Summary
Vision is a highly complex process that begins when light enters the eye. It is subject to physical laws while also depending on stimulus processing in the brain. The most important functional units in vision are the dioptric apparatus (composed of the cornea, aqueous humor, lens, and vitreous body) and the retina. The dioptric apparatus is responsible for projecting a reduced, inverted image onto the retina. The retina converts light stimuli into electrical signals (transduction) and transmits them to the brain. The path for optical perception from the retina to the brain is called the visual pathway.
For details on anatomy, see "Eye and orbit."
For details on the physiology of the pupil, see "Physiology and abnormalities of the pupil."
Dioptric apparatus
For an object to be perceived clearly, light rays reflected from it must converge at a single point on the retina. The dioptric apparatus (composed of the cornea, aqueous humor, lens, and vitreous body) performs the necessary light refraction. These components, which have different refractive powers, are arranged in series and collectively function as a converging lens . The light rays thus create a reduced and inverted image on the retina, which is then interpreted by the brain, allowing for conscious vision.
Physical principles
For more details, see "Optics."
When light strikes the interface between two media with different optical densities at an angle, the light is refracted (i.e., it changes its direction of propagation).
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Refractive index (n)
- Describes a material property (optical density) that slows light rays (compared to their speed in a vacuum)
- The higher the refractive index, the greater the optical density, and the slower the light propagates in the medium being examined.
- Air: n = 1.0003
- Water: n = 1.33
- Cornea: n = 1.376
- Lens: n = 1.413
- Aqueous humor: n = 1.336
- Vitreous body: n = 1.336
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Refraction (= bending of light): change in the ray's direction
- Occurs at the interface between two materials with different refractive indices (n)
- The greater the difference in the refractive index, the more strongly the light is refracted
- Focal point (F): point at which the refracted light rays, which strike the lens parallel to the optical axis, intersect
- Focal length (f): distance from the center of the lens to the focal point in meters (m)
- Refractive power (D): reciprocal of the focal length in diopters (dpt) (D = 1/f → 1 dpt = 1/m)
The more the refractive indices of two media differ, the more the direction of the light ray is changed at the interface!
Refractive media
- Comprises the cornea, lens, aqueous humor, and vitreous body
- Direct and refract light to the posterior region of the retina
- Alterations in the structure of the refractory media can lead to impairment of vision (e.g., cataract).
- Minor deviations in eye anatomy that affect refractive media (e.g., axial eye length, cornea curvature) can result in the development of refractive errors.
- Total refractive power of the resting eye: approx. +58–60 dpt
The cornea has the strongest refractive power of the dioptric apparatus because it represents the interface between air and a fluid medium (highly different refractive indices)!
Accommodation (eye)
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Definition: adjustment of the eyes to different distances (near vision versus far vision)
- Primarily mediated by the lens, which changes convexity to adapt the refractive power
- Changes occur via contraction of ciliary muscles and are mediated by the Edinger-Westphal nuclei bilaterally.
- Constricted ciliary muscle → relaxed ciliary processes → curvature of lens increases → increase in lens refractive power
- Relaxed ciliary muscle → tense ciliary processes → curvature of lens decreases → decrease in lens refractive power
Accommodation reflex
- The synkinetic constriction of the pupil (miosis), convergence of the eyes, and accommodation of lens convexity in response to a suddenly closer object
- Miosis: contraction of the iris sphincter muscle
- Convergence: contraction of both medial rectus muscles (mediated by the CN III) → simultaneous inward movement of both eyes
- Accommodation: preganglionic parasympathetic fibers from Edinger-Westphal nucleus travel with the oculomotor nerve → ciliary muscle contraction → increased lens convexity
- Near point: the closest point that can still be seen clearly (is approx. 10 cm in a young adult with normal vision)
- Total refractive power of the accommodated eye: up to 73 dpt
Mechanism of far point vision
- The simultaneous dilation of the pupil (mydriasis), divergence of the eyes, and flattening of the lens in response to shifting gaze from a near to a distant object
- Mydriasis: contraction of the iris dilator muscle → increased pupil size to allow more light in
- Divergence: relaxation of both medial rectus muscles → eyes move outward to parallel
- Lens flattening (disaccommodation): ciliary muscle relaxation → passive tensile forces from Bruch's membrane and the sclera pull zonular fibers taut → lens is pulled flat → decreased lens convexity
- Far point: the most distant point that can still be seen clearly (located at infinity for a person with normal vision)
- Total refractive power of the resting eye: approx. +58–60 dpt
Since near point accommodation is an active process (contraction of the ciliary muscle), the resting state of the eye is far point accommodation!
