CHAPTER 4: SENSATION AND PERCEPTION
On and Off Sensation
-Beyond the 5 senses: smell, taste, touch, sound, sight, we have our kinaesthetic/ proprioceptive sense, ie.
Our understanding of where we & our body are located in space which stops working when we are drunk
preventing us from touching our nose w/ our eyes closed. Our vestibular sense/system provides us w/
information about our movement through space which stops working if we are on a ship that is rolling &
pitching on rough waters. Cutaneous senses/somatosenses is basically the touch sense incl pressure,
vibration, pain, temperature & position.
-We use our senses in 2 related processes: sensation (the act of using our sensory systems to detect stimuli
present in environment) & perception (once acquired sensory info is interpreted in the context of past &
present stimuli; also involves recognition & identification of a sensory stimulus)
Common Features of Sensation & Perception
-Each of the senses has a set of specialized sensory receptor cells which convert a specific form of
environmental stimuli into neural stimuli. Sensory transduction is the form of communication used in our
brains & the nervous system which is the process of converting the specific environmental stimuli to neural
-The limits of the senses: Tresholds:
-The absolute threshold is the minimal stimulus necessary for detection by an individual. Absolute
thresholds vary from person to person but in most cases are small. The difference threshold or just
noticeable diff (JND) is the minimal diff b/w 2 stimuli necessary for detection of a diff b/w the 2. Eg. How
much do you turn up the radio to notice it’s louder? JND depends on how loud it was in the 1 place.
Weber’s law states that JND of a stimulus is a constant proportion of the stimulus intensity, ie. The stronger
the stimulus the larger the change has to be.
Sensory System Physical stimuli Sense Absolute threshold
Olfactory (smell) Odorants (airborne Smell A drop of perfume diffused through a
chemicals) 6-rm apartment
Somatosensory (touch, Pressure or damage to Touch An insect’s wing falling on you
heat, pain) the skin cheek from a height of ~1cm.
Gustatory (taste) Chemicals (typically in Taste 5 mL of sugar in 9L of H2O
Auditory (hearing) Sound waves Hearing The tick of a watch 6 m away in a
Visual (sight) Light (photons) Sight A candle flame 50 km away on a
clear, dark night
-Surrounded by stimuli: Sensory Adaptation:
-Our senses are generally organized to detect change which makes sense b/c most stimuli were
exposed to aren’t important enough to warrant our attention. To combat the possibility of being unable to
focus on salient/important cues, sensory adaptation occurs which is the process whereby repeated/continued
stimulation of a sensory cell leads to a reduced sensory response. Eg. pressure from our clothing on our
skin, sensory cells respond less & less. All sensory systems exhibit some form of adaptation but sense of
smell seems esp prone to this response. Our ability to detect odours gradually fades away when we are in
their presence for a prolonged period but sensory adaptation can be overcome by providing a much stronger
-Processing Sensory Info: sensation & perception almost always happen together. Perception can occur via
bottom-up processing which begins w/ physical stimuli from the environment & proceeds through
transduction of those stimuli into neural impulses and then move onto successively more complex brain
regions. Top-down processing is perception processes led by cognitive processes, eg. memory/expectations,
ie previously acquired knowledge influences perception. In other words, bottom-up processing is driven by
incoming sensory info that the world provides vs. top-down processing is affected by our knowledge &
-Our perceptual expectations/perceptual sets prepares us to perceive certain things in particular
ways, so it is the readiness to interpret a certain stimulus in a certain way. Aside from our vision, our
experiences, cultures, contexts, etc. affect our perceptual sets. Perceptual sets may come into play when we
are faced w/ ambiguous stimuli—those that can be interpreted in diff ways, eg. Fig 4-2, woman & witch. Typically perception involves both bottom-up & top-down processing occurring simultaneously which lets
us rapidly recognize familiar faces, familiar songs, smell/taste of a familiar food, etc.
Before You Go On
1. What is sensory transduction?
2. What is the absolute threshold & what is the JND?
3. Compare & contrast bottom-up & top-down processing?
The Chemical Senses: Smell & Taste
-Smell & taste are usually called chemical senses b/c they involve responses to particular chemicals. Smell,
our olfactory sense & taste, our gustatory sense emerged early in our evolutionary history; smell is more
sensitive & more important in less complex animals who use it for finding food & avoiding predators. Still,
sense of taste & smell has a great contribution to safety, social communication & overall quality of life
though is often underestimated.
