AP Psych

Unit 3
Sensation and
Perception
• INTRODUCTION TO SENSATION AND
PERCEPTION

• What do you see ?

• What do you perceieve ?
What do you see ?
What do you perceieve ?
INTRODUCTION TO SENSATION AND
PERCEPTION

• To study sensation is to study the relationship between

physical stimulation and its psychological effects.

• Sensation is the process of taking in information from the

environment.

• Perception refers to the way in which we recognize, interpret,

and organize our sensations.
 THRESHOLDS

ABSOLUTE THRESHOLD
• In psychophysics, the branch of psychology that deals with
the effects of physical stimuli on sensory response,
researchers determine the smallest amount of sound,
pressure, taste, or other stimuli that an individual can detect.
Psychologists conducting this type of experiment are
attempting to determine the absolute threshold—the
minimum amount of stimulation needed to detect a stimulus
(and cause the neuron to fire) 50 percent of the time. At the
absolute threshold, we cannot detect lower levels of stimuli,
but we can detect higher levels.

This can be applied to all our senses:

•The minimum intensity of light we can see
•The lowest volume of a sound we can hear
•The smallest concentration of particles we can smell
•The smallest concentration of particles we can taste
•The lightest touch we can feel
• But what is that “50% of the time” part of the
definition for? Why not 100% of the time?
• That is because our absolute threshold can vary
according to external and internal factors like
background noise, expectation, motivation and
physical condition. It is easier to hear a sound
when we are in perfect health, expecting to hear it
in a quiet room than when we are tired, unaware
of it and in a noisy street.
SIGNAL DETECTION THEORY

• In a typical absolute-threshold experiment, an experimenter plays a

series of tones of varying volume to determine at exactly what
volume the participant first reports that she can hear the tone.
• Another approach to measuring detection thresholds involves
signal detection theory (SDT). This theory takes into consideration
that there are four possible outcomes on each trial in a detection
experiment: the signal (stimulus) is either present or it is not, and
the participants respond that they can detect a signal or they cannot.
• Therefore, we have the following four possibilities:
SIGNAL DETECTION THEORY

• In a typical absolute-threshold experiment, an experimenter plays a

series of tones of varying volume to determine at exactly what
volume the participant first reports that she can hear the tone.
• Another approach to measuring detection thresholds involves
signal detection theory (SDT). This theory takes into consideration
that there are four possible outcomes on each trial in a detection
experiment: the signal (stimulus) is either present or it is not, and
the participants respond that they can detect a signal or they cannot.
• Therefore, we have the following four possibilities:
• Hit—the signal was present, and the participant reported sensing it.
Miss—the signal was present, but the participant did not sense it.
• False alarm—the signal was absent, but the participant reported
sensing it.
• Correct rejection—the signal was absent, and the participant did
not report sensing it.

• SDT takes into account response bias, moods, feelings, and

decision- making strategies that affect our likelihood of having a
given response.
DIFFERENCE THRESHOLD OR DISCRIMINATION
TRESHOLD OR JUST NOTICEABLE DIFFERENCE

• Another type of threshold is the discrimination threshold,

which is the point at which one can distinguish the
difference between two stimuli. The minimum amount of
distance between two stimuli that can be detected as
distinct is called the just noticeable difference (JND) or
difference threshold.
• Measure : the experiment might involve playing pairs of
tones of varying volumes. The participants would try to
determine if the tones that they heard were the same or
different.

• Examples:
• The smallest difference in sound for us to perceive
a change in the radio’s volume
• The minimum difference in weight for us to
perceive a change between two piles of sand
• The minimum difference of light intensity for us to
perceive a difference between two light bulbs
• The smallest difference of quantity of salt in a soup
for us to perceive a difference in taste
• The minimum difference of quantity of perfume for
us to perceive a difference in something’s smell
• DIFFERENCE THRESHOLD AND WEBER’s LAW

To quantify the difference threshold, psycho-physicist Ernst

Weber developed what is known as the Weber’s Law.

Ernst Weber (1795–1878) noticed that at low weights, say 10

gr, it was easy to notice 5g increases or decreases in weight;

however, at high weights, say 100 gr, participants were not

well able to judge 5gr differences. The observation that the

Just Noticeable Difference is a proportion of stimulus

intensity is called Weber’s law. Simply put, this law states

that the higher the intensity of a stimulus, the larger the

differences must be to be noticed. => the more it will need

to change so we can notice a difference

Gustav Fechner (1801–1887), the founder of psychophysics,

in addition to contributing to Weber’s Law determined that

the perceived brightness/loudness of a sensation is

proportional to the logarithm of its actual intensity.

=> While the difference threshold or the just noticeable
difference between two stimuli means detecting differences
in stimulation levels, the absolute threshold refers to the
smallest detectable level of stimulation.
• SUBLIMINAL PERCEPTION

Subliminal stimuli are not detectable 50% of the time.

They are below your absolute threshold.

Subliminal perception is a form of preconscious processing

that occurs when we are presented with stimuli so rapidly that
we are not consciously aware of them. When later presented
with the same stimuli for a longer period of time, we
recognize them more quickly than stimuli we were not
subliminally exposed to. Clearly, there was some
preconscious processing, known as priming, occurring even
if we were not aware of it. Another example of preconscious
information processing can be seen in the tip-of- the-tongue
phenomenon, in which we try to recall something that we
already know is available but is not easily available for
conscious awareness. This phenomenon demonstrates that
certain preconscious information may be available to the
conscious mind but quite difficult to access.
PERCEPTUAL PROCESSES
• sensory mechanisms => how environmental stimuli affect the
receptor systems.

