Human Eye

Neural Pathways

Transform and transmit those messages from the eye to the brain

Brain

Visual centers process and interpret the messages, especially to guide behavior

Compound Eye

Work very differently from our eyes, possess a single optical system

Light Sensitive Spots (Animals)

Tells their owners whether it’s day or night. Very different from humans eyes, allows creatures to sense and respond to light presence or absence

Precursors for variety of eyes

Which mammals have light sensitive spots

Worm, hydra, & more advanced animals

Arguments of evolution of the Eye

Complex structure of our eyes led some people to conclude that is simply couldn’t have arisen by natural selection.

Argue: Hand in God creation

Others: Plausible scenarios evolution of eyes, natural selection intertwined with chance mutations

Electromagnetic Spectrum

1. Light travels fast in straight lines, letting us register distant events almost instantly and with geometry preserved

2. Sun floods our world with visible light, ensuring a reliable supply of

3. Light interacts with object surfaces in ways that reveals their structure- reflection and absorption carry info and shape texture and color

Electromagnetic Spectrum Range

Between Ultraviolet and Infrared

Why Should Eyes Use Light

Vision depends on light because it’s abundant, well-behaved and informative

Electromagnetic Radiation Travels

Any creature that can detect such radiation can pick up information from distant sources with essentially no lag

Electromagnetic Radiation Traveling in Straight Lines

Images created by this radiation preserve important geometrical characteristics of the objects that reflected or emitted that radiation toward the eyes

Frequency of Electromagentic Radiation

Depends on oscillation frequency of the material that emits the radiation. Electromagnetic radiation can be arranged along continuum, a spectrum according to oscillation frequency.

Wavelength

(Light energy freq or oscillation rate can be converted in units of wavelength)

Defined by how far the radiation travels between successive oscillation. High oscillation rates mean radiation travels a very short distance between oscillations hence short wavelength

Why do the eyes rely solely on this one very narrow portion of the entire electromagnetic spectrum, the part of we call tonight?

Eyes use light as medium. Abundance ensures poppy to exploit any light sensing apparatus they have.

Would Elctromagentic Spectrum work for early vertebrate eyes?

Most of sunlight’s energy in this short wavelength region of the spectrum is absorbed by molecules in earth’s atmosphere

How is Light Useful

A source of information about the world because light interacts with the surface molecules of many objects useful for us to sense

Inverse Optics

Light delivers information to the eye, but the real target is the object not the light itself. Because surface properties, distance and illumination vary, the same object casts many different patterns on the retina. Ambiguity means the eye and brain must work back ward from patterns of light to infer The object

Backward (For Inverse Optics)

Because surface properties, distance and illumination vary, the same object casts many different patterns on the retina. Ambiguity means the eye and brain must work back ward from patterns of light to infer

Difference Eyes Have their own advantages

Various eye placement schemes. Each strategy carries its advantage: frontal eyes improve depth perception, but frontal eyes make it possible to take in more of visual world at one time

Where should Eyes go?

Those needing excellent depth perception to stalk and capture have considerable binocular overlap of the two visual fields. Those. Needing a more panoramic view of the environment to watch predators have little binocular overlap

Why Should The Eye be Able to Move?

Humans lack panoramic vision, what we see at any given moment is limited. There is no guarantee your eye will always be direct toward things in environment that you need to see

Strategies Animals Use to See

Can love their bodies, heads or eyes.

(Ex. Owls can only rely on their head [turn 90 degrees] to see since they cannot move their eyes)

How the Human Eyes Move

Every muscle in your body works by contracting and thereby pulling on the structure or structures to which the muscle is attached.

Mach Theory (Truth)

We’re accustomed to moving our eyes and heads, we seldom realize how limited our actual view of the world is at any given moment

How can Eyes Be Protected? (Orbit)

Partially protected by its location within orbit. Within the orbit each eyeball is surrounded by a substantial fat cushion. Without it blows to the head would be directed to the eye

How Can Eyes Be Protected? (Eyelids)

Movable folds tissue. Position of upper lid relative to lower one determines the opening through which the front of the eye is visible

Protecting the Eye Lids Blinks

Cleans and moisten the front of the eye to keep it from drying out. Also excite reflexive blinks that protect the eye when it is touched by a foreign object when an object is on a collision course with the head

Sclera

Tightly packed, interwoven protein fibers that are aligned parallel to surface. Needs to be tough because pressure inside the eyeball is double that of the atmosphere

Cornea

The transparency of this is crucial for vision and formation of images on the back of the eye. Allows light to enter the eyeball unimpeded.

Middle Vascular Coat

Photoreceptors that turn light into neural signals. W/o their nourishing blood supply, vital photoreceptor cells would die from lack of oxygen

Amber Chamber

Toward front of eye. Aqueous humor serves several essential maintenance functions: transports oxygen and nutrients.