-Smell & Taste: How they work
-Smells around us: sensation in the olfactory system begins when odorants enter the nose, they are
then converted to neural signals at sensory receptors (in a lock & key fashion) in our nasal mucosa; these
receptors are on the cilia of olfactory receptor neurons (sensory receptor cells that convert chemical signals
from odorants into neural impulses that ravel to the brain). Only certain airborne chemicals bind to specific
receptors & when enough of the odorants have bound to receptors, it sets off an action potential in the
olfactory receptor neuron which sends the message to other neurons & in turn the brain. Continuous
binding of odorants will fatigue the olfactory receptor neurons to which they bind, ie. Cells will stop
responding to odorant unless its given a chance to recover or fire again or if the stimulus is inc in
magnitude. Eg. restaurant example.
-Types of Taste: in humans, the sense of taste & smell are closely tied together and what
constitutes a flavour. Taste is itself independent of smell. Papillae are the bumps on the tongue that contain
clumps of taste buds (clusters of sensory receptor cells—60 to 100 each—that convert chemical signals
from food into neural impulses that travel to the brain). Taste receptor cells contain cilia that contain the
actual receptors; cilia extends through the pores of the taste receptor & are exposed to the contents of your
mouth. 5 major kinds of taste receptors: sweet, sour, bitter, salty & umami (taste of MSG). Each of these
receptors uses a slightly diff mechanisms for transduction of chemicals in food to neural impulses, eg. salt
activates taste receptors by sending Na+ into channels on taste receptor cells & since they are + change, the
electrical charge of the taste receptor becomes more +. Taste buds are not evenly distributed across the
tongue but most tastes can be recognized to a greater/lesser degree on most parts of the top of the tongue.
-Eating: It’s more than Smell & Taste: the overall sensations we experience when eating food
doesn’t only depend on flavour, much of the info we get about food is delivered via tactile senses. The
consistency of a food is not relayed to the brain via taste receptors but by inputs from touch receptors in the
tongue. Also, the sensation we get from eating a spicy meal comes from a component of the tactile system
that communicates info about pain. Capsaicin, from chilli peppers activates pain receptors located on the
tongue & these pain impulses w/ tactile info about food & flavour assoc’d w/ food can combine to produce
the spicyness pleasurable to many people. When you burn your tongue from eating hot foods, you might
notice the next day your ability to taste has returned & this is b/c of the remarkable regenerative
characteristics of your taste buds. Taste receptors generally turnover in matter of days & happens even
faster when damaged. Under normal circumstances our olfactory receptor neurons are also constantly
turning over & this fast turn over is important b/c both taste & smell are constantly exposed to external
-Smell & Taste: What happens in the Brain?
-Signals from our olfactory receptor neurons travel to the brain via olfactory nerve & then info
carried there travels to the olfactory bulb which is the 1 region where olfactory info reaches the brain on
its way from the nose. Then message is sent to regions of the cerebral cortex important for recognizing or
discriminating among odours, incl. the piriform cortex, a part of the olfactory cortex. The ability of our
cortex to recognize patterns of inputs from various olfactory receptors is most likely responsible for our
detection of certain odours. Studies have shown that the piriform cortex is plastic/ changeable in adulthood,
ie. Parts of it that normally recognize specific odorants can change w/ experience, remapping this brain
region. Some molecules are so closely related that untrained humans can’t discriminate them (2 are just
below JND) but if exposure to one is paired w/ shock, humans can be taught to discriminate b/w the odours.
A great example of top-down processing; learning about assoc’ns b/w odours & other experiences can influence our ability to perceive sensory info in the future. The areas of the piriform cortext that are
activated by each of the prev indistinguishable molecules may also become more distinct from each other.