• perceptual processes => how our mind interprets these stimuli.

• two main processes of perception:

• Bottom-up processing achieves recognition of an object by
breaking it down into its component parts. It relies heavily on the
sensory receptors. Bottom-up processing is the brain’s analysis
and acknowledgement of the raw data. : => Bottom-up processing is
when the information acquired in our sense receptors (sight, hearing, taste,
touch and smell) goes to our brain to be interpreted

• Top-down processing: when our brain uses information that has

already been brought by sensory systems to organize our
experiences and expectations => perception guided by experience
and high level processing.

• For example, let’s think about the first time a person tastes the
sourness of a lemon: the neurons firing to alert the brain of the
presence of some taste in the mouth is a bottom-up process,
whereas labeling it “sour” is the top-down process.
• However, the next time the person sees a lemon, they might
salivate or wince before ever tasting the lemon. This is top-down
processing because the expectation based on experience
influences the perception of the lemon.
• Top-down processing can be a factor in optical illusions when
people see what they expect to see rather than what is actually in
front of them.
Perceptual Processes

Wiseman. Pan Books 2009

Wiseman. I-perception 2016
 Perceptual Processes

• Perception is bounded by the inherent

properties of the visual apparatus
• Perception depends on attention to
direct the limited ressources =>
Attention processes bias competitive
• Perception depends on experience
• Perception depends on expectations
 Perceptual Processes

…achromatic or grayscale, but the light

source that your brain interprets to be
on the scene has got this blueish
component.
You brain says, 'the light source that I'm
viewing these strawberries under has
some blue component to it, so I'm going
to subtract that automatically from
every pixel.' And when you take grey
pixels and subtract out this blue bias,
you end up with red.

Kitaoka Akioshi, Ritsumeikan University.

Perceptual Processes
The visual system needs to determine the color
of objects in the world. In this case the problem
is to determine the gray shade of the checks on
the floor. Just measuring the light coming from a
surface (the luminance) is not enough: a cast
shadow will dim a surface, so that a white surface
in shadow may be reflecting less light than a
black surface in full light. The visual system uses
several tricks to determine where the shadows
are and how to compensate for them, in order to
determine the shade of gray "paint" that belongs
to the surface.
The first trick is based on local contrast. In
shadow or not, a check that is lighter than its
neighboring checks is probably lighter than
average, and vice versa. In the figure, the light
check in shadow is surrounded by darker checks.
Thus, even though the check is physically dark, it
is light when compared to its neighbors. The dark
checks outside the shadow, conversely, are
surrounded by lighter checks, so they look dark
by comparison.
A second trick is based on the fact that shadows
often have soft edges, while paint boundaries
(like the checks) often have sharp edges. The
visual system tends to ignore gradual changes in
light level, so that it can determine the color of
the surfaces without being misled by shadows. In
this figure, the shadow looks like a shadow, both
because it is fuzzy and because the shadow
casting object is visible.
The "paintness" of the checks is aided by the
form of the "X-junctions" formed by 4 abutting
checks. This type of junction is usually a signal
that all the edges should be interpreted as
changes in surface color rather than in terms of
shadows or lighting.
As with many so-called illusions, this effect really
demonstrates the success rather than the failure
of the visual system. The visual system is not very
good at being a physical light meter, but that is
not its purpose. The important task is to break
the image information down into meaningful
components, and thereby perceive the nature of
the objects in view.[3]
Perceptual Processes

Perceptual decision guided by

semantic association

• Same sensory stimulus,

• Different perception
 Perceptual Processes
• Visual perception is quite complex. We need to perceive depth, size,
shape, and motion.
• Depth perception is facilitated by various perceptual cues. Because of
the limited ability of the brain to process information, it must take
certain shortcuts and educated guesses based on how the world is
normally structured. As such, the brain uses these cues but can also fall
victim to illusions.
• Visual perception cues can be divided into monocular and binocular
cues.

• Monocular depth cues are those that we need only one eye to see. As
such, they can be depicted in two-dimensional representations.

• MONOCULAR CUES :

Relative size refers to the fact that

images that are farther from us project a
smaller image on the retina than do
those that are closer to us => Therefore,
we expect an object that appears closer
much larger than another to be farther
to us.

Related to this idea is the idea of texture

gradient. Textures, or the patterns of
distribution of objects, appear to grow more
dense as distance increases. If we are looking
at pebbles in the distance, they appear smooth
and uniform, but close up may appear jagged
and rough. Another monocular depth cue is
interposition, also known as occlusion,
which occurs when a near object partially
blocks the view of an object behind it.
 Perceptual Processes

Linear perspective is a monocular

cue based on the perception that
parallel lines seem to draw closer
together as the lines recede into the
distance.
=> parallel lines appear to meet in the
distance

The place where the rails seem to join

is called the vanishing point. This is
the point at which the two lines
become indistinguishable from a
single line and then disappear.
Objects present near the vanishing
point are assumed to be farther away
than those along the tracks at a point
where they diverge greatly.

