Psychology Content Category 6A: Sensing the environment
This content category has 5 major topics: Sensory Processing, Vision,
Hearing, Other Senses, and Perception.
Sensory Processing
All of our knowledge of the outside world comes to us through one of several sensory systems devoted to
detecting a specific type of environmental stimulus and transmitting that detection to the brain. This initial topic
first covers concepts, methods and mathematical principles that govern the study of sensation along with a
general overview of sensory receptors.
•
Sensation - The topic of sensation deals with how stimulation from the environment (in terms of light, air
vibrations, pressure, odors, etc.) is detected, transduced (or converted from stimulation to neural impulses),
and transmitted to the brain. This first section covers topics from psychophysics, which attempts to
quantify our psychological experience of physical stimuli.
o Thresholds
 A threshold is the amount of stimulus that needs to be detected before you notice the stimulus.
 Absolute threshold
• As seen in figure 1, an absolute
threshold is the stimulus intensity
above which you will detect a
stimulus at a better than chance
(50%) level.
 Difference threshold
• A difference threshold is also known
as the just noticeable difference.
This is how much a stimulus needs
to increase or decrease in intensity
so that you will notice a change in
the stimulus’ intensity.
• Difference thresholds change with
Figure 1: Absolute Threshold
stimulus intensity; as the stimulus
gets more intense, the difference
threshold increases)
o Example: If you go camping in a quiet alpine forest, you notice (and may get freaked out
by) lots of very quiet sounds. But if you’re sitting in the crowd at Mile High stadium
during a Broncos game, you probably won’t notice if one additional person starts
cheering with the crowd.
o Weber’s Law
 This law attempts to quantify the just noticeable difference.
 The law is the form of an equation: ΔI/I=k, where ΔI represents the difference threshold, I
represents the initial stimulus intensity and k is a constant (known as the Weber fraction).

o
In English, this means that the just noticeable difference for a stimulus is proportional to the
magnitude of the stimulus and is a constant ratio across magnitudes.
 It turns out that k (the ratio) is logarithmic for human vision and hearing (although Weber’s law
isn’t accurate for extremely high and low intensity sounds).
Signal detection theory
 This theory was an advance on psychophysics research that relied only on thresholds. Signal
detection theory incorporates both thresholds and individual differences in human judgment.
• Different people have different standards for judgment; some are very careful, some are
very concerned about never making a mistake, and some are very concerned about never
missing an opportunity.
• In a stimulus detection task (like being asked to indicate if they heard any sound in a
particular time interval), loss averse or careful people are going to be more biased to
respond “no” while those that don’t want miss something may be more biased to respond
“yes”.
• Signal detection theory allows psychologists to quantify how good people are at detecting
stimuli and quantify and response biases they have.
• Signal detection theory states that detecting a stimulus requires making a judgment about
whether it is there or not, and this judgment is based on subjective interpretations of
ambiguous stimuli.
• Signal detection theory methodology
o In signal detection studies, participants are asked to indicate whether or not a stimulus
(sound, object, or something else) was present
o As can be seen in Figure 2, there are four possible outcomes for any trial.
 Correct answers are hits (saying “yes” when the stimulus is present) and correct
rejections (saying “no” when the stimulus is absent).
 Incorrect answers can be separated into False Alarms (saying “yes” when the
stimulus is absent) and Misses (saying “no” when the stimulus is present).
o Response bias is the extent to which an individual is likely to answer “yes.”
Figure 2: Possible Trial Outcomes in Signal Detection Theory
o
Sensory adaptation
 Our sensory receptors and brain circuits have evolved to detect changes in our environment.
 This means that when a stimulus’ level is not changing, then our sensory processes become
less sensitive to that stimulus
 Psychologically, this is experienced as “getting used” to a particular stimulus or ceasing to pay
attention to that stimulus.