-The olfactory bulb also sends info to the amygdala (which is important for emotions & fears) &
indirectly to the hippocampus (important for learning & memory). Many ppl report some smells remind
them of past events & this ability of smells to call up memories is prob related in part to the olfactory
connections in the amygdala & hippocampus. Taste receptor cells don’t have axons b/c they’re not neurons,
instead they connect w/ sensory neurons in the tongue to send info to our brains via thalamus & eventually
the cerebral cortex. The thalamus is a relay station for many incoming sensory info w/ the exception of the
olfactory system. Taste info is integrated w/ reward circuits in the brain & rewarding tastes seem to be
processed separately from aversive tastes. Rewarding tastes, eg. salty & sweet, activate overlapping areas in
the taste cortex vs. aversive tastes, eg. bitter & sour, activate regions that overlap less w/ rewarding tastes &
more w/ one another in the taste cortex. Taste & smell info is processed through separate pathways but
converge in the prefrontal cortex. The insula of the prefrontal cortex is assoc’d w/ emotion of disgust which
becomes activated when we smell/taste something bad & if we view repulsive images.
Tying it Together: Your Brain & Behaviour: Eating Pizza
-Photoreceptors in the eye transmit info about the appearance of the pizza to the brain via the optic nerve,
passing through the brainstem, then thalamus & then the visual cortex. Taste receptor cells & sensory cells
that respond to touch & temp are activated on the tonge; these nerves carry impulses to the brain by passing
through the brainstem thalamussensory cortex (gustatory cortex & somatosensory cortex). Taste &
smell produces flavour. Olfactory receptor neurons transducer pizza odorants & send info on to the
olfactory bulb bypassing the thalamus. Info about taste, smell, texture, temp & appearance is integrated in
assoc’n regions of the neocortex & theses + memories related to pizza=your perception of the pizza slice.
-Somatosensory cortex=a large part of somatosensory cortex is devoted to processing info about
texture, temp, pain from the tongue & this info is critical for enjoyment of food.
-Olfactory cortex=when you eat something delicious & close your eyes, you may be maximizing
the experience by inc activity in certain parts of the cortex, incl taste & smell cortex; when your eyes are
open, activity in parts of the cortex serving nonvisual senses dec.
-Burning your tongue=taste buds contain taste receptor cells that continually regenerate; quicker if
-Smell & Taste: How We Develop?
-The sense of smell is relatively well-developed at birth & seems to be in place even before birth.
Newborns are capable of distinguishing their mother & showed a learned preference to the odours of their
mom’s amniotic fluid. After birth, they quickly learn to recognize the smell of their own mother’s milk
which has a calming effect on infants when they are experiencing brief, minor painful stimulus; exposure to
vanillin also has the same calming effect as the mom’s milk odour. Ability to taste is also well-formed at
birth. Newborns show an innate preference for sugar & aversion to bitter/sour tastes. Researchers have
shown than by ~7yrs old, children develop a preference for sour but aversion to bitter tastes remain until
young adulthood. Many of these developmental changes are due to learning but there is evidence
suggesting that the gustatory system itself changes overtime. We have a higher concentration of taste buds
as newborns & it gradually dec w/ time; children have taste buds on their palates, inside cheeks & back of
mouths. This high # of taste buds may explain why some children are picky eaters b/c the more the taste
buds, the more neural impulses formed compared to if adults ate the same food. Some researchers suggest
this may be adaptive in helping us survive to avoid risk of poisoning. It’s likely that inc exposure to certain
foods esp when paired w/ pos social interactions & encouragement from parents result in remapping of prev
aversive taste info on the gustatory cortex.
-Smell & Taste: How We Differ?
-Human vary greatly in their ability to detect certain odours, some more insensitive & some are
sensitive; these individual diff may be related to learning. Exposure to particular odours during childhood
lessens rxn to those odours later on. Research suggests that females are more sensitive to smell than males
& this sensitivity varies w/ the stage of the menstrual cycle; around time of ovulation, women are more
sensitive to odours than during stages of cycle & their ability to detect diff odours also diminishes after
menopause. The exact biological mechanisms of these are not known but it may be b/c of reproductive
hormones, eg. estrogen which alter the excitability of firing of olfactory neurons. Researchers group people
into 3 groups based on taste sensitivity: non-tasters (25%), medium tasters (50%) & supertasters (25%);
they are distinguished based on their ability to detect & respond negatively to a specific bitter substance. Supertasters feel the more repulsed or disgusted by the bitter chemical & these functional diff are due to
variations of the concentration of taste buds in the tongue. There are more women supertasters than men.