Amplitude of accommodation
The amplitude of accommodation is the maximum change in the eye’s refractive power when shifting focus from far to near.
- Calculation: amplitude of accommodation (dpt) = 1/near point (m) – 1/far point (m)
In youth, the amplitude of accommodation is still about 14 dpt, but in old age, it can drop to about 1 dpt (presbyopia)!
Refractive errors
- Emmetropia: a physiologic state of vision in which rays of light entering the relaxed eye are focused on the retina, resulting in clear vision
- Refractive errors (ametropia): conditions in which light rays entering the eye are not focused on the retina, leading to blurred vision. This results from an incongruity between the eye's optical power and its axial length. Includes:
Cells of the retina
Photoreceptors
- Two different types: cones and rods (overall ratio approx. 1:20)
- Located in the light-perceiving part of the retina (pars optica)
- Consist of an axon, cell body, and a light-sensitive process with an outer and inner segment
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Visual pigments: light-absorbing molecules located in the outer segments
- Stimulation by incident light energy → chemical conformational change
Retinal cones
- Primarily located within the fovea centralis
- Chromatic (contain pigments for blue, red, and green light): photoreceptors specialized for color vision, bright light, and visual acuity (phototopic vision)
- Contain three types of photopsins with different spectral sensitivities (enabling the distinction of colors)
- Resolution: high in the fovea centralis
Retinal rods
- Primarily located in the parafoveal zone, not present in the fovea centralis
- Achromatic: photoreceptors specialized for night vision (dim light) and motion (scotopic vision)
- Contain rhodopsin: a G protein-coupled receptor consisting of opsin and 11-cis-retinal
- Resolution: rather low
There are no rods in the fovea centralis, only cones!
Retinal bipolar cells
Bipolar cells receive information from photoreceptors and transmit it to ganglion cells.
- On-cone bipolar cells
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Off-cone bipolar cells
- Mainly possess an ionotropic glutamate receptor (iGLUR) and are excited by glutamate released by cones in the dark
- Inhibited when little or no glutamate is released (light signal)
- Rod bipolar cells
Retinal ganglion cells
Ganglion cells receive information from bipolar cells and transmit it to the brain via their axons, which form the optic nerve.
On- and off-center ganglion cells
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Connection of cones (direct pathway)
- On-center ganglion cells: illumination in the center of the cone's receptive field → decreased glutamate release → depolarization (excitation) of the on-cone bipolar cells and increased glutamate release from these cells → depolarization (excitation) of the on-center ganglion cells; illumination in the periphery of the receptive field inhibits them (partly via horizontal cells)
- Off-center ganglion cells: illumination in the center of the cone's receptive field → decreased glutamate release → hyperpolarization (inhibition) of the off-cone bipolar cells and decreased glutamate release from these cells → hyperpolarization (inhibition) of the off-center ganglion cells; illumination in the periphery of the receptive field excites them (partly via horizontal cells)
- Connection of rods (indirect pathway): illumination of rods → depolarization (excitation) of the rod bipolar cells → depolarization (excitation) of the amacrine cells → excitation of the on-cone bipolar cells or inhibition of the off-cone bipolar cells → depolarization of the on-center ganglion cells or hyperpolarization of the off-center ganglion cells
Morphological groups
The ganglion cells are divided into three morphologically and functionally different groups. The three cell types form the origin of different systems, which in turn fulfill different functions in vision.
