Table of Contents
Overview of the Visual Sense
The visual sense (vision) allows organisms to detect and interpret light from their environment. It is based on:
- A physical stimulus: electromagnetic radiation (light)
- Sensors: light-sensitive cells (photoreceptors)
- Optical structures: that guide and focus light
- Neural processing: that converts signals into images and visual perceptions
This chapter focuses on how typical eyes are built and function, with humans and vertebrates as the main example, and short comparisons to other animal eye types.
Physical Basis of Vision: Light as Stimulus
Wavelength and Visible Spectrum
Light is a form of electromagnetic radiation. For humans:
- Visible light ranges approximately from $400\,\text{nm}$ (violet) to $700\,\text{nm}$ (red)
- Different wavelengths are perceived as different colors
- Short wavelengths: violet, blue
- Medium wavelengths: green, yellow
- Long wavelengths: orange, red
Many animals see a different range:
- Bees and many insects see ultraviolet (shorter than $400\,\text{nm}$)
- Some snakes sense infrared (longer than $700\,\text{nm}$) with special organs (not eyes in the strict sense)
Intensity and Contrast
The intensity (brightness) of light is also important:
- Low intensity: twilight, night
- High intensity: bright daylight
Eyes must cope with varying intensities and detect contrast—differences in brightness or color between adjacent areas. Contrast is crucial for recognizing edges, shapes, and movement.
Basic Structure of the Vertebrate Eye (Example: Human Eye)
The vertebrate eye is often described as a camera-type eye: it forms a sharp image on a light-sensitive surface, similar to a camera on a sensor or film.
Protective Structures and Outer Layers
Bony Orbit, Eyelids, and Lacrimal Apparatus
- The bony orbit (eye socket) protects the eye against mechanical damage.
- Eyelids:
- Spread the tear film
- Protect against drying and foreign bodies
- Enable blinking and closing of the eyes
- Lacrimal (tear) glands:
- Produce tear fluid that:
- Keeps the cornea moist and smooth
- Washes away dust and microorganisms
- Contains disinfecting substances (e.g. lysozyme)
Sclera and Cornea
- Sclera (“white of the eye”)
- Tough, opaque connective tissue
- Maintains the shape of the eyeball
- Serves as attachment for eye muscles
- Cornea
- Transparent continuation of the sclera at the front
- Strongly curved: provides most of the eye’s refractive power
- Avascular (no blood vessels) to remain clear; receives nutrients from tear fluid and aqueous humor
- Very sensitive because of dense nerve supply
Middle Layer: Choroid, Ciliary Body, and Iris
- Choroid
- Rich in blood vessels
- Provides nutrition to outer layers of the retina
- Darkly pigmented to absorb stray light, improving image sharpness
- Ciliary body
- Contains the ciliary muscle
- Produces aqueous humor
- Via zonular fibers (suspensory ligaments) connects to the lens and enables accommodation (see below)
- Iris
- Colored part of the eye (eye color determined by pigment in the iris)
- Central opening: pupil
- Contains circular and radial muscles:
- In bright light: circular muscles contract → pupil constricts
- In dim light: radial muscles contract → pupil dilates
- Pupil adjustment regulates the amount of light reaching the retina and improves depth of field
Inner Optical Structures: Aqueous Humor, Lens, Vitreous Body
Aqueous Humor
- Clear fluid in the anterior and posterior chambers (between cornea, iris, lens)
- Functions:
- Supplies nutrients to cornea and lens
- Maintains intraocular pressure, helping maintain shape
- Continuously produced and drained; imbalance can lead to increased pressure (glaucoma)
Lens
- Biconvex, transparent, flexible structure behind the pupil
- No blood vessels; receives nutrients from aqueous humor
- Main function: fine focusing of the image on the retina
Accommodation
Accommodation is the eye’s ability to adjust its focusing power for different distances:
- Distant vision:
- Ciliary muscle relaxed
- Zonular fibers tense
- Lens flatter (less curved) → lower refractive power
- Near vision:
- Ciliary muscle contracts
- Zonular fibers relax
- Lens becomes rounder (more curved) → higher refractive power
With age, the lens becomes less elastic (presbyopia): near vision becomes worse, reading glasses are needed.
