is triggered by the sudden incidence of light into a pupil. The eye reacts with miosis (direct reaction). The contralateral pupil also contracts due to the central crossover of the stimulus (consensual reaction).

Afferent supply: optic nerve

Efferent supply: parasympathetic fibers via the oculomotor nerve.

E. Corneal Reflex

The corneal reflex is triggered by touching the cornea, which leads to reflex lid closure. Afferent supply: trigeminal nerve

Efferent supply: mainly the facial nerve.

B. Color vision

Relative absorption

Wavelength (nm)

a Absorption spectra of the different cone types

Adaptation/Color Vision

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