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Definitions:
The detection of physical energy by our sense
organs, which then relay information to the
brain.
The process by which our sense organs
translate environmental stimulation into
neural impulses to prepare it for processing
by the nervous system.
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 The two basic requirements for all sensation
are a stimulus and a receptor.
◦ Stimulus: The particular form of energy that the
sense organ is designed to detect.
◦ Receptor: A sensory neuron that responds to
specific stimuli such as light waves, sound
waves, or chemicals in substances.
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 To activate a sensory receptor, a stimulus
must be at or above the receptor's absolute
threshold.
 Absolute threshold refers to the minimum
amount of stimulus energy required to
activate a receptor
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 Sensory receptors eventually cease to respond to
stimuli that are continually present. In other
words, they become accustomed to the stimuli
(except when they are too intense).
 Example: after a while you no longer notice the
sensation of pressure on your skin from a watch
or sock you are wearing.
 Sensory adaptation is
essential in allowing us
to focus our attention.
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 JND refers to the smallest change in the intensity
of a stimulus that we can detect.
 The stronger the stimulus, the greater the
change needed for a change in stimulus to be
noticeable.
 Example: Imagine how much
light you would need to add
to a very brightly lit room vs
a dark room to notice a
change in illumination.
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The Power of Vision
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 The stimulus detected by the visual system is
light, a form of electromagnetic energy.
 Humans respond to a narrow range of
wavelengths of light.
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[Insert Figure JK97fig3.3.eye_structure.jpg]
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 Cornea – a curved, transparent layer, covering
the iris and pupil. It bends incoming
light, focusing it at the back of the eye.
 Iris: The circular arrangement of smooth
muscles that contract and expand to control
the size of the pupil.
 Pupil: The opening in the iris that admits light
waves into the eye.
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 Lens: The transparent structure inside the eye
that changes shape to focus images on the
retina, allowing us to fine-tune the visual
image.
 Retina: Tissue at the back of the eyeball;
contains rods and cones which are the
receptors for vision.
◦ Rods – allow us to see in light of low intensity;
enable us to see basic shapes and forms
◦ Cones – allow us to see colour; more sensitive to
detail
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 Optic Nerve: Nerve that travels from the
retina to the brain, conveying visual
information.
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 Light waves strike the cornea and pass
through the pupil to the lens.
 The lens focuses the light waves onto the
retina which contains the light-sensitive
receptors.
 Messages from the receptors are transmitted
to the brain via the optic nerve which exits at
the back of the eyeball.
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 The point at which the optic nerve exits the
eye is a blind spot.
 No light receptors are located at this point so
if an image is focused on this region, it will
not be seen.
 Try the activity on the next slide to find your
blind spot!
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 Make a small dot on the left side of a piece of
paper and a small + on the right side (as
shown below), with about 6-8 inches between
them.
 Hold the paper about 20 inches away, close
your right eye and stare at the +. Slowly move
the paper closer, all the time focusing on the
+.
 After a while, the dot will disappear – this is
when the dot falls on the blind spot in your
left eye.
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The Power of Hearing
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 The stimulus detected by the auditory system
is sound, a form of mechanical energy.
 Sound waves vary in frequency, which is
measured in cycles per second, or Hertz (Hz).
 Humans can detect frequencies ranging form
about 20 to 20,000 Hz.
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 Outer ear: the visible portion of the ear that
collects sound waves and funnels them onto
the eardrum.
 Eardrum: the membrane between the outer
ear and the inner ear.
 Middle ear: an air-filled cavity containing
three small bones known as the ossicles (the
hammer, anvil and stirrup) which conduct
vibrations.
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 Inner ear: the part of the ear that contains the
cochlea, vestibule, and semicircular canals.
 Cochlea – contains the receptors for sound.
 Auditory nerve: carries auditory information
from the ear to the brain.
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 Sound waves enter the outer ear, pass
through the auditory canal, and set up
vibrations of the eardrum.
 The three bones of the middle ear transmit
the vibrations to the cochlea through its
oval window.
 The auditory nerve carries messages from
the hearing receptors inside the cochlea to
the brain.
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 The four basic tastes are sweet, salty, sour
and bitter.
 In recent years, scientists have considered a
fifth taste – umami
 The taste receptors are called taste buds and
can be found in the bumps on the tongue
(papillae) and at the back of the throat.
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Smell (Olfaction)
Olfactory epithelium:
the part of the nasal
passage that contains
receptor cells for smell.
Odorants: chemical
molecules that
stimulate the receptor
cells for smell.
When olfactory receptors come in contact with
odorants, action potentials are triggered in
olfactory neurons.

 The system that helps us to detect touch and
pain is the somatosensory system.
 The somatosensory system responds to various
types of stimuli, for example, pressure and
temperature.
 We sense touch, temperature and pain with nerve
endings located in the skin.
 Different parts of the body have varying degrees
of sensitivity to touch.
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 The sense of bodily position is called proprioception
(or kinesthetic sense).
 Proprioception helps us to keep track of where we are
and move efficiently.
 The receptors for body position are called
proprioceptors, which help us to sense muscle
strength and force.
 Two types of proprioceptors: stretch receptors
located in the muscles, and force detectors
embedded in the tendons.
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 Our sense of balance or equilibrium is called
the vestibular sense.
 It depends on hair-like
receptors in the inner
ear's three semicircular
canals and vestibular
sacs.
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