This heightened sensitivity to both smell & taste is likely to have adaptive significance in women; since the
chemicals in women’s diet are passed on to their children when they are pregnant/nursing, the ability to
detect & avoid pot harmful odours & tastes may have contributed to the survival of the species by
protecting infants from toxicants. The number of taste buds starts to decline at ~50yrs & sense of smell at
~60yrs resulting in loss of interest in food which can be counteracted by adding spices & modified food
-Smell & Taste Disorders: When Things Go Wrong
-True taste disorders are rare. Most of those who complain that they cannot taste are actually
suffering from problems with their olfactory vs. gustatory. Anosmia is the inability to smell. They can still
taste sweet, sour, bitter, salty, umami but can’t detect other flavours b/c that req additional info provided by
food odorants. Ageusia is the inability to taste; they can still taste sweet, sour, bitter, salty & umami but
can’t detect flavour requiring odorants from food; usually results from head trauma or problems during oral
surgery. Head trauma is also the leading cause of anosmia; sometimes the nerves carrying olfactory info
from the olfactory receptor neurons to the olfactory bulb are sheered, cutting of that pathway. People w/
Alzheimer’s also suffer from diminished sense of smell maybe due to degeneration of olfactory receptor
neurons & neurons located in the olfactory bulb. Although, humans can survive w/o sense of smell, their
quality of life greatly decreases.
-Migraines, Epilepsy & Sensory systems: A specific odour can initiate a migraine. Patients w/
reflex epilepsy will experience a seizure only after exposure to a specific odour. This also applies to if they
are exposed to stimuli from other sensory systems, eg. touch, sound, light. Some people experience
hallucinations called auras before/during migraines, headaches or epileptic seizures; they involve any of the
sensory systems, eg. strange light or unpleasant smells. The involvement of diff senses indicates which
brain circuits are compromised in these conditions, eg. unpleasant smell aura may indicate compromised
Before You Go On
4. What five tastes have specific recpetors?
5. Which parts of the brain are involved in sensing/perceiving odours?
6. What are supertasters?
7. How are smell & taste involved w/ migraines & epileptic seizures?
Tying it Together Ques
1. Explain how multiple senses are involved in our eating experiences.
2. Explain how the taste of food may be enhanced when closing our eyes.
3. Which areas of the brain are responsible for integrating info about various components of eating?
The Tactile or Cutaneous Sense: Touch, Pressure, Pain & Vibration
-As with the chemical senses, there are rewarding & aversive types of tactile stimuli. Our skin contains
various sensory receptors to register diff types of physical stimuli:
Pain & Temperature Fine touch & pressure
-Free nerve endings: located mostly near the surface -Meissner’s corpuscle: transduce info about
of the sin & function to detect touch, pressure, pain sensitive touch & are found in hairless regions of
& temperature. Free nerve endings for pain are the body, eg. fingertips, lips & palms.
located near the surface of skin (sharp/dull) pain & -Merkel’s disc: transduce info about light to
free nerve endings for temp are located deeper near moderate pressure on the skin
adipose tissue layer (heat/cold) -Ruffini’s end organ: located deep in the skin &
they register heavy pressure & movement of the
-Hair receptors: flutter/steady skin indentation
-Pacinian corpuscle: are also buried deep w/in the
skin & respond o vibrations & heavy pressure.
-Pressure on the skin activates free nerve endings that gives us a sense of being touched. Certain parts of
the body are more sensitive, eg. the elbow vs, face hands maybe b/c of diff densities of nerve endings; areas
that are more sensitive have more free nerve endings. We can also experience sensory adaptation resulting
in reduced tactile sensation from depression of the skin that continues for a prolonged period, eg. dressing
up. -Tactile Senses: What Happens in The Brain?
-Our brain uses various related processes to help us perceive general info about non-painful touch
sensations, incl. pressure, temp, touch but pain perception is also an important function.
-The touching brain: when we touch something or something touched us, our free nerve endings
send tactile info into the spinal cord & then signal travels up the spinal cord & into the brain where touch
info is 1 received by the thalamus & then routed from there to the somatosensory cortex (in the parietal
lobe). Our brain processes contralaterally so if you touched something in your left hand, info is eventually
processed by the right somatosensory cortex. The somatosensory cortex doesn’t have equal representation
of all parts of the body, eg. greater proportion dedicated to the hand which makes sense b/c its specialized
in object manipulation & we need to process info in great detail. Other animals have somatosensory
systems that are adapted to provide high-resolution tactile info from the parts of their bodies important to
their daily lives, eg. star-nosed mole, ie. The nose representation in their somatosensory cortex is
proportionately larger vs. its other parts. Info about pressure & vibration in animals & humans are generally
transmitted in a similar way after being converted to neural impulses by special receptors.