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M-cells (= magnocellular system)
- Number: about 10% of ganglion cells
- Conduction speed: fast
- Function: detection of motion and distance of objects
- Transmission: to the primary visual cortex
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P-cells (= parvocellular system)
- Number: about 80% of ganglion cells
- Conduction speed: slow
- Structure: smaller receptive fields → high spatial and low temporal resolution
- Function: detection of color and form of objects
- Transmission: to the primary visual cortex
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K-cells (= koniocellular system)
- Number: about 10% of ganglion cells
- Conduction speed: slow
- Function: detection of color
- Transmission: to the lateral geniculate nucleus
Intrinsically photosensitive ganglion cells
- Some ganglion cells contain a visual pigment (melanopsin) and are thus photosensitive.
- Information is not used for vision but for light-induced synchronization of the circadian rhythm, control of pupillary movement, and reflexive eye movements.
Signal convergence
- Definition: a mechanism by which multiple presynaptic neurons transmit signals to a smaller number of postsynaptic neurons
- In the retina, multiple photoreceptors transmit signals to a smaller number of ganglion cells.
- The degree of convergence determines the size and sensitivity of the receptive field
- Fovea: low convergence
- A single cone connects to one bipolar cell and one ganglion cell
- Creates extremely small receptive fields → maximizes visual acuity
- Periphery: high convergence
- Many rods connect to a single ganglion cell
- Creates large receptive fields → maximizes light sensitivity at the expense of resolution
- Fovea: low convergence
Retinal information processing
Signal transduction within the photoreceptors
The mechanism of signal transduction is detailed below using rods as the example. The process in cones is identical, except they utilize photopsins instead of rhodopsin as the visual pigment.
Rods in darkness (resting state)
- Rods are depolarized (resting potential of approx. -40 mV)
- "Dark current": constant influx of sodium and calcium through open cGMP-dependent channels (CNG channels) maintains membrane depolarization → continuous release of glutamate
Rods during illumination (phototransduction)
- Light absorption → isomerization of 11-cis-retinal to all-trans-retinal → conformational change forms meta-rhodopsin II ("active rhodopsin")
- Meta-rhodopsin II binds G protein transducin → exchange of GDP for GTP on the α-subunit → α-subunit dissociates
- Activated α-subunit stimulates cGMP-dependent phosphodiesterase (PDE) → PDE hydrolyzes cGMP to GMP
- Decreased cGMP concentration → CNG channels close → influx of sodium and calcium stops
- Hyperpolarization of the cell (approx. -70 mV) → inhibition of glutamate release (signal transmission to bipolar cells)
Rods after illumination (recovery phase)
- Light influx ceases → transducin α-subunit hydrolyzes its bound CTP to GDP → transducin deactivates → PDE deactivates
- Low cytosolic calcium concentration stimulates guanylate cyclase → resynthesis of cGMP → CNG channels reopen
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Rhodopsin deactivation:
- Rhodopsin kinase phosphorylates meta-rhodopsin II.
- Arrestin binds to phosphorylated rhodopsin, blocking further activation of transducin ("signal termination").
- Pigment regeneration:
- All-trans-retinal detaches from opsin
- Enzymatic conversion to 11-cis-retinal in cells of the pigment epithelium
- 11-cis-retinal returns to the photoreceptor and recombines with opsin to regenerate functional rhodopsin.
Photoreceptors react to a light stimulus with hyperpolarization!
Vitamin A deficiency directly impairs the regeneration of rhodopsin. Consequently, the earliest manifestation is night blindness, as the rod cells are the first to be affected by the lack of pigment. While often caused by insufficient dietary intake, this deficiency can also be triggered by infections, such as measles, which rapidly deplete the body's vitamin A stores.
Adaptation
Adaptation is the ability of the retina to adjust to different light conditions.
Dark adaptation (scotopic vision)
- Transition from cone to rod vision: The light-sensitive rods take over vision in the dark when the minimum perception threshold of the cones is reached (in a graphical adaptation curve, this can be seen after about 10 minutes at the so-called rod-cone break).