Vitreous Body (Vitreous Humor)
- Transparent, gel-like mass filling the large posterior cavity behind the lens
- Maintains the spherical shape of the eye
- Provides optical clarity and presses retina gently against the choroid
Innermost Layer: Retina
The retina is the light-sensitive layer lining the inner surface of the eye.
General Layering
From outside (toward sclera) to inside (toward vitreous):
- Pigment epithelium
- Darkly pigmented
- Absorbs stray light
- Supports photoreceptors (e.g. recycling of visual pigments)
- Photoreceptor layer:
- Contains rods and cones
- Several layers of interneurons and bipolar cells
- Ganglion cells
- Their axons form the optic nerve
Note: Light passes through neuron layers before reaching photoreceptors (inverted retina).
Photoreceptor Types: Rods and Cones
- Rods
- Very light-sensitive: enable scotopic (night) vision
- Do not distinguish colors: image is black–white–gray
- Higher density in the peripheral retina
- Good for detecting movement and shapes in low light
- Cones
- Less light-sensitive
- Responsible for photopic (daylight) vision and color vision
- Provide high spatial resolution (sharp vision)
- Concentrated in the fovea centralis
Fovea and Blind Spot
- Fovea centralis (yellow spot):
- Small depression in the retina near the visual axis
- Contains almost exclusively cones with minimal overlapping connections
- Area of sharpest vision and highest color discrimination
- Blind spot (optic disc):
- Point where axons of ganglion cells leave the eye as the optic nerve
- No photoreceptors present → no image can be formed here
- Normally not perceived as a “hole” because the brain fills in missing information using surrounding context and the second eye
Visual Pigments and Phototransduction (Principle Only)
Visual Pigments
- Photoreceptors contain light-sensitive visual pigments
- All visual pigments consist of:
- A protein (opsin)
- A light-sensitive molecule (usually retinal, derived from vitamin A)
Rods contain rhodopsin, cones have three types of opsins with different peak sensitivities (short, medium, long wavelengths).
From Light to Nerve Signal (Qualitative Principle)
In simplified form:
- Absorption of a photon changes the structure of retinal in the pigment.
- This triggers a cascade in the photoreceptor cell.
- The cell’s membrane potential changes and modulates neurotransmitter release.
- Bipolar and ganglion cells convert graded signals into action potentials.
- These impulses travel through the optic nerve to the brain.
Details of action potentials and synaptic transmission belong to the chapters on excitability and nervous systems.
Image Formation and Optical Defects
Refraction and Sharp Imaging
The eye forms a reduced, upside-down image on the retina. Focusing is achieved by the combined refractive power of:
- Cornea (largest contribution)
- Lens (variable contribution via accommodation)
For a sharp image:
- Light from each point in the environment must be focused on a corresponding point in the retina.
- Deviations in eye shape or refractive power cause refractive errors.
Common Refractive Errors (Principle)
Myopia (Nearsightedness)
- Eyeball too long or total refractive power too strong
- Light from distant objects focuses in front of the retina
- Symptoms: distant objects appear blurry, near objects sharp
- Correction: diverging (concave) lenses in glasses or contact lenses
Hyperopia (Farsightedness)
- Eyeball too short or total refractive power too weak
- Light from near objects focuses behind the retina (if unaccommodated)
- Symptoms: near objects appear blurry, distant objects relatively sharp (especially in young people who can accommodate)
- Correction: converging (convex) lenses
Astigmatism (Corneal Irregularity)
- Cornea not evenly curved (e.g. more curved in one direction than another)
- Image lines in certain directions are blurred or distorted
- Correction: cylindrical lenses
Light and Dark Adaptation
Eyes must function across a huge range of light intensities. Adaptation occurs via:
Fast Mechanism: Pupil Adjustment
- Bright light → pupil constriction (miosis)
- Dim light → pupil dilation (mydriasis)
- Acts within seconds, but only moderately changes total light input
Slow Mechanisms: Photoreceptor and Neural Adaptation
- In darkness:
- Rod pigments regenerate → sensitivity increases
- More convergence of signals from rods to ganglion cells → better light sensitivity but lower spatial resolution
- In strong light:
- Photopigments partially “bleach” → reduced sensitivity
- Cones dominate → high resolution and color vision
Full dark adaptation can take around 20–30 minutes in humans.