• Aerial perspective, another

perceptual cue, is based on the
observation that atmospheric
moisture and dust tend to
obscure objects in the distance
more than they do nearby
objects.
=> objects that are further away
look blurry and lighter
Perceptual Processes

• Relative clarity is a perceptual clue that explains why

less distinct, fuzzy images appear to be more distant.
ÞObjects that appear sharp, clear, and detailed are seen as closer than
more hazy objects.

Motion parallax (or relative motion) is the difference

in the apparent movement of objects at different
distances, when the observer is in motion. For example,
when riding on a train, a person sees distant objects out of
a window as seeming to move fairly slowly; they may
appear to move in the same direction as the train. Nearer
objects seem to move more quickly and in the opposite
direction to the movement of the train. Note that motion
parallax differs from other monocular depth cues in that it
requires motion and cannot be represented in a two-
dimensional image.
 Perceptual Processes
BINOCULAR CUES :

• Binocular depth cues rely on both eyes viewing an image. They

result from the fact that each eye sees a given image from a slightly
different angle.
• > As an object becomes closer or farther, both binocular depth cues
operate to help us judge distance

• Stereopsis refers to the three-dimensional image of the world

resulting from binocular vision.

• Retinal convergence is a depth cue that results from the fact that
your eyes must turn inward slightly to focus on near objects. The
closer the object, the more the eyes must turn inward.

• The complement to stereopsis is binocular disparity, which results

from the fact that the closer an object is, the less similar the
information arriving at each eye will be.

This process can be demonstrated by covering one

eye, then the other, while looking at something
directly in front of you. This procedure reveals
two very different views of the object. Repeat this
procedure with an object across the room,
however, and the two views appear more similar.
Perceptual Processes

• The Gestalt Approach

• the visual system also needs to perceive and recognize

form: that is, size and shape (on top of depth). The
Gestalt approach to form perception is based on a
top-down theory. This view holds that most
perceptual stimuli can be broken down into figure-
ground relationships. Figures are those things that
stand out, whereas the ground is the field against
which the figures stand out. The famous vase-face
example shows us that figure and ground are often
reversible.
Perceptual Processes
• Some basic Gestalt principles of figure detection include the
following:
• Proximity—the tendency to see objects near each other as
forming groups

• Similarity—the tendency to prefer grouping like objects

together
Perceptual Processes
• Symmetry—the tendency to perceive forms that make up
mirror images

• Continuity—the tendency to perceive fluid or continuous

forms, rather than jagged or irregular ones
Perceptual Processes
• Closure—the tendency to see closed objects rather than
those that are incomplete

• These Gestalt principles represent the Law of Prägnanz

(« good figure »), or minimum tendency, meaning that we
tend to see objects in their simplest forms.

For example, when presented with the Olympic logo, you see overlapping
circles rather than an assortment of curved, connected lines.

• A different theory of form recognition is based on a feature

detector approach. This approach differs from the Law of
Prägnanz, which reduces an image to its simplest form, by
positing that organisms respond to specific aspects of a
particular stimulus.
For example, when driving a car, we use feature detection to
anticipate the movement of other cars and pedestrians that
demand our immediate attention, helping us to be more aware
of the environment.
Perceptual Processes
• Constancy is another important perceptual process.
Constancy means that we know that a stimulus remains the
same size, shape, brightness, weight, and/or volume even
though it does not appear to. People who have never seen
airplanes on the ground will have trouble perceiving the
actual size of a plane because of their experience with the
size of the object when airborne. The ability to achieve
constancy, which is innate, and the experience, which is
learned, both contribute to our development of the various
types of constancy.
=> tendency to perceive an object that you are familiar with as
having a constant shape, size, and brightness despite any changes
in stimuli that occur
Eg: If you take two identical white sheets of paper and place one in the sun and
the other in the shade, then for the observer they will both be the same color –
white. This shows the constancy of lightness

Eg: The height of the same person from a distance of 3.5 and 10 meters is
perceived by us as unchanged, although the size of the image of this person on
the retina will be different depending on the distance.
Perceptual Processes

• One of the most complex abilities we have is

motion detection.
• We perceive motion through two processes.
• One records the changing position of an object as it
moves across the retina.
• The other tracks how we move our heads to follow
the stimulus.
• In both cases, the brain interprets the
information with special motion detectors.
• A related issue is the perception of apparent
motion.

• Examples of apparent movement include

blinking lights on a roadside arrow, which give
the appearance of movement (phi
phenomenon);

• a motion picture, where still pictures move at a

fast enough pace to imply movement
(stroboscopic effect);

• and still light that appears to twinkle in

darkness (autokinetic effect).
Sensory Adaptation
• Our sensory systems need to do more than simply detect the
presence and absence of stimulation.
• They also need to do more than detect the intensity or
quality of stimuli.
• A key feature of our sensory systems is that they are
dynamic: they detect changes in stimuli intensity and
quality.
• Two processes are used in responding to changing stimuli:
Sensory Adaptation
• Our sensory systems need to do more than simply detect the
presence and absence of stimulation.
• They also need to do more than detect the intensity or
quality of stimuli.
• A key feature of our sensory systems is that they are
dynamic: they detect changes in stimuli intensity and
quality.
• Two processes are used in responding to changing stimuli:
adaptation and habituation.

