EYE AND ITS VISION-BASIC CLINICAL SCIENCES

Both Light and Sound Have Oscillating Waves

• Sound and light are waves in which the amplitude oscillates with time
  • For pure tones or colors the oscillation is a sine wave with a fixed wavelength and frequency
  • The wavelength is the distance between 2 peaks of the wave
  • Frequency is the number of waves per second
  • Wavelength and frequency are related:
  • Frequency X Wavelength = Velocity of Wave

    

• Wavelength is associated with the pitch of a sound or the color of light
• Amplitude is related to the loudness of sound or the brightness of light
• Note thate light is almost a million times faster than sound and can travel through a vacuum. (Remember that light travels through the vacuum of space from the sun to earth).
• Even though it is slower sound has some advantages as a signal: we can use it in the dark for one. Also sound passes quite well through liquids and solids even when they are opaque.
• Both are eyes and ears are exquisitely sensitive. The eye can detect a single packet (photon) of light (the smallest possible unit)! The ear can detect a vibration the fraction of a diameter of a hydrogen atom!

Light Waves Have Wavelengths From 400 to 700 Nanometers

• Light is a form of electromagnetic waves– travels
• through space
• Each color is associated with a different wavelength
  • Violet colors have wavelengths around 400 nanometers (nm)
  • Yellow colors are close to 580 nm
  • Red colors have wavelengths around 700 nm
• White light is a mixture of the colors of the rainbow; a prism splits white light into the various colors:

  

Basic Eye Anatomy

This diagram is modified from one in The Sourcebook of Medical Illustration, edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).

The Eye Has 3 Essential Parts

• Optical apparatus for collecting and focusing light (also contols for adjusting amount of light entering the eye)
• Light sensitive detector system (for both light & dark and color)
• Nerve pathways routing signals to the brain for interpretation

The Iris Controls the Amount of Light Entering the Eye

• The iris constricts and dilates to cotrol the amount of light entering the eye (pupillary light reflex)
• The pupil is the hole in the center of the iris
• The iris has 2 rings of muscles: sphincter (constrict pupil) and dilator (open pupil) muscles
• The iris sphincter is controlled by the oculomotor nerve (cranial nerve III: parasympathetic)
• The dilator muscle is controlled by sympathetic nerves

Light is Focused by the Cornea and Lens

• Most of the focusing is done by the cornea which is a transparent section of the sclera
• Cornea is non-adjustable (fixed focus)
• Lens (very elastic) gives fine adjustment to focus
• Lenses give inverted images

To Focus Light the Lens Must Change Shape

• Camera lens focuses by moving closer to or farther away from film
• Eye lens focuses by changing shape
• Lenses bend or refract light waves
• The more curved the lens surface the more bending (refraction)
• The lens is held in place by suspensory ligaments
• Contraction of ciliary muscle relaxes ligaments -> lens curvature increases ( focuses for near vision)
• Relaxation of ciliary muscle -> lens flattens, less curvature (focuses for far vision)
• Ciliary muscle is also controlled by the oculomotor nerve (parasympathetic response is focus for near vision)

Most of Us Have Focusing Problems During Our Lifetime

• Farsightedness (hyperopia): eyeball too short -> near focus poor
• Nearsightedness (myopia): eyeball too long -> far focus poor
• Presbyopia: lens becomes stiff with age, cannot change shape

Six Muscles Turn the Eyeball:

This diagram is modified from one in The Sourcebook of Medical Illustration, edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).

In Near Vision the Eye Accomodates

• Focus for near vision
• Pupil constriction- only center of lens used -> better depth focus
• Convergence- keeps both eyes focused on the same object

The Retina Has 2 Types of Light Detectors: Rods & Cones

• Rods detect light and dark. About 120 million in eye
• Cones detect colors. About 6 million in eye.
• Both rods and cones have light-sensitive pigments derived from vitamin A (rhodopsin, iodopsin)

Color Vision is Best in the Fovea Centralis

• Fovea centralis is a depression in the retina ; has the greatest density of cones
• Fovea also has the greatest visual acuity (ability to distinguish 2 points as separate points)

Three Types of Cones Account for Color Vision

• There are red, green and blue sensitive cones
• These 3 types can produce sensations of all the colors in the rainbow
• A deficiency in one of the types of cones causes color blindness

Generator Potentials of Rods and Cones Activate the Optic Nerve

• When rods or cones are struck by light they produce a small generator potential
• This is not an action potential- it is not propagated
• If the generator potentials are large enough they will depolarized fibers of the optic nerve (ganglion cells) and produce action potentials that will travel to the brain
• The generator potentials are modified in complicated ways by other cells in the retinal layer

Some Visual Nerve Fibers Cross in the Optic Chiasma

• The optic chiasma is a prominent X-shaped landmark on the ventral side of the brain
• Fibers from the area of retina closest to the nose cross over; fibers from the lateral part of the retina do not cross
• Because of this crossover the right optic tract carries sensation from the left visual field & vice versa

The Occipital Cortex is the Primary Area for Vision

• Optic fibers go to the thalamus (lateral geniculate body); after synapsing they go to the occipital lobe of the cortex
• Light impulses are mapped out in 2 dimensiona on the occipital cortex
• Brain constructs a 3 dimensional image from the sensory data
• Fovea centralis has a much larger area on cortex than nerves from other parts of the retina

Visual Reflexes are Handled in the Superior Colliculus (Midbrain)

• Some of the fibers in the thalamus branch and go to the superior colliculus in the midbrain
• The superior colliculus is involved in light reflexes (pupil reflex)

Visual Pathways of the Brain (Occipital Cortex on Left)

• Label the occipital cortex, lateral geniculate, superior colliculus, optic chiasma, retina and lens. Color the fibers so you can see which ones cross over.

This diagram is modified from one in The Sourcebook of Medical Illustration, edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).

More Information

Mark McCourt of North Dakota State University has an excellent set of internet notes on the physiology and psychology of vision.

John Krantz of Hanover College has an interesting website on Vision in Art.