Glaucoma

Increased intraocular pressure. The single most common cause of binding eye disease in N.A. can be detected if it’s early enough

Aqueous Humor

Helps maintain the shape of eyeball. If too little fluid is present in anterior chamber the eye may become deformed. Excess aqueous accumulates either because of overproduction or improper drainage out of anger chamber

Iris

Circular section of tissue that gives your eyes its characteristic color. Two layers an outer layer containing pigment and an inner layer containing blood vessels. If the outer layer is heavily pigmented the iris will appear brown. If outer layer light pigment inter layer will be kinda visible out one will look more lightly colored

Pupil

Opening between two sets of muscles. When circular band of muscles contracts, the pupil gets smaller. Changes in pupil size regular amount of light reaching the back of the eye. Pupil size varies in response events stimulating the automatic nervous system. Larger pupils more light, smaller pupils less.

Depth of Field

The range of distance over which objects will appear sharply focused varies in rekey with the size of the pupil. Sharp range of vision

(Crystalline) Lens

Right behind Iris, 3 distinct parts: elastic covering, capsule, epithelial layer. Moderates flow aqueous humor into the lens helping lens retain transparency to light. Elastic Capsule molds shape of lens - varies curvature, lens optical power. Clear seeing = must be transparent, readily with minimal loss.

Cataract

Lens opacity (or reduced transparency). Minor barely reducing light transmission others undermine vision to the point of blindness.

Vitreous Chamber

Larger of 2 chambers is bounded by lens in front and by retina on the sides in the rear. Filled with transparent fluid, gel-like substance consistency of egg white. Adheres to inner wall of eyeball.

Retina

Resembles very thin fragile meshwork. Light had to pass through a whole network of neurons before reaching photoreceptors the cells responsible for converting light energy into neural signals. Direct extensions of central nervous system

Neural Signals (Retina)

Generated by photoreceptors. Turn pass through a network of cells - bipolar machine and horizontal cells - that collect and recombine the photo receptor

Philosopher Hermann von Helmholtz realized..

It’s possible to illuminate eye while at the same time looking into it

Ophthalmoscope

Allows us to visualize the interior of the living human eye, including its retina and blood supply

Macula

looking directly at objects, such as words on page on this page. Images of those objects are centered within this of each eye

Optic Disc

Located toward left hand. Eye region where optic nerve fibers exit the retina carrying information to the brain. Blood vessels within resting enter and leave the globe at the optic disc. Normally, has lovely pinkish color because of small blood vessels on its surface which nourish part of optic nerve

What sorts of things might interrupt the retina’s blood supply?

Feeder arteries can become clogged, blocking the flow of blood to and within the inner retina. Happens in arteriosclerosis (hardening of arteries) and sometimes sickle cell disease

Pigment Epithelium

Which forms a barrier through which chordal blood must pass to nourish the outer segments of photoreceptors. Transfers oxygen, nourishment and vitamin A from choroidal circulation. Also responsible for disposing of metabolic waste products, molecular garbage shed by receptors taken up and recyclers

Diabetes

Common disease that can affect retina’s blood supply. Disease is marked by disordered insulin metabolism that causes too much sugar to acclimate in the diabetic’s blood

Retinal Image

The raw material on which the retina operates and high quality vision is impossible without high quality raw material

What determines the fidelity of retinal image?

Image quality is governed by the interactions between patterns of light arriving at the eye and various ocular structures that influence the passage of that light to the back of the eye, where photoreceptors are located

Emitted Light

Light originates from a source such as the sun or a light bulb. This how light gets information to convey.

Optical Structure

Conveys potential information about those objects and composition of their surfaces

Sharpness Depends on two factors…

Crystalline Lens and Optical Power (Other factor is the size of the eyeball, particularly the eyeball’s length from front to back)

Optical Power

ability to bend. (Or refract light). Not fixed, but changing its shape, the crystalline lens changes its optical power hence overall focus of eye

Accommodation

Helps you see objects clearly, regardless of their distance from your eye

Divergent Light

Stone corresponds to our point of light and radiating out from that point is a set of spherical wave fronts. Light that spreads out in this way. Cannot form a well focused image a point sunless something is done to reverse its divergence

How powerful should those optics be to produce a sharp retinal image of that distinct object?

Optics power must match eyeball’s length - specifically. The distance from the lens’s to the retina

Hyperopia

Can work around accommodating increasing the eye’s optic power. Enables the Hyperion to produce focused images of objects, provided the eye is not too hyperopic and the objects are not too close.

Myopia

An object at optical infinity would be sharply focused somewhere in front of the retina not at the plane of the retina. Means you cannot see distant objects in sharp focus without glasses or contact lenses. Reveals considerable genetic influence on development of myopia

Why sort of corrective lens would a myopic eye need?