• Adaptation is an unconscious, temporary change in

response to environmental stimuli.
• Sensory adaptation is diminished sensitivity to stimuli as a
consquence of constant stimulation
An example of this process is our adaptation to being in
darkness. At first, it is difficult to see, but our visual system
soon adapts to the lack of light.
Think of another example
Sensory Adaptation
• Our sensory systems need to do more than simply detect the
presence and absence of stimulation.
• They also need to do more than detect the intensity or
quality of stimuli.
• A key feature of our sensory systems is that they are
dynamic: they detect changes in stimuli intensity and
quality.
• Two processes are used in responding to changing stimuli:
adaptation and habituation.
• Adaptation is an unconscious, temporary change in
response to environmental stimuli.
• Sensory adaptation is diminished sensitivity to stimuli as a
consequence of constant stimulation
An example of this process is our adaptation to being in
darkness. At first, it is difficult to see, but our visual system
soon adapts to the lack of light.
• Sensory adaptation to differing stimuli leaves our sensory
systems at various adaptation levels. The adaptation level is
the new reference standard of stimulation against which
new stimuli are judged.
A familiar example is that of the swimming pool. If you enter
a 75-degree swimming pool directly from an air-conditioned
room, it will feel warm, as your adaptation level is set for the
cold room. If, however, you are on a hot beach and then enter
the same pool, it will feel cold, as your adaptation level is set
for the heat of the beach.
• Habituation is the process by which we become
accustomed to a stimulus, and notice it less and less
over time.
• Dishabituation occurs when a change in the stimulus,
even a small change, causes us to notice it again.
Dishabituation also occurs when a stimulus is removed
and then re- presented.
A good example of this pair of processes is in the noise
from an air conditioner. We may notice a noisy air
conditioner when we first enter a room, but after a few
minutes, we barely even notice it; we have habituated to
the noise. However, when the air conditioner’s
compressor turns on, slightly altering the sound being
generated, we once again notice the noise. This noticing
is dishabituation. Although habituation is not typically a
conscious process, we can control it under certain
circumstances. If, in the examples above, we are
unaware of the air-conditioner noise, but then someone
asks us whether the noise of the air conditioner sounds
like something else, we can force ourselves to
dishabituate, and again notice the noise.

This control over our information processing is the key

to distinguishing habituation from sensory adaptation:
you cannot control sensory adaptation; for example,
you cannot force your eyes to adapt to darkness by
mere force of will. You can, however, force yourself to
pay attention to things to which you have habituated.
Sensory Adaptation vs. Habituation

Sensory adaptation and habituation both involve

reduced attention to a stimulus, but the two concepts
have important differences.

Sensory adaptation is an automatic, involuntary process

that involves becoming less sensitive to sensory
stimulation. => no control

Habituation is a behavioral phenomenon involving a

decreased response to something that occurs over time.
While it may occur without much thought, it does have
an element of conscious control.
Sensory Adaptation & Habituation

• Imagine that you just walked into your favorite

Italian restaurant. The delicious smell of garlic and
tomatoes is almost overwhelming when you first
walk through the door. You sit down to wait for a
table, and after a few minutes, the scents dissipate
until you barely notice them

• Sensory adaptation or habituation ?

Sensory Adaptation & Habituation

• Smokers are not bothered by the smell of

tobacco smoke the way nonsmokers are, because
smokers are accustomed to the odor. Their sensory
receptors respond less to the stimuli (the smell of
smoke) because they experience it often

• Sensory adaptation or habituation ?

Sensory Adaptation & Habituation

if you order the same dish every time you eat at a

restaurant, you might find yourself enjoying it less
after you become accustomed to it.

• Sensory adaptation or habituation ?

Sensory Adaptation & Habituation

• With the first bite of a very flavorful dish, you'll

notice the strong saltiness, sourness, or sweetness
of the food. But after a few mouthfuls, your taste
buds will adapt, and the flavor will not be as
pronounced.

• Sensory adaptation or habituation ?

Attention

• Attention => processing through cognition of a select

portion of the massive amount of information incoming
from the senses and contained in memory.

• In common terms, attention is what allows us to focus on

one small aspect of our perceptual world, such as a
conversation, while constantly being assailed by massive
input to all of our sensory systems. Attention serves as a
bottleneck or funnel, that channels out some information in
order to focus on other information.
• This process is essential because the brain is not equipped to
process and pay attention to all of the information it is
presented with at a particular moment.

• The fact that the brain must take shortcuts and focus on
particular information is a key issue in perception, which
explains why the brain can be tricked through illusions.
 Attention

• A good example of attention in action is selective attention,

by which we try to attend to one thing while ignoring
another. For example, we try to attend to a movie, while
trying to ignore the people having a conversation behind us.
Attention

• An example of selective attention is called the

“cocktail party phenomenon,” which refers to our
ability to carry on and follow a single conversation in
a room full of conversations. At the same time, our
attention can quickly be drawn to another
conversation by key stimuli, such as someone saying
our names. This recognition of our names is a
demonstration that, although we are not paying very
much attention to those other conversations, we are
definitely attending to information we are not
consciously aware of at that moment.
This phenomenon has been studied in the laboratory
with headphones, by playing a different message in
each of a participant’s ears. The participant is instructed
to repeat only one of the conversations. This repetition
is referred to as shadowing. The message played into
the nonshadowed ear is largely ignored, however.
Changes in that message or key words, like names, can
draw attention to that message.
 Attention model helpful to understand
selective attention, not in the AP manual
• Perception depends on attention to direct
the limited ressources => Attention
processes bias competitive interactions
- What determines salience or relevance ?