- Dilation of the pupil: allows more light to enter in the dark
- Rhodopsin regeneration: The more rhodopsin is present, the higher the light sensitivity.
- Spatial summation: expansion of receptive fields in the dark due to a decrease in lateral inhibition → higher light sensitivity of the neurons
- Temporal summation: Subthreshold stimuli become suprathreshold if they persist over a longer period.
- Purkinje effect: A blue dot appears brighter in the dark than a red dot because in the dark, it is mainly the rods that are active, which have an absorption maximum in the short-wave (approx. 500 nm) blue-green range
- Decrease in critical flicker fusion frequency
- Decrease in visual acuity
- Decrease in contrast sensitivity
Light adaptation (photopic vision)
- Transition from rod to cone vision: perception occurs exclusively via the cone cells
- Constriction of the pupil
- Rhodopsin bleaching
- Increase in critical flicker fusion frequency
- Increase in visual acuity
- Increase in contrast sensitivity
"Twilight vision" (mesopic vision)
- Both rods and cones are active.
Since the point of sharpest vision is in the fovea centralis and only cones are found there, it is not possible to fixate properly from the point of pure rod vision!
Color vision
Visible light has a wavelength between about 400 nm (blue-violet) and 750 nm (red) . A blue object, for example, appears blue because it absorbs wavelengths other than the blue spectrum; the blue wavelengths are thus reflected.
- Principle: There are three different cones for color vision (red, blue, and green) with different opsins, the excitation of which creates the different color impressions
- Dispersion: The refraction of light depends on the wavelength of the light.
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Cone types
- S cones (short-wavelength cones): absorb blue-violet light and have an absorption maximum at about 420 nm
- M cones (medium-wavelength cones): absorb blue-green to yellow light and have an absorption maximum at about 535 nm
- L cones (long-wavelength cones): absorb yellow to red light and have an absorption maximum at about 565 nm
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Forms of color mixing
- Additive color mixing: mixing of light (e.g., on a television) of different wavelengths → a new color impression is created (the spectrum after mixing is larger than before)
- Subtractive color mixing: mixing of paints (e.g., in painting) of different wavelengths → a new color impression is created (the spectrum after mixing is smaller than before, as the wavelengths of both colors are absorbed)
If wavelengths of the entire spectrum are mixed, the color impression "white" is created!
Color anomalies
The term color anomaly refers to a genetic disorder of color vision in which one (or more) of the three cone types is not normally functional. Color anomalies are usually inherited in an X-linked recessive manner, which is why men are primarily affected. In so-called dichromacy, only two (of the physiological three) cone types in the retina are normally functional. The most common is deuteranomaly (green weakness), which occurs in about 5% of the male population. Deuteranopia (green blindness), protanomaly (red weakness), and protanopia (red blindness) are somewhat rarer, each with a frequency of 1% in the male population. In a so-called red-green weakness, those affected can still perceive the colors "red" and "green," but they confuse them.
Visual field
The visual field is the area that can be seen while fixating a single point, with the eyes and head held still . The visual field is measured using perimetry. For clinical relevance, see "Overview of visual field defects" .
- Monocular visual field: the total area seen by one eye (∼ 150° horizontal)
- Binocular visual field: the central ∼ 120° where the fields of both eyes overlap, allowing for stereopsis (depth perception)
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Perimetry (visual field testing)
- Perimetry can test both achromatic (white light) and chromatic stimuli.
- Color detection reflects the retina’s color sensitivity: at the far periphery, only achromatic (color-blind) detection is possible; sensitivity to blue, then red, and finally green appears progressively toward the center.
Color vision narrows as you move outward. Blue has the widest field, followed by red, with green being the most central.
Visual pathway
The visual pathway consists of four primary neurons and conducts the optical stimulus from the retina to the visual cortex of the brain.
Visual pathway
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Retina
- First-order neuron: rods and cones (photoreceptors)
- Second-order neuron: bipolar cells
- Transmission from photoreceptors occurs via glutamate.
