Perception of stimuli
Option E.2
Assessment statements
• E.2.1 Outline the diversity of stimuli that can be
detected by human sensory receptors, including
mechanoreceptors, chemoreceptors, thermoreceptors
and photoreceptors.
• E.2.2 Label a diagram of the structure of the human
eye.
• E.2.3 Annotate a diagram of the retina to show the cell
types and the direction in which light moves.
• E.2.4 Compare rod and cone cells.
• E.2.5 Explain the processing of visual stimuli, including
edge enhancement and contralateral processing.
• E.2.6 Label a diagram of the ear.
• E.2.7 Explain how sound is perceived by the ear,
including the roles of the eardrum, bones of the
middle ear, oval and round windows, and the hair cells
of the cochlea.
Sensory receptors and diversity of stimuli
• We have learned to link certain tastes,
sights and sounds with emotions
• Sensory cells send messages to certain
parts of the brain that control emotion
and memory
• Nerve impulses arriving at the brain
results in sensation
Mechanoreceptors
• Stimulated by mechanical force or
some type of pressure
• Sense of touch is due to pressure
receptors
• Pressure receptors are also found in:
– Arteries (detect change in blood pressure)
– Lungs (stretch receptors respond to the
degree of lung inflation)
– Arms and legs (proprioceptors which tell
us position and help maintain posture)
– Inner ear (pressure receptors sensitive to
waves of fluid moving over them)
Chemoreceptors
•
•
•
•
Respond to chemical substances
Allows us to taste and smell
Found in blood vessels and monitor pH
Pain receptors respond to chemicals
released by damaged tissues
Thermoreceptors
• Respond to change in temperature
Photoreceptors
• Respond to light energy
• Found in our eyes
• Rod cells respond to dim light resulting
in black and white vision
• Cone cells respond to bright light and
give us color vision
TOK
• To what extent are we dependent on
technology for our knowledge of
biology?
• http://physics.ucsd.edu/neurophysics
/publications/New_Scientist.pdf
Structure of the human eye
Posterior chamber
pupil
fovea
Suspensory
ligament
Ciliary muscle
Functions of parts of the eye
Part
Function
Iris
Regulates the size of the pupil
Pupil
Admits light
Retina
Contains receptors for vision
Aqueous humour
Transmits light rays and supports the eyeball
Vitreous humour
Transmits light rays and supports the eyeball
Rods
Allow black and white vision in dim light
Cones
Allow color vision in bright light
Fovea
An area of densely packed cone cells where vision is most acute
Lens
Focuses the light rays
Sclera
Protects and supports the eyeball
Cornea
Focusing begins here
Choroid
Absorbs stray light
Conjunctiva
Covers the sclera and cornea and keeps eye moist
Optic nerve
Transmits impulses to the brain
Eye lid
Protects the eye
Cow eye dissection
• http://www.exploratorium.edu/learni
ng_studio/cow_eye/
The retina
• Vision begins when light
enters the eye and is
focused on the
photoreceptor cells of
the retina
• Both rods and cones
synapse with their own
bipolar neurons
• Each bipolar neuron
synapses with a
ganglion cell
• Axons of the ganglion
cell make up the optic
nerve which carries the
message of vision to the
brain
Annotation of retina diagram
•
•
•
•
Rods are photoreceptor cells
which are sensitive to light
and function well in dim light;
synapse with a bipolar neuron
Cones are photoreceptor cells
which are activated by bright
light; synapse with a bipolar
neuron
Bipolar neurons are cells in
the retina which carry
impulses from a rod or a cone
to a ganglion cell of the optic
nerve; called bipolar b/c they
each have two processes
extending from the cell body
Ganglion cells are the cell
bodies of the optic nerve;
synapse with the bipolar
neurons and send the
impulses to the brain
Comparison of rods and cones
Rods
Cones
Cells are more sensitive to light
and function well in dim light
Cells are less sensitive to light and
function well in bright light
Only one type of rod is found in
the retina. It can absorb all
wavelengths of visible light.
Three types of cones are found in
the retina. One type is sensitive to
red light, one type to blue light and
one type to green light.
The impulses from a group of rod
cells pass to a single nerve fiber in
the optic nerve.
The impulse from a single cone
cells passes to a single nerve fiber
in the optic nerve.
Color fun:
http://www.colourtherapyhealing.com/colour/colour_fun/
Processing visual stimuli
• When we look at an object, light rays
pass through the pupil and are focused
by the cornea, lens and the humours
• Image focused on the retina is upside
down and reversed from left to right
• The brain must correct the position of
the image so that it is right side up and
not reversed
• It must also coordinate the images
coming from the left and right eye
Edge enhancement
• Complex
structure of
vision is exposed
by studying
illusions
• Why do you see
some grey blobs in
the white area
between the black
squares?
• Theory is that the
areas where you see
grey are in your
peripheral vision,
where there are
fewer light-sensitive
cells than in the
center of your retina
• It demonstrates that you have a
special mechanism for seeing edges –
it is called edge enhancement
• Theorized that light-sensitive
receptors in your eye switch off their
neighboring receptors making the
edges look more distinct
Contralateral processing
• Opposite side processing is due to the
optic chiasma
• Nerve fibers bringing information from
the right half of each visual field
converge at the optic chiasma and pass
to the left side of the brain
• Nerve fibers bringing information from
the left half of each visual field converge
at the optic chiasma and pass to the
right half of the brain
• Information eventually ends in the visual
cortex of the brain and information is
shared to form a complete visual image
• Image received by visual cortex is both
inverted and reversed
• Brain must correct in the cerebral cortex
• Process of two sides of the brain working
together can be illustrated by the abnormal
perceptions of patients with brain lesions
• Patients with right side brain lesions, when
looking at an object from above, does not
recognize that object and will deny what it is
• Patients with left brain lesions can describe
the function of the object but cannot come
up with the name of the object
• It takes both sides of the brain working
together to have correct “vision” which is
able to recognize an object and understand
what it is
Vision 2020
• Joint initiative of the World Health
Organization and the International
Agency for the Prevention of Blindness
whose goal it is to eliminate avoidable
blindness worldwide by 2020
• Aim is to give every one in the world
the right to sight
Videos
• How the eye works
• Laser surgery
Structure of the ear
pinna
Oval
window
Round
window
How sound is perceived by the ear
• Outer ear catches sound waves
(vibrations of air molecules)
• Sound waves travel down auditory/ear
canal and cause the tympanic
membrane (eardrum) to move back
and forth slightly
• Bones of the ear – malleus, incus, and
stapes – receive vibrations from the
tympanic membrane and multiply
them approximately 20 times
• Stapes strikes the oval window
causing it to vibrate
• Vibration is passed to the fluid of the
cochlea which causes hair cells within
the cochlea to vibrate
• These receptors release a chemical
message across a synapse to the
sensory neuron of the auditory nerve
and is carried to the brain
• The wave in the fluid of the cochlea
dissapates as it reaches the round
window
• Loud noises cause the fluid to vibrate
to a higher degree and the hair cells
bend even more
• Brain interprets this as higher volume
• Pitch is a function of sound wave
frequency
– Short, high-frequency waves produce
high-pitched sound
– Long, low-frequency waves produce lowpitched sound
• Sound which is sensed by the brain is
processes in the auditory area of the
cerbral cortex
Videos
• Sense of hearing
• Cochlear implant