- Current models of selective attention propose 3 major sources of salience, of

prioritization of signals :

- One source of relevance is Physical salience : A property of a stimulus that

stands out from its neighbours dues its lowel level physcial features . Such as
colour, contrast…
- This is a stimulus driven attention.

Awh, Belopolsky & Theeuwes, TICS 2012

 Attention model helpful to understand
selective attention, not in the AP manual
• For example if you just change the color of one
stimulus among identical stimulus, it pops out
from the rest.
• And automatically capture attention.

- This is an attentional Selection process that

depends only on the physical properties of the
stimulus itself.
Attention model helpful to understand
selective attention, not in the AP manual

- This source of salience depends of the internal goal

which determines which stimulus should be
attended to.
-attention is voluntarily allocated toward items in a
goal-driven manner , this is not an implicit source of
salience.
- Such attention is manipulated explicitly.
- => Attention is attracted by stimuli relevant to a
task
Attention model helpful to understand
selective attention, not in the AP manual
• Eg. The basketball game
• Task: count how many passes in the white team

• In the original study, a gorilla stops in the middle of the display,

turns to face the camera, thumps its chest, and then continue
walking across the field of view.

• Approximately 60 % of the observers fail to notice the gorilla.

• When attention is directed toward a task, observers fail to notice
an unexpected object, even if it appears at fixation.
• Attention filters out the processing of objects non relevant to the
task.
Attention model helpful to understand
selective attention, not in the AP manual

- These sources include a broader class of processes,

such as working memory, rewards and emotion,
assumptions…

- This category emphasizes the role of learning and

of memory in the deployment of attention (and
perception).

- BUT they share one core feature: in each case, past

selection episodes are recapitulated in subsequent
trials when the relevant context is encountered
again.
Attention model helpful to understand
selective attention, not in the AP manual
• The example of emotion

• there is a selection bias toward previously rewarded

signals, or emotional signals.

• What we glean from the spider is not directly

available on its sensory surface (chikazoe)
We may tune attention to affectively salient stimuli
because of our history of emotional experience with
them. Via experience via learning ( markovic)

The reason why the spider is emotional does not depend on its
physical surface.

• It may be that the attention to emotional signals

increase their physical salience.
Attention

• There are two main types of theories explaining

selective attention: filter theories and attentional
resource theories. Filter theories propose that stimuli
must pass through some form of screen or filter to
enter into attention. Donald Broadbent proposed a
filter at the receptor level. However, the notion of a
filter at this level has generally been discarded based
on findings showing that meaningful stimuli, such as
our own names, can catch our attention. Therefore,
the filter must be at a higher processing level than that
of the receptors because meaning has already been
processed.
Attention
• Attentional resource theories, in contrast, posit that we
have only a fixed amount of attention, and this resource can
be divided up as is required in a given situation. So, if you
are deeply engrossed in this book, you are giving it nearly
all of your attentional resources. Only strong stimulation
could capture your attention. This theory is also inadequate,
however, because all attention is not equal. For example, a
conversation occurring near you is more likely to interfere
with your reading than is some other nonverbal noise.
Attention

• Divided attention, trying to focus on more than one task at

a time, is most difficult when attending to two or more
stimuli that activate the same sense, as in watching TV and
reading. The ability to successfully divide attention declines
with age. Inattentional blindness, also known as change
blindness, demonstrates a potential weakness of selective
attention. Sometimes, when people focus too intently on
specific stimuli, they can miss the bigger picture going on
around them. There are many entertaining videos available
online that display this phenomenon.
RECEPTOR PROCESSES
Sensory organs have specialized cells, known as receptor cells,
which are designed to detect specific types of energy. For
example, the visual system has specialized receptor cells for
detecting light waves.

The area from which our receptor cells receive input is the
receptive field. Incoming forms of energy to which our receptors
are sensitive include mechanical (such as in touch),
electromagnetic (such as in vision), and chemical (such as in taste).

No matter what the form of the input at the level of the receptor,
it must first be converted into the electrochemical form of
communication used by the nervous system.

Through a process called transduction, the receptors convert the

input, or stimulus, into neural impulses, which are sent to the
brain.

For example, when we hear something, tiny receptor cells in the

inner ear first convert mechanical vibrations into electrochemical
signals. These signals are then carried by neurons to the brain.
Transduction takes place at the level of the receptor cells, and
then the neural message is passed to the nervous system. The
incoming information from all of our senses, except for smell,
travels to the sensory neurons of the thalamus. The thalamus, as
you may recall from the neuroanatomy section, redirects this
information to various sensory cortices in the cerebral cortex
where it is processed. The thalamus may also filter out some
sensory inputs, an adaptive mechanism for humans because it
means that they will not be overwhelmed by incoming sensory
information. It is at the level of the thalamus that the contralateral
shift occurs, in which much of the sensory input from one side of
the body travels to the opposite side of the brain. Olfaction, or the
sense of smell, travels in a more direct path to the cerebral cortex,
without stopping at or being relayed by the thalamus.
• SENSORY MECHANISMS
• Sensory receptors deal with a wide range of stimuli, and we
experience a wide variety of input within each given sensory
dimension. Imagine, for example, the gamut of colors and
intensities that the eye can sense and relate to the brain.
• Sensory coding is the process by which receptors convey such a
range of information to the brain.
• Every stimulus has two dimensions:
• what it is (its qualitative dimension) and
• how much of it there is (its quantitative dimension).

