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Visual Acuity, Photosensors

Visual acuity is an important measure of eye function. Under well-lighted conditions, the normal eye should be able to distinguish two points as separate when the light rays emitted by the point objects converge at an angle (a) of 1 min (1/60 degree) (^ A and p. 346). Visual acuity is calculated as 1/a (min1), and is 1/1 in subjects with normal vision.

Visual acuity testing is generally performed using charts with letters or other optotypes (e.g., Landolt rings) of various sizes used to simulate different distances to the test subject. The letters or rings are usually displayed at a distance of 5 m (^ A). Visual acuity is normal (1/1) if the patient recognizes letters or ring openings seen at an angle of 1 min from a distance of 5 m. Example: It should be possible to identify the direction of the opening of the middle ring from a distance of 5 m and that of the left ring from a distance of 8.5 m (^ A). If only the opening of the left ring can be localized from the test distance of 5 m, the visual acuity is 5/8.5 = 0.59.

Photosensors or photoreceptors. The lightsensitive sensors of the eye consist of approximately 6 • 106 rods and 20 times as many cones (^ p. 345 E) distributed at variable densities throughout the retina (^ B1). (Certain ganglion cells also contain a light-sensitive pigment; ^ p.334). The fovea centralis is exclusively filled with cones, and their density rapidly decreases towards the periphery. Rods predominate 20-30 degrees away from the fovea centralis. Approaching the periphery of the retina, the density of the rods decreases continuously from 1.5 x 105/mm2 (maximum) to about one-third this value. No photosensors are present on the optic disk, which is therefore referred to as the blind spot in the visual field.

Clear visualization of an object in daylight requires that the gaze be fixed on it, i.e., that an image of the object is produced in the fovea centralis. Sudden motion in the periphery of the visual field triggers a reflex saccade (^ p. 360), which shifts the image of the object into the fovea centralis. Thereby, the retinal area with the highest visual acuity is selected (^ B2, yellow peak), which lies 5 degrees temporal to the optical axis. Visual acuity decreases rapidly when moving outward from the fovea (^ B2, yellow field), reflecting the decreasing density of cone distribution (^ B1, red curve). In a dark-adapted eye, on the other hand, the sensitivity of the retina (^ B2, blue curve) is completely dependent on the rod distribution (^ B1, purple curve). The color-sensitive cones are therefore used for visual perception in daylight or good lighting (day vision, photopic vision), while the black and white-sensitive cones are used to visualize objects in darkness (dim-light vision, night vision, scotop-tic vision). The high light sensitivity in night vision is associated with a high loss of visual acuity (^ p. 354).

Photosensor Function

Light-absorbing visual pigments and a variety of enzymes and transmitters in retinal rods and cones (^ C1) mediate the conversion of light stimuli into electrical stimuli; this is called photoelectric transduction. The membranous disks of the retinal rods contain rhodopsin (^ C2), a photosensitive purple-red chromo-protein (visual purple). Rhodopsin consists of the integral membrane protein opsin and the aldehyde 11-cis-retinal. The latter is bound to a lysine residue of opsin which is embedded in this protein; it is stably kept in place by weak interactions with two other amino acid residues. Photic stimuli trigger a primary photochemical reaction in rhodopsin (duration, 2-10-14s) in which 11-cis-retinal is converted to all-trans-retinal (^ C3). Even without continued photic stimulation, the reaction yields bathorhodopsin, the intermediates lumirho-dopsin and metarhodopsin I, and finally meta-rhodopsin II within roughly 10-3s (^ D1).

Metarhodopsin II (MR-II) reacts with a Gs-protein (^ p. 274) called transducin (Gt-pro-tein), which breaks down into as and sub-units once GDP has been replaced by GTP (^ D1). Activated as-GTP now binds the inhibitory subunit of cGMP phosphodiesterase (1PDE) (^ D2). The consequently disinhibited phosphodiesterase (PDE) then lowers the cytosolic concentration of cyclic guanosine monophosphate (cGMP). The activation of a single retinal rhodopsin molecule by a quantum of light can induce the hydrolysis of up to 106 cGMP molecules per second. The reaction cascade therefore has tremendous amplifying power.

Landolt rings

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