Could correct the myopic eye by adding a concave lens (which causes light to diverge combating myopic eyes tendency to make light converge too much)

Presbyopia

Old sight, old lens is sluggish in executing even the small shape changes that are still within it’s capabilities

Astigmatism

Caused when cornea is more strongly curved along one axis than another. These differences curvature mean that cornea cannot sharply focused 2 different line orientations well simultaneously

LASIK Surgery

Procedure involved re sculpting the cornea by shaving thin layers from it, modifying the corneas shape and the way it bends light

Photoreceptors

Two types: Rods (100-120 mil) & Cones (5-7 mil)

The spatial distribution of activity representing object

Geography of Rods & Cones

Cones: Predominate in central vision

Rods: Predominate in peripheral vision

Plots density of rods and cones in samples taken from different parts of the retina, center of macula. Gap corresponds to complete absence of photoreceptors at the optic disk, part of retina where nerve fibers exit eye

Photo receptors in each eye convert…

The optical image on the retina into a neural image that will be transmitted to the brain. However, the density of photoreceptors varies regionally throughout the retina

Albinism

Skin, hair and eyes lack pigmentation, have a genetic disorder

Melanin

Particularly abundant in the macular region of normal retina

Light Signals

Presence in the retina by interacting with special light sensitive molecules contained within photoreceptors

Absence of light

Electrical current flows into the stack of discs within the photoreceptors outer layer

Presence of Light

Through its effect on the photopigments and the electrical current flow / reduced the rate of glutamate released into the synapse

Rods

More sensitive than cones. Fixed amount of light can produce a much larger electrical response in rods than cones. Require less light than cones to produce reliable neural signals

Cones

Receive enough light to produce phototransduction, continue functions even when illumination level is raised to extremely high levels. Operate within a very narrow range of light level and one the upper limit of that range is exceeded

An Engineering Approach: Purpose-First Understanding

To gather light, focus it, transduce it into neural signals to guide behavior. Those purposes constrain it’s construction

Possible Tasks of Light-Sensing Systems e.g. Vertebrate Eyes

• Signal Presence v. Absence of light

• Convey intensity of light

• Convey direction of light

• Convey wavelength info

• Convey contrast info

• Produce high resolution images of a scene

Light & Evolution

• All eyes are sensitive to a very narrow portion of the electromagnetic spectrum

• Early evolution occurred in water so eyes rely on biochemical mechanism (Opsin &+ chromosphere) sensitive to wavelengths that can penetrate water

Cornea

• Clear structure at eye’s front

• Key element in eyes optics

• It’s optical properties closely match those of water

• Underwater your eye effectively lacks a cornea

Characteristics of an Image

An image contains spatial information about the light emitted by/reflected from a surface: The spatial distribution of photons (light) from the surface

Neural Connectivity

Arrays of neural signals represent the object; convergence within array loses information

Emmetropia (“well proportioned eye”)

I the state of an eye in which a faraway object is in sharp focus (a high quality image on retina) with the lens in a neural (relaxed) state, with no accommodation

Ametropia (“badly proportioned eye”) - different types

Astigmatism, Hyperopia, Myopia & Presbyopia

Accommodation & Pseudomyopia

• Crystaline lens sits in a elastic capsule, held under sneaking by sensory ligaments

• When ciliary muscles relaxes, the ligaments tension flattens the lens→ aids focus at distance

• When the ciliary muscle contracts, ligament tension is released the fattens → aids focus at near

• Too much near work can temporarily lock the muscle in contraction

Principle 1: Isolate Incompatible Functions in 3 layers

• Strength v. clarity

• Blood supply v. Clarity of Optical Path

• Dynamic Change Turnover v. Static Enclosure/Fixed Form

Principle 2: Protect the Most Vulnerable

Nourish & protect the retina

Principle 3: Metabolic Effciency

Don’t just see; see effiecently Every feature of the retina helps to conserve energy

Protection implies convering

Covering implies thicknesss, some opacity

Light sensitivity implies transparency

Conflicts with protection

Nutrition depends on a fairly opaque substance (blood)

Conflicts with light sensitivity

Outer Layer

= ~ 1mm thick

Function:,Protection/Optical Power

Balances toughness & transparency

Specialized as sclera (white) & cornea (transparent)

Middle Layer (Incl. Chorid)

=~ 0.2 mm thick

Functions: Nutrition, structural support, garbage disposal

Specialized into iris, vascular regions

Inner Layer (Mainly Retina)

=~0.5mm

Fucnctions: Sensitivity to light, transforms photon capture into neural signals that are sent to the brain

Specialized into ROS sensorineural retina

Cataract reduces Transperacy

Accumulation of water &/or denaturing of lens protein

Causes: Diabetes, ocular tumor, age related changes

Symptoms: progressive loss in quality of vision, reduced retinal illuminance, scattering of light

Photopigment

Light sensitive molecule

Type in humans: rhodopsin (rods) photospins (cones), melanospin (iPRGCs)

Light acts in molecule & changes its shape

How does photopigment & photoreceptors react?

Photopigment absorbs photons, changes the amount of neurotransmitter output from the photoreceptor

Retina Main Tasks

Detective & report the distribution of photons - the distribution over space & time

Detect Light

Capture light

Report the Spatial distribution of photons

Transduce the captured light into neural signals in a way that preserves the spatial distribution of light into the retina

How to fuel the outer retina?

Photoreceptors are far from the central retinal arety

How to avoid wasting energy?

Processing every “pixel” at maximum resolution is inefficient (not needed)