• The qualitative dimension is coded and expressed by which neurons

are firing. For example, neurons firing in the occipital lobe would
indicate that the sensory information is light, and neurons firing in
the temporal lobe might indicate that the sensory stimulus is sound.
In contrast, the quantitative information is coded by the number of
cells firing. Bright lights and loud noises involve the excitation of
more neurons than those brought on by dim lights and quiet noises.
The wavelengths of light and frequency of sound are perceived as
hue and pitch, respectively. The physical characteristic of amplitude
is perceived as brightness for light and loudness for sound.
Similarly, the physical trait of complexity is known as saturation,
when dealing with light, and timbre, when referring to sound.
Sensory neurons respond to differing environmental stimuli by
altering their firing rate and the regularity of their firing pattern.
• Single-cell recording is a technique by which the firing rate
and pattern of a single receptor cell can be measured in
response to varying sensory input.
• Visual Mechanisms
• Visual sensation occurs when the eye receives light input
from the outside world.

• In the human eye, an image forms on the retina

Distal stimulus: the object as it exists in the environment

Proximal stimulus : the image of that object on the retina

Because of the shape of the retina and the positions of the

cornea and the lens, the proximal stimulus is inverted.

The brain, through perceptual processes, is then capable of

interpreting this image correctly.

Distal Proximal
stimulus stimulus
Vision
Vision

• First, light passes through the cornea, which is a protective

layer on the outside of the eye covering the pupil and iris.
• The pupil is a small adjustable opening in the center of the
eye through which light passes.
• The iris is a ring of muscle tissue that forms the colored
portion of the eye around the pupil and controls the size of
the pupil opening by expanding or contracting the pupil.

• Just under the cornea is the lens. The curvature of the lens
changes to accommodate for distance. These changes are
called accommodations => help focus images on the retina
• The retina is at the back of the eye and serves as the screen
onto which the proximal stimulus is projected.
Vision
• The retina is light-sensitive. It is covered with receptors known as
rods and cones, plus layers of neurons that begin the processing of
visual information.

• Rods and cones are receptor cells that will receive the incoming
light waves and operate transduction: the process of changing
physical energy to neural impulses so that the brain can understand
them
• Rods (120 milliion), located on the periphery of the retina, are
sensitive to low light adn detect black, white and gray.
• Cones ( 6 million), concentrated in the center of the retina, or
fovea, are sensitive to bright light and color vision. => central fine
detail vision
Vision

This is an estimate of what the retina only codes

with a good 1° of central vision, and weaker
peripheral vision.

Rosenholtz. Annual Review of Vision Science 2016

Vision

• After light stimulates the receptors, this

information passes through horizontal cells
to bipolar and amacrine cells. Some low-
level information processing may occur here.
• The stimulation then travels to the ganglion
cells of the optic nerves => axons of the
ganglion cells leaving through the back of
the eye to the brain.
Vision

• Where the optic nerve exits the retina,

humans have a blind spot because there are
no photoreceptors there.
Vision

• The optic nerves cross at the optic chiasm, sending half of

the information from each visual field to the opposite side
of the brain.
• Each visual field includes information from both the left
and right eye.
• From here, information travels to the primary visual cortex
areas for processing.
Vision

• The brain processes the information received

from vision—color, movement, depth, and
form—in parallel, not serial, fashion.
• In other words, the brain is simultaneously
identifying the patterns of what is seen.

• Serial processing occurs when the brain

computes information step-by-step in a
methodical and linear matter,
• parallel processing happens when the brain
computes multiple pieces of information
simultaneously.
• Over time and through practice, serial
processes can turn into parallel processes,
just as riding a bike initially requires a
person to consider each decision, but later is
done seemingly automatically.
Vision

• Some Cells in the visual cortex, called feature cells or

feature detectors, respond selectively to various
components of a visual image, such as orientation of lines,
colour, and movement.

• Example - in the above image, the feature cells that are

being measured respond to vertical lines. They fire the
most in response to vertical lines, basically not at all for
horizontal lines, and a little bit for diagonal lines.

• Feature detector neurons “see” different parts of the

pattern, such as a line set at a specific angle to the
background. Like pieces of a jigsaw puzzle, these parts are
amalgamated to produce the pattern in the environment.
This process starts at the back of the occipital lobe and
moves forward.
Vision

• As the information moves forward, it becomes more

complex and integrated. This process, by which information
becomes more complex as it travels through the sensory
system, is known as convergence and occurs across all
sensory systems.
• Once lines and colors have been sensed, the information
travels through two pathways: the dorsal stream and the
ventral stream. The ventral stream is the “what” pathway
that connects to the prefrontal cortex, allowing a person to
recognize an object. The dorsal stream is the “where”
pathway that integrates visual information with the other
senses through a connection to the somatosensory cortex at
the top of the brain.
Vision

• David Hubel (1926–2013) and Torsten

Wiesel (1924–), through experiments with
cats, determined that mammals, including
humans, will develop normal vision along
these lines so long as any impairments are
corrected during the critical period, the first
months after birth.