Color Vision
Principle of Trichromatic Vision in Humans
Humans normally have three types of cones:
- S-cones: sensitive to short wavelengths (blue)
- M-cones: sensitive to medium wavelengths (green)
- L-cones: sensitive to long wavelengths (red)
Perceived color depends on the relative activation of these cone types. Examples:
- Pure blue: mainly S-cones active
- Yellow: strong activation of M- and L-cones, little S-cone activity
- White: strong, roughly equal activation of all three cone types
- Black: very low activation of all cones
Color Vision Deficiencies (Principle)
Color vision disorders often result from missing or altered cone types:
- Red–green color blindness (most common):
- Affects M or L cones
- Often X-linked: more common in males
- Complete color blindness (achromatopsia):
- Very rare; mainly rod vision, poor visual acuity, photophobia
Visual Pathways and Simple Processing Principles
(Advanced neural processing is dealt with in the chapters on nervous systems and information processing; here only a brief overview.)
Path from Eye to Brain
- Ganglion cell axons form the optic nerve of each eye.
- At the optic chiasm, fibers from the nasal half of each retina cross to the opposite side.
- After the chiasm, fibers continue as optic tracts to brain regions including:
- Lateral geniculate nucleus (thalamus)
- Superior colliculus (midbrain) for movement and orientation reflexes
- From the thalamus, signals are relayed to the primary visual cortex in the occipital lobe.
Result: Each brain hemisphere primarily processes information from the contralateral (opposite) half of the visual field of both eyes.
Binocular Vision and Depth Perception
- In animals with frontally placed eyes (e.g. humans, many predators), visual fields of both eyes overlap strongly.
- The brain compares slightly different images from both eyes:
- This binocular disparity enables stereoscopic depth perception (3D perception).
- In animals with laterally positioned eyes (e.g. many prey species), overlap is smaller:
- Wide field of view (better for detecting predators)
- Less precise depth perception directly in front
Besides stereopsis, the brain also uses:
- Accommodation
- Convergence (eye muscle signals)
- Perspective, size, shadows, and motion parallax
to assess distances.
Variability of Eyes in the Animal Kingdom (Overview)
Camera-Type Eyes in Vertebrates and Cephalopods
- Vertebrate camera eye:
- Inverted retina
- Blind spot at optic nerve exit
- Cephalopod (e.g. squid) eye:
- Also camera-type, but:
- Photoreceptors located in front of nerves → no inversion
- No blind spot
- Example of convergent evolution: similar solution to similar task, but independent origin
Compound Eyes in Arthropods
- Found in insects, crustaceans, many other arthropods
- Composed of many small functional units (ommatidia), each with its own lens and photoreceptors
- Features:
- Very large field of view
- Excellent motion detection
- Typically lower spatial resolution than vertebrate camera eyes (but some insects have specialized regions with higher acuity)
Simple Light Sensitivity
- Many animals and larvae have only simple eyespots:
- Can distinguish light from dark and direction of light
- Sufficient for basic orientation (e.g. moving away from harmful light)
Summary
- The visual sense is based on the detection of light by photoreceptors and the formation of a focused image on the retina.
- The vertebrate eye is a camera-type eye with cornea, lens, iris, and retina; rods and cones are specialized for low-light vision and color/daylight vision, respectively.
- Accommodation and pupil reactions adjust the optical and light conditions; adaptation allows functioning over a wide range of brightness.
- Color vision in humans relies on three cone types and their relative activation.
- Neural pathways from the retina to the brain and binocular viewing enable image interpretation and depth perception.
- Across animals, eyes show great diversity (from light-sensitive spots to compound eyes), reflecting different ecological demands.