•
Sensory receptors
o Other sections in this category will go into specific detail for the receptors for the major sensory
systems. As a general overview, however, receptors are cells that are specialized to detect a type
of physical, chemical, or wave stimulus; translate that stimulus to electrical impulses; and transmit
those impulses to nerves.
o Sensory pathways
 Other sections will go into more detail into the specific sensory pathways, but in general sensory
information reaches the brain by first being detected by a receptor cell. The receptor cell then
excites nerve cells which then go directly into the specific brain area that begins to interpret the
stimulus or connect or (if the receptor is relatively far away from the brain) connects to other
nerve cells which then connect to the brain.
o Types of sensory receptors
 Figure 3 shows the various types of receptors specialized for detecting most of the various
forms of sensation that humans process. In addition to the receptors listed, we also have
receptors (in the skin) for pain and temperature.
Figure 3: Types of Sensory Receptors
Vision
Figure 4: Structure of the Eye
•
Structure and function of the eye (figure 4)
• Cornea – the cornea is the transparent outer covering over the lens. It serves to focus incoming light.
• Pupil – the pupil is the dark circle at the center of the eye and is actually an empty hole in the iris
above the lens. Light goes through the pupil to the lens.
• Iris – the iris is the colored part of the eye. It serves to regulate the amount of light that goes through
the pupil to the lens. By dilating and contracting, it can allow more or less light through allow us some
adjustability of our visual acuity in different amounts of light.
 The iris also is sensitive to emotional content – it dilates when we see something we like.
• Lens – The lens serves to focus light and transmit it to the light sensitive cells on the retina.
• Sclera – The sclera is the white part of the eye. It serves as a protective outer covering.
Figure 5: Detailed Structure of the Retina
•
Retina (Figure 5 & Figure 6)
o The retina is the part of the eye that transduces photons into nerve impulses. It is located at
the back of the eye. Light focused from the lens falls onto the retina.
o There are two types of photosensitive cells: rods and cones. These are located at the very back
of the retina (figure 6), which means that photons must pass by the optic nerve and other retinal
cells before being sensed by the rods and cones.
o Rods- Rods are sensitive to very low levels of illumination, and are therefore responsible for
night vision. They also do not do a very good job of discriminating fine details or color. They
are concentrated at the edges of the retina. The eye has approximately 120 million rods.
o Cones are most densely concentrated at the fovea. Unlike rods, they do a good job
discriminating color and fine details. There are three types of cones, each most sensitive to a
different wavelength of light (figure 7); combinations of activation of these cones help produce
our experience of color.
There are approximately 6
million cones in the eye.
o Bipolar cells (along with
amacrine and horizontal
cells, which are not shown in
figure 5), take the input from
rods and cones and perform
some initial computations.
This is then passed along to
the ganglion cells.
o Ganglion cells take input
and, once a threshold is
reached, produce action
potentials that cause
stimulation along their axons.
These axons are bundled
together and exit the eye
through the back of the
retina. The bundle of axons
is the optic nerve.
Figure 6: Comparison of Rods and Cones
Blind Spot – there are no
rods or cones where the
optic nerve exits the eye. For this reason you have a blind spot in each of your visual fields.
You do not experience a sensation of blindness, however, because the brain guesses what is
most likely to be in the part of your visual field and fills in your visual experience accordingly.
• The fovea is the center of the retina. It is the area where cones are the densest and is therefore the
area where your visual acuity is greatest.
 Color vision is determined by the wavelengths of the light that is detected by the cones.
o Wavelengths (figure 7) - There are three types of cones, each maximally sensitive to a different
wavelength of light. Color vision results from the combination of activity of the different cones
(this is known as trichromatic theory).
o “S” cones are most sensitive to short wavelengths of lights (figure 7). This wavelength of light is
experienced as blue to violet.
o “M” cones are most sensitive to medium wavelengths (which are experienced as yellow to green)
o “L” cones are most sensitive to longer wavelengths of light (which are experienced as red to
orange).o
o
Visible light is only a small part of the entire electromagnetic spectrum. While gamma rays can
have wavelengths as short as 10-5 nm, and radio waves have wavelengths as long as 1013 nm,
visible light in only in the range of 400 nm to 700 nm.