• For more information read the story of Hubel and

Wiesel exp.
• https://embryo.asu.edu/pages/david-h-hubel-and-to
rsten-n-wiesels-research-optical-development-kitte
ns
Vision

• We only see visible light, which corresponds

to ROYGBIV. The longest corresponds to red
and the shortest corresponds to purple

• Theories of color vision

• Two different processes contribute to our
ability to see in color. The first is based on
the Young-Helmholtz or trichromatic
theory. Another theory is known as
opponent process theory
Vision

the Young-Helmholtz or trichromatic theory

• three different types of cones in the eye (red, green, and

blue) are responsible for converting light into electrical
signals. These three types of cones each respond to
different wavelengths of light, which can be combined to
create all the colors we see in the visual spectrum.
However, this does not tell the whole story
Vision
• Color blindness responds to this theory, as well. Most color
blindness occurs in males, which provides strong evidence
that this is a sex-linked genetic condition. Dichromats are
people who cannot distinguish along the red/green or
blue/yellow continuums. Monochromats see only in shades
of black and white (this is much more rare). Most color
blindness is genetic.
Fun fact

• the mantis shrimp has sixteen different types of

cone, and six polarisation channels. This allows
them to see things that are invisible – and
unimaginable – to humans
Vision

• opponent process theory,

• cells within the thalamus respond to opponent pairs of
receptor sets—namely, black/white, red/green, and
blue/yellow.
• If one color of the set is activated, the other is essentially
turned off.
• Eg when you are looking at the color green red is being inhibited
Vision

• For example, when you stare at a red dot on a page and

then you turn away to a blank piece of white paper, you will
see a green dot on the blank piece of paper because the
red receptors have become fatigued and, in comparison,
the green receptors are now more active. This is known as
an afterimage.

• How does after image prove Opponent Process?

• ● You stare at a light source that has green, yellow and
dark.
• ● Stair at it for a while tires those receptors.
• ● When you then look at white light (which is composed
of all the
• spectrums of light) those receptors are tired and no
longer fire.
• ● Because they don’t fire in their place you see their
opponent pair
• ● For the Yellow you experience blue
• ● For the Green you experience red
• ● For the dark you experience white.
•
Vision
• Auditory Mechanisms
• Auditory input, in the form of sound waves, enters the ear
by passing through the outer ear, the part of the ear that is
on the outside of your head, and into the ear canal.
• Passing through accessory structures to sense receptors,
vibrating air triggers nerve impulses that the brain decodes
as sounds
Audition

• The outer ear collects and magnifies sound waves. The

vibrations then enter the middle ear, first vibrating the
tympanic membrane (or eardrum).

• Vibration of the tympanic membrane vibrates the ossicles,

the three tiny bones that comprise the middle ear.
• Malleus (hammer) / Incus/ Stapes
• The last of the three ossicles is the stapes, which vibrates
against the oval window.
• The oval window is the beginning of the inner ear, the
membrane-covered opening of the cochlea. It vibrates
when it receives the sound waves and causes the fluid
inside the cochlea to move
Audition

• The vibrations further jiggle the cochlea.

• Within the cochlea are receptor cells, known as hair cells,
so named for their hair-like cilia which move in response to
the vibrations.
• The hair cells line a structure in the cochlea called the
basilar membrane.
• => The motion of the sound vibration against the oval
window of the
cochlea causes ripples in the basilar membrane, bending
the hair cells lining its surface
Audition

• From the cochlea, sound energy is transferred to the

auditory nerve and then to the temporal lobe of the
auditory cortex
Audition

• . The inner ear is also responsible

• for balance and contains vestibular sacs, which have
receptors sensitive to tilting.
• Various theories have been suggested for how hearing occurs.
• Current thinking relies on the work of Georg von Békésy, which
asserts that a traveling wave energizes the basilar membrane
(membrane within the cochlea).
• As frequencies get higher, so do the peaks of the traveling wave,
increasing the stimulation of the receptors for hearing.

Þ Place theory = sound waves generate activity at

different places along the basilar membrane.
we hear different pitches because different
sound waves trigger activity at different
places along the cochlea’s basilar
membrane.
Thus, the brain determines a sound’s pitch
by recognizing the specific area (on the
membrane) that is generating the neural
signal.
Audition

• This accounts for recognition of sound above 150 Hz.

However, humans can hear from 20 to 20,000 Hz.
• The volley principle— which states that receptor cells fire
alternatively, increasing their firing capacity—appears to
account for the reception of sound in the lower ranges =>
neurons fire in succession so fast that they can create a
combined frequency above 1000 waves/ second.

Þ Frequency theory (or temporal theory) in hearing states

that we sense pitch because the rate of neural impulses
is equal to the frequency of a particular sound.

the brain reads pitch by monitoring the

frequency of neural impulses traveling up
the auditory nerve
Audition

• Place theory best explains how we sense high pitches.

• Frequency theory, extended by the volley principle, also
explains how we sense low pitches.
• Finally, some combination of place and frequency
theories likely explains how we sense pitches in the
intermediate range.

