HOW HEARING WORKS
A BRIEF OVERVIEW
WHAT IS SOUND?
• If a tree falls in the woods and
there is nobody, or nothing to hear
it, does it make a sound?
• The answer is no- but that is only
because sound is an interpretation
of vibrations.
• If there is nothing to interpret those
vibrations, there cannot be sound.
WHAT IS HEARING?
• When a vibration is made, the brain
can interpret that as being a sound.
• It must have an amplitude (loudness)
large enough to cause the mechanisms
in the ear to react
• It must also be within the frequencies
(pitch) of sound that our ears and
brains can react to and interpret
HEARING PHYSIOLOGY
• There are several parts of the ear and the
brain that are involved in hearing.
• The outer ear (pinna)
• Dish shaped to help gather sounds and funnel
them into the ear canal
• The curves and ridges in your ear allow you to
determine the direction sound is coming from
• The ear drum (tympanic membrane)
• A thin membrane of tissue that vibrates,
sending the vibrations further into the ear
HEARING PHYSIOLOGY
• The Middle Ear (ossicles)
• Comprised of three separate bones
(the smallest bones in the human
body!) that receive the vibrations from
the eardrum and amplify the signal
• The eardrum vibrates and causes the
malleus (hammer) to hit the incus (anvil).
That causes the stapes (stirrup) to move in
and out, transferring the vibrations further
into the ear.
HEARING PHYSIOLOGY
• The Cochlea
• Vibrations from the stapes cause the
vibrations to further amplify inside this
pressurized, fluid-filled tube.
• Thousands of specially tuned fibers
are activated by resonant
frequencies and pass the information
on.
HEARING PHYSIOLOGY
• Inner Ear
• The organ of corti contains hair cells that are
activated when the fibers in the cochlea
are activated by resonant frequencies.
• The hair cells send electrical impulses to the
cochlear nerve, and into the brain.
• In the brain, the electrical impulses are
translated into sounds- pitch is determined
by what hair cells are activated, and
volume is determined by how many cells
are activated.
VISION
A BRIEF OVERVIEW
ANATOMY OF THE EYE
• The sclera
• Rigid top layer of the eye (the white part)
• Responsible for keeping the eye’s shape
• The cornea is part of the sclera, and is the
top lens responsible for focusing light
• The extraocular muscles
• Attached to the sclera and responsible for
the eye’s movement
ANATOMY OF THE EYE
ANATOMY OF THE EYE
• The choroid
• Second layer of the eye, containing the
vessels that supply the eye with blood.
• Also contains the iris, which is the diaphragm
that gets larger or smaller to let more or less
light in to the pupil.
• Contains the ciliary body, which is the
muscles that are attached to the lens.
• The ciliary body will contact or relax to
control the size of the lens.
ANATOMY OF THE EYE
ANATOMY OF THE EYE
• The retina
• The inner area of the eye responsible for sensing
light.
• Rod cells
• Senses grayscale and low-light environments
• Cone cells
• Senses color
• Only have 3 different cones in the human eye. Senses red,
blue and green light.
• Macula
• Only contains cone cells and is responsible for seeing fine
detail.
ANATOMY OF THE EYE
ANATOMY OF THE EYE
• Rhodopsin, a chemical that converts light
into an electrical signal, is contained in the
retina.
• A mucous membrane covers the inside of
the eyelid and the top of the sclera. This
membrane is called conjunctiva.
• The eye is housed in the orbital cavity in the
skull. The eye is held in the socket by the
muscles that are also used for movement
of the eye.
PHYSIOLOGY OF THE EYE
• When light enters the eye, it passes
through the cornea, lens, two layers of
viscous liquids and finally reaches the
retina.
• The cornea, lens and layers of liquid are
incrementally focusing and fine-tuning the
image entering our eye.
• The retina is the light sensing part of the
eye, and contains 2 types of
photosensitive (light sensing) cells- Rods
and Cones.
PHYSIOLOGY OF THE EYE
• Rods sense the light and dark
• We have 2 types of rod cells
• Shadows, gray scale, shading, perception of
depth are all results of the rods
• Cones sense the color
• We have 3 different cone cells
• Senses Red, Green and Blue light
• Deficiencies in the cones or their function
can result in differing degrees of
colorblindness
PHYSIOLOGY OF THE EYE
• Rods contain rhodopsin at the tips, which
allow them to sense the different intensities of
light
• Rhodopsin is made largely of a form of vitamin A
• Rods are responsible for our ability to see in the dark
• Cones contain color pigments, which allow
them to sense color
• The inside of the retina is lined with a black
substance called melanin
• It is used to lessen reflections, much like how a
camera is lined in black for the same purpose
PHYSIOLOGY OF THE EYE
• The central area of the retina is called the
macula, which contains only cones. This area
is responsible for sharp, contrasting details in
color.
• When light hits the rhodopsin, it causes a series
of physical and chemical changes. We call
this “bleaching.”
• This is why sometimes your vision is dark when you go
from a dark place to a light place quickly. It takes
about a second for all of these reactions to take
place.
PHYSIOLOGY OF THE EYE
• The cones of the eye sense color
• When light hits the cones, they vibrate with the frequency of
light
• Different cones have different sensitivities to light, and will
activate depending on the color of light entering the eye
PHYSIOLOGY OF THE EYE
• When the rods and cones are activated, they send
an electrical signal to the sensory cortex in the brain
• Inside the sensory cortex, the signal is translated into an
image
• We have gotten pretty good at reading brain waves and
patterns, so good that we can read a brain wave and
nearly reconstruct the image a person is looking at!
DISEASES OF THE EYE
• Macular degeneration
• The slow breaking down of the macula. Causes blindness
• Glaucoma
• The inability for your eye to drain fluid. Causes pressure to build
in the eye balls.
• Conjunctivitis
• An bacterial infection of the conjunctiva (mucous membrane)
of the eye
• Colorblindness
• A genetic deficiency, located on the X chromosome, that
causes one or more cones to be either absent or not function
WHAT NUMBER DO YOU SEE?
WHAT NUMBER DO YOU SEE?
WHAT NUMBER DO YOU SEE?