Figure 7: Trichromatic Vision
•
Opponent-process is another theory
that describes the way that color is
perceived. Some colors appear to be
opposites. For example it is very
difficult to imagine a “reddish green,
while it is comparatively easy to
imagine a “yellowish green.” Staring
too long at a red object also leaves a
green afterimage in your vision.
o Colors that can’t mix are
considered to be opponents.
o This has nothing to do with the
cones; instead, it involves how
input from the cones is processed
by the ganglion cells.
o One type of ganglion is excited by
S cones and inhibited by L cones
and M cones. This creates the
perception that blue and yellow
(which is a combination of red and
green) are opponents.”
o Another type of ganglion is excited
by L cones and inhibited by M
cones, which creates the
perception that red and green are
opponents.
Figure 8: How visual information gets from the eyes to the brain
Figure 9: Dorsal and Ventral processing streams
•
•
We describe colors using the variables of hue, saturation, and brightness.
o Hue is the position of the color in the color spectrum, and is determined by the wavelength of
the light that is reflected by an object.
 Shorter wavelengths look violet, purple, and blue while longer wavelengths look yellow,
orange, or red.
o Saturation is the vividness or purity of the hue. It is determined by the mixture of wavelengths
(the fewer the number of wavelengths reflected by an object, the greater the saturation).
o Brightness is the intensity or luminance of a color. The more light that is reflected by an object,
the brighter the object’s color will appear.
Visual processing
 The eyes are at the front (anterior) of the brain while the primary visual cortex is at the back (posterior)
of the brain. This means that visual information must travel the length of the brain before it undergoes
significant processing.
 Each retina has a separate optic nerve. These two nerves meet close to the midpoint of the brain at
the optic chiasm (figure 8). At this point, most (but not all) of the fibers in each optic nerve cross over
to the other hemisphere.
• This means that most of the input from the right eye and the right visual field is processed in the
left hemisphere (and vice versa).
• After the chiasm, the visual input travels to the thalamus and then to the right and left visual cortex in
the occipital lobes of each hemisphere.


Parallel processing means that several different types of qualities are processed simultaneously for
each object. These qualities include color, motion, shape, and depth.
• The two best studies forms of parallel processing involve shape and motion, which provide
information about what an object is and where it is (and where it is going). These are each
processed by a separate pathway in the brain (figure 9)
• The dorsal “where stream” projects from the primary visual cortex into the parietal lobe.
• The ventral “what stream” projects from the primary visual cortex into the temporal lobe.
Feature detection refers to the phenomenon that certain groups of neurons in the primary visual cortex
seem to preferentially respond to the presence of certain features.
• For example, some neurons respond strongly to the presence of horizontal lines, while others
respond to lines that are at 45 degree angles. Others respond to vertical lines and still further
neurons respond to every other angle in between.
• Therefore, different neurons show a selective response to certain features of the visual input.
• Other types of feature detectors include edge detectors and length detectors.
• This is important because it suggests that vision is a constructive process.
o Entire scenes are not transmitted as a whole.
o On the contrary, the visual system somewhat seems to break up visual input into its
constituent parts and then reconstruct it using feature detectors and parallel processing.
Hearing
Our sense of hearing is used to detect
sound waves in the air and translate
the information about the waves’
frequencies and amplitudes into
information about pitch, volume, and
duration.
•
Auditory processing (figures 10
and 11)
 Outer Ear
• The auditory canal is
shaped to channel sound
waves to the eardrum
• The eardrum vibrates when
struck by sound waves, and
these vibrations cause the
ossicles to move.
Figure 10: The outer, middle, and inner ear
 Middle Ear
• The ossicles are three
bones named for the tools they somewhat resemble (the hammer, anvil, & stirrup). The ossicles
are connected at one end to the eardrum and at the other end to the inner ear (the cochlea) at the
oval window. The ossicles transmit vibrations from the eardrum to the cochlea
 Inner Ear
• The cochlea is a snail-shaped, fluid-filled structure. Vibrations from the ossicles cause pressure
changes in the cochlea which causes the cochlear fluid to move.