• Deafness can occur from damage to the ear structure or

the neural pathway.
• Conductive deafness refers to injury to the outer or middle
ear structures, such as the eardrum. Impairment of some
structure or structures from the cochlea to the auditory
cortex results in sensorineural, or nerve, deafness.
• Conductive deafness and milder forms of sensorineural
deafness may be addressed successfully with hearing aids.
However, profound sensorineural hearing loss may require
a cochlear implant, which stimulates the auditory nerve
directly.
• Olfaction (smell) is a chemical sense.
• Scent molecules reach the olfactory epithelium, deep in the
nasal cavity.
• The scent molecules contact receptor cells at this
location.
• Axons from these receptors project directly to the
olfactory bulbs of the brain.
• From there, information travels to the olfactory cortex and the
limbic system. Because the amygdala and hippocampus connect
to olfactory nerves, it is easy to understand why certain smells
trigger memories.

The sense of smell (olfaction) is the only one of

the five
senses that does not pass neural information
through the
thalamus.
Gustation

• Gustation (taste) is also a chemical sense. The tongue is

coated with small protrusions known as papillae. Located on
the papillae are the taste buds, the receptors for gustatory
information.
Gustation

• There are five basic tastes:

• sweet, salty, bitter, sour, and umami (savory).
• These five tastes may have evolved for specific reasons. For
example, sweetness, which we tend to like, is often
accompanied by calories. Most poisonous plants, in
contrast, taste bitter, a taste we generally do not like.
Gustation
• Information from the taste buds travels to the medulla
oblongata and then to the pons and the thalamus. This
information is then relayed to the gustatory areas of the
cerebral cortex, as well as the hypothalamus and limbic
system.
Touch

• The skin has cutaneous and tactile receptors that provide

information about pressure, pain, and temperature.

Just remember nociceptors: receptors for pain

Touch

• The receptor cells sensitive to pressure and movement are fast-

conducting myelinated neurons, which send information to the
spinal cord.
• From here, the information goes to the medulla oblongata, the
thalamus, and finally, to the somatosensory cortex.
• Pain information is sent via two types of neurons;
• C fibers are unmyelinated and responsible for the throbbing sense
of chronic pain,
• while myelinated A-delta fibers send information about acute pain.
The pain signal first reaches the spinal cord and triggers the release
of “substance P” (a neuropeptide, or chemical signal similar to a
neurotransmitter, that alerts the spinal cord to the presence of a
painful stimulus). The signal then travels to the thalamus and to
the cingulate cortex, which is responsible for attention. Once pain
is perceived, the brain begins to reduce the intensity of the signal
through a process known as “pain-gating.” A signal is sent from the
brain to opiate receptors in the spinal cord, which reduces the
sensation of pain. This information projects to the limbic system
and then to the somatosensory cortex.
• The receptor cells for temperature can be divided into cold fibers,
which fire in response to cold stimuli, and warm fibers, which are
sensitive to warm stimuli.
Pain
Our experience of pain reflects both bottom-up sensations and top-down
cognition : Pain is a biopsychosocial event
Biology
Sensory receptors called nociceptors—mostly in your skin, but also in your
muscles and organs—detect hurtful temperatures, pressure, or chemicals.
• Sensory receptors (nociceptors) respond to potentially damaging stimuli by
sending an impulse to the spinal cord, which passes the message to the
brain, which interprets the signal as pain
• The gate-control theory states that the spinal cord contains a neurological
“gate” that blocks pain signals or allows them to pass on to the brain.
• The “gate” is opened by the activity of pain signals traveling up small nerve
fibers and is closed by activity in larger fibers (such as massage) or by
information coming from the brain (such as distracting thoughts).
Psychology
• Pain is impacted by how much attention we give to it. If we distract our
minds with other thoughts, the pain feels as if it has diminished.
• Our memories of pain may be edited from the actual pain we felt.
Socio-cultural
People overlook a pain’s duration and recall two moments: pain’s peak
moment and how much pain is felt at the end.
We tend to perceive more pain when others seem to be experiencing pain
• We get cues on how to perceive pain from our culture’s views on pain.
Balance

• Other senses include the vestibular sense, which provides

the sensation of balance. This sense is located in the
semicircular canals of the inner ear.

• The vestibular organs are fluid-filled and have hair cells, similar to
the ones found in the auditory system, which respond to
movement of the head and gravitational forces. When these hair
cells are stimulated, they send signals to the brain via the vestibular
nerve. Although we may not be consciously aware of our vestibular
system’s sensory information under normal circumstances, its
importance is apparent when we experience motion sickness
and/or dizziness related to infections of the inner ear
Position and Movement

• Proprioception is the sense of the relative positioning of

neighboring parts of the body, and the sense of the strength of
effort needed for movement.
• Kinesthesia is the perception of the body’s movement using
sensory organs, which are known as proprioceptors, in joints and
muscles

Þ Receptors for kinesthesia are proprioceptors

Þ Proprioception relies on mechanoreceptors located in deep
tissues such as muscles and tendons. However, low-threshold
mechanoreceptors located in the skin and hair follicles may
also contribute to proprioception, in addition to touch
• Synesthesia is a neurological condition in which
stimulation of one sense leads to automatic activation of
another sense;

• for example, one might “hear” colors.