- On-bipolar cells: have metabotropic (G-protein coupled) glutamate receptors
- Off-bipolar cells: have ionotropic glutamate receptors
- Third-order neuron: ganglion cells; axons form the optic nerve
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Optic nerve
- Carries fibers from both the temporal (lateral) and nasal (medial) retinal areas of the ipsilateral eye
- Maintains retinotopic organization
- Temporal retinal fibers (carrying info from the nasal/medial visual field) run laterally.
- Nasal retinal fibers (carrying info from the temporal/lateral visual field) run medially.
- Leaves the eyeball at the optic disc, which contains no photoreceptors and creates the "blind spot"
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Optic chiasm
- Located superior to the pituitary gland
- Fibers from the nasal retina (carrying information from the temporal visual field) cross over to the contralateral side.
- Fibers from the temporal retina (carrying information from the nasal visual field) remain uncrossed.
- Optic tract: Each optic tract carries contralateral nasal fibers and ipsilateral temporal fibers.
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Lateral geniculate nucleus (LGN)
- The optic tracts terminate in the LGN of the thalamus
- Cell bodies in the LGN are the fourth-order neurons of the pathway
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Optic radiation
- Consists of the axons from the LGN neurons
- Carries visual information to the ipsilateral primary visual cortex (area striata)
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Primary visual cortex
- Processes the initial visual information
- Located at the posterior aspect of the occipital lobe
- Each primary visual cortex receives and processes all visual information from the contralateral visual field.
Disorders of the visual pathway
Interruptions to the visual pathway from neurological disorders or eye diseases produce predictable visual impairments, as the specific visual field defect directly corresponds to the anatomical location of the lesion. These disorders are classified into three main sections: prechiasmal disorders, affecting the optic nerve, which cause unilateral visual defects on the same side as the lesion; chiasmal disorders, affecting the optic chiasm, which typically cause bitemporal hemianopsia (loss of both temporal visual fields); and retrochiasmal disorders, affecting the pathway from the optic tract to the visual cortex, which lead to a homonymous hemianopsia on the opposite side (contralateral) of the lesion (e.g., a left optic tract lesion causes a right homonymous hemianopsia).
Central visual processing
Visual cortex
See also "Occipital lobe" in "The cerebral cortex, meninges, basal ganglia, and ventricular system."
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Primary visual cortex (= striate cortex, V1)
- Located in the occipital lobe (Brodmann area 17) along the calcarine sulcus
- Retinotopically organized, with cortical magnification of the fovea
- First cortical level of conscious visual perception
- Layer IV shows a macroscopically visible dense band of myelinated fibers (line of Gennari)
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Secondary visual cortex (V2)
- Corresponds mainly to Brodmann areas 18 and 19
- Surrounds V1 (V2 forms a horseshoe around the calcarine sulcus)
- Responsible for further processing and interpretation of visual features (form, color, motion)
- After initial processing in V1–V2 and further analysis in the extrastriate visual areas (V3–V5), visual information is sent to higher-order association regions depending on the type of information being processed (e.g., the amygdala for emotion-related vision, the hippocampus for memory processing).
Subcortical visual pathways
- Pathways that receive input directly from the retina and bypass V1
- Superior colliculus pathway: reflexive eye movements, orienting responses
- Retinohypothalamic pathway (via suprachiasmatic nucleus): regulation of circadian rhythms
- Pretectal pathway (via Edinger-Westphal nucleus): control of the pupillary light reflex
Parallel processing
- Refers to the brain’s ability to simultaneously analyze multiple components of a visual scene, rather than processing them sequentially
- Two types of retinal ganglion cells initiate separate visual pathways:
Feature detection
- Feature detection theory: states that complex stimuli can be broken down into individual features, each processed by specialized neurons (feature detectors)
- Feature detector neurons in the primary visual cortex respond selectively to basic visual components (e.g., lines, edges, angles, and motion).
- Features are then integrated through serial processing to form a coherent perception of the object as a whole