• The basilar membrane is a thin membrane running through the center of the cochlea. When the
cochlear fluid moves, this causes the basilar membrane to move. On the basilar membrane are
hair cells
which are
moved when
the basilar
membrane
moves. This
movement
stimulates
the hair cells,
which in turn
stimulate the
auditory
nerve. The
hair cells are
the main
receptor cells
of the
•
auditory
Figure 11: Signal Transduction in the inner ear.
system.
The auditory nerve is connected to the hair cells and connects the auditory system to the brain.

•
Auditory pathways in the brain
• Nerve impulses from the auditory nerve make their way to the thalamus, a structure in the midbrain
located just above the brainstem. The thalamus is involved in consciousness, sleep, and the
relaying of sensory and motor signals throughout the cortex. The thalamus has strong connections
to the primary auditory cortex.
• Located in the temporal lobe in both hemispheres of the brain, the primary auditory cortex. Neurons
in different parts of the auditory cortex respond selectively to different frequencies (and therefore
lead to the conscious experience of different pitches). Further processing allows humans to group,
recognize, and interpret the sounds into something meaningful.
• Damage to the primary auditory cortex results in the loss of conscious awareness of sound,
although individuals with such damage can react reflexively and unconsciously to sound due to the
processing in the thalamus.
Sensory reception by hair cells
 As discussed above, the hair cells are located inside the cochlea on the basilar membrane. The
hair cells can move, and when the ossicles cause vibrations in the cochlea (transmitted via the oval
window), the fluid inside the cochlea moves, which causes the hair cells to move, which releases
neurotransmitters that cause the auditory nerve to fire.
• Hair cells are grouped in
bundles, and are
connected to each at
their tips by proteins
called tip links. When
the hair cells are moved
in the inner ears, the tip
links are stretched,
which causes the tip link
to mechanically lift open
a potassium channel in
the hair cell membrane
(figure 12). This
depolarizes the hair cell,
which releases
neurotransmitters across
the synapse to the
auditory nerve.
Figure 12: Hair cell structure and function
Other Senses
Sight and hearing are the best known and best studied senses, but humans have several other systems to
acquire sensory information from the environment, including somatosensation, taste, smell, the kinesthetic
sense, and the vestibular sense.
•
Somatosensation
o Pain perception
 Pain serves as a warning
system that tells you to
stop or change what you
are doing.
 Pain receptors can be
found in many places in
the body including skin,
membranes around
muscles and joints,
muscles, and organs.
 There are two main types
of pain receptors (as can
be seen in figure 13)
• Fast fibers are
myelinated and
communicate sharp,
Figure 13: Pain Receptors
immediate pain.
They are activated by strong physical pressure and extreme temperature changes.
• Slow fibers are unmyelinated and communicate chronic, dull pain. They are activated by the
chemical changes that occur in skin when it is damaged.
Figure 14: Taste from tongue to brain
•
Taste is a chemical sense. Your taste bud receptors (inside your papillae; figure 14) bind to different
molecules that then are translated into different tastes.
o Taste buds/chemoreceptors that detect specific chemicals : There are five basic types of taste buds
devoted to detecting different combinations of basic qualities. All of your taste experiences start with
different amounts of activation of these receptors.
• Sweet, salty, sour, bitter, and umami
• Umami is Japanese for “savory” and results from the detection of glutamate.

o
•
The different taste bud receptors (contrary to popular belief) are not concentrated in particular
regions of the tongue, but instead are spread relatively uniformly across the tongue.
Supertasters are individuals that have a particularly high number of taste buds (often 6 times that of
non-supertasters).
 They are more likely to be highly aware of flavors and textures, experience pain when eating
spicy foods, and report disliking bitter foods and drinks (like coffee or alcohol).
Smell
o Smell (like taste) is a chemical sense. Olfactory receptors bind to molecules and send the signal to
the olfactory bulb and then on to the amygdala and prefrontal cortex.
 The olfactory epithelium (figure 15) is a lyaer of tissue deep in the nasal cavity which contains
the olfactory receptors.
 When
olfactory
receptors
detect an
odor
molecule
they
send
their
input to
the
olfactory
bulb, the
area for
odor
processi
ng in the
brain.
Figure 15: Olfactory epithelium and olfactory bulb
Because
the olfactory bulb is so close to the olfactory receptors, smells have the most direct route to
the brain.
 The prefrontal cortex areas help process the pleasantness/unpleasantness of the odors,
while the amygdala is involved in processing the emotional and memory content of the odors
(and explains why smells can evoke strong feelings and memories).
o Olfactory cells/chemoreceptors that detect specific chemicals
 Each olfactory receptor is sensitive to only one type of molecule. However, unlike taste
(where there are only five types of receptors), there are thousands of different types of
receptors for odor molecules.
 Although we have so many receptors for sensing olfactory information, humans are
surprisingly bad at perceiving and identifying specific smells.
• Humans can quickly and easily identify whether a smell is pleasant or unpleasant, but
can have an accuracy rate (for identifying even common food items) of less than 50%.
 Humans (like other animals) also have olfactory receptors to detect pheromones (chemicals
that animals release to trigger behavioral and physiological changes in other animals), but it
is unclear to what extent (if any) that pheromones affect human behavior.
•
Kinesthetic sense
o Kinesthetic sense is your perception of your body in space.
o This sense is crucial for coordination and voluntary movement.
o If you keep your eyes closed and are able to touch your finger to your nose without missing, you have
successfully used your kinesthetic sense.
o Information for the kinesthetic sense comes from receptors in your joints, muscles, and tendons.
•
Vestibular sense
o Your vestibular sense is your sense of balance.
o It uses receptors in the semicircular canals of the ear (figure 16) that can detect the level of liquid in the
canals which the brain can use to interpret the head’s rotation.
o This explains why ear infections can affect the sense of balance.
o Seasickness/motion sickness involves the vestibular sense, but not exclusively. It arises when the
visual system and vestibular system provide conflicting and irreconcilable signals.
Figure 16: The semicircular canals in the inner ear are the
site of receptors that provide you information about your
balance.
Perception
Our understanding of the outside world is not a direct record of the input from our sensory processing organs.
Instead, our perceptions are a selective record of the environment around us, influenced by our goals,
motivations, attention, and the basic processes of the brain.
•
•
Bottom-up/Top-down processing
• Both of these types of processing affect our pattern recognition, or our ability to recognize,
contextualize, and understand our perceptual input.
• Bottom-up processing refers to pattern recognition that is driven by features of the stimulus as well
as how the brain processes those features. Information is then transmitted up to higher levels of
mental processing.
 This is sometimes called data-driven processing.
 Examples of bottom-up processing include the perceptual organization principles discussed
below.
• Top-down processing refers to pattern recognition that is driven by higher-level processes such as
attention, context, or expectations. These higher-level processes then transmit information down to
lower levels of mental processing to guide perception.
 This is sometimes called schema-driven processing.
 Example: In the adjacent figure, the same shape is interpreted
as both an “h” and an “a.” The surrounding context of the other
letters helps guide the perception of the ambiguous letters.
Figure 17: Example of topPerceptual organization
down processing effects.
• There are a number of cues both in the body and environment that
the brain uses to judge depth/distance, motion, and our perception of the unity and constancy of
environmental objects.
• Depth/Distance
 Depth perception cues can be either monocular cues (which are available to either eye alone) or
binocular cues (which depend on comparing input from both eyes).
 Binocular Cues
• Binocular disparity
o Human’s eyes are set apart, and therefore have a slightly different view of the world. By
comparing how differentially far apart two objects appear to each eye, the brain can
calculate the distance to each object and their relative depth.
• Convergence
o When we look at objects that are close, our eye muscles turn inward. The brain can use
the flex in each eye muscle to compute how close an object is.
 Monocular Cues
• Occlusion – in general, objects that are closer block and occlude objects that are further
away.
• Relative Size – If you know that two objects are relatively the same size, then the object that
projects a smaller image on the retina is usually farther off.
• Familiar Size – We can use the known size of familiar objects to judge how far away they
are.
• Linear perspective is the phenomenon where parallel lines seem to distantly converge.
• Texture gradient – If an object has a uniform texture or pattern, the closer parts of that
pattern will appear to be larger than the farther parts of that pattern.
•
•
•
Size
 Cues for size are intimately related to cues for distance. Most of these cues rely on interpreting
the meaning of how large an object appears on the retina. By knowing an object’s true size, you
can interpret its distance, and vice versa.
 When these cues are ambiguous or hard to interpret, visual
illusions (such as the Ponzo Illusion, figure 18) can result.
Motion
 Motion aftereffects
• Also known as the waterfall effect, these aftereffects occur
when you look at a moving object for a long time and then look
at a still object which then appears to be moving in the
opposite direction.
o This is excellent evidence of motion-sensitive neurons in
the brain. By staring at the moving object, the neurons that Figure 18: The Ponzo illusion
are sensitive to motion in that direction are adapting to the
motion (getting fatigued and used to it). When you look away from the motion, the
neurons that respond to motion in the other direction are less fatigued, and therefore
more able to fire, which convinces your brain that the still object is actually moving in the
other direction.
 Stroboscopic motion
• This explains how movies (on film) appear to move despite being composed of individual
still frames. When two or more images are presented in rapid succession (about 60
milliseconds apart), they appear to show continuous motion.
• Constancy
 Usually, changing an object’s angle, distance, or illumination does not cause us to change how
we perceive the object’s size, shape, color, or lightness. This is because the brain computes
size, shape, color, and lightness in a relative fashion (for example, lightness is computed by
comparing how much light is reflected from an object with how much light is reflected by the
background). If this was computed in an absolute fashion, we would think that (for example) an
object’s actual color and lightness had changed if it was moved
from the sun to the shade.
Gestalt principles
• Gestalt psychologists were an early group of researchers who
emphasized that in perception, the “whole is more than the sum of
its parts.” They proposed that the brain uses several principles to
organize sensory information.
• Good continuation
 The mind tends to think that shapes of similar size, color, and
orientation are part of the same object when they are obscured
behind another object. Therefore, in figure 19, we see a
continuous long green rectangle rather than two shorter green
rectangles.
Figure 19: Gestalt principle of
good continuation
•
•
•
•
•
Figure/Ground
 The mind easily distinguishes between things that are in the foreground and things that are in
the background as well as between the focal and important parts of a scene versus the
unimportant parts. When this is ambiguous, you get illusions like the reversible figure illusion
(figure 20) that looks like both two faces looking at each other (the purple areas) and a
candlestick (the white area).
Proximity
 We tend to see things that are close to each other as being part of the same entity. Therefore,
in part (b) in figure 22, we see four lines of four green dots, rather than one group of sixteen
dots or sixteen individual dots.
Similarity
 We tend to group things that
are similar to each other
together in the same group.
Therefore, in part (c) in
figure 22, we are more likely
to interpret the figure on the
left as vertical columns of
alternating red circles and
squares and the figure on
the right ad horizontal rows
of alternating red circles and
Figure 21: The Gestalt principle of
figure ground illustrated by the
squares.
Figure 20: The Gestalt
reversible figure illusion
principle of illusory contours.
Closure
 We tend to “fill in the blanks”
and complete figures that have gaps, as you can see in part (a) in figure 22, where we easily
see a beige triangle, purple square, and blue horse despite the many gaps in the figures.
Illusory contours
 The mind tends to interpret the existence of the parts of a figure that are not actually there but
only implied (to the point that as you see in figure 21, unless you closely attend to the white
space, you may “fill in” the sides of the blue square in the white spaces even though they aren’t
actually drawn).
Figure 22: Examples of the Gestalt principles of (a) Closure (b) Proximity and (c) Similarity
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