-Pain & the brain: painful sensations are also transmitted to the brain via free nerve endings. Pain
travels to the brain via 1) fast pathways: uses more myelinated axons to carry signals; msgs about sharp,
localized pain travel along here directly up the spinal cord, into the thalamus & into the somatosensory
cortex; this allows us to respond quickly w/ a withdrawal cortex. 2) slow pathways: uses more
unmyelinated axons—these inputs communicate w/ brain regions involved in processing emotions, eg.
burning pain. The pain system also shows evidence of sensory adaptation, eg. spicy foods. The sensation
from eating chilli peppers is mostly due to activation of pain fibers on the tongue; when spicy foods are 1
ingested, the pain response seems great but gradually diminishes as the meal progresses & this is due to
adaptation of the pain fibers & a subsequent dec in their activity. But when pain is assoc’d w/ actual tissue
damage or abnormality in the pain system, pain can be persistent & debilitating.
-Tactile Senses: How We Develop?
-Tactile senses are also generally in place at birth. Studies shows that fetuses can respond to touch
of a hair at relatively early stages in prenatal development but the ability to recognize/respond to diff
somatosensory stimuli only occurs after birth & involves further brain development & learning. For
children, one of the most enjoyable types of somatosensory input is tickling. The rxn we have to tickling is
bc of activation of the somatosensory pathway in an uneven, uncontrollable & unexpected manner; the
sensory systems are organized to detect change but also most-tuned to surprising & unexpected stimuli.
When you move your body or produce tactile sensations yourself, it is less likely noticeable than if it was
produced by another individual, like if a cat jumps onto your lap while you have your eyes closed, your rxn
would greater than if your eyes were open. This differential response to surprising stimuli appears to be a
defense mechanism that has adaptive significance which is maybe why being tickled by someone else
illicits a more effective emotional rxn than if you tickled yourself. Our enjoyment of being tickled dec w/
age maybe b/c adults are better at anticipating stimuli & thus are more difficult to surprise or may reflect a
change in tactile thresholds (becoming less sensitive) w/ age.
-Tactile Senses: How We Differ?
-Humans differ greatly in their ability to detect physical stimuli on the skin & to the degree to
which they find tactile stimulation pleasurable or aversive. Pain management for surgical procedures &
other medical conditions received the most attention. There are dramatic diff in both the threshold to detect
pain & degree to which it causes emotional suffering, eg. studies have shown Japanese ppl have lower pain
threshold than Caucasions; this also applies to detection of non-painful stimuli. Learning plays some role
but groups of ppl also differ in the actual sensation & perception of pain due to physical differences.
Studies have shown women have a lower threshold for detecting pain than men, they report greater
intensity of pain to the same stimulus & one interpretation of this is that women weren’t “toughened up”
but in fact, research suggests that women may have ~2x as much pain receptors in their facial skin than
men but it is not yet known whether this diff applies to diff parts of the body. People exposed to high temp
stimulus exhibited varied responses: those who reported feeling pain showed changes in activity of
thalamus, somatosensory cortex & cingulate cortex & those who didn’t feel pain, showed similar activity in
thalamus but not the cortical regions. These suggest that different activation of brain circuitry may also
underlie varied responses to pain stimuli. The gate control theory of pain is the theory that certain patterns
of neural activity can close a “gate” to keep pain info from travelling to parts of the brain where it is
perceived. -Tactile Senses: When Things Go Wrong
-Chronic Pain: pain that lasts longer than 3 months. Research indicates that ~1/6 of the popn
suffers from chronic pain; in all cases, pain management is critical since prolonged pain can interfere w/
normal functioning & may lead to depression/suicide. Researchers have identified 2 naturally produced
chemicals by our nervous system that have pain-relieving properties: endorphins & enkephalins. They are
classed as opiates which incl pain-killing drugs,e g. morphine, heroines, etc. When opiates are naturally
present in the nervous system, they are referred to as endogenous opiates which are released by neurons
after PA, stress & sexual experience (runner’s high). Doctors use opiate drugs that mimic thes for pain
relief but it has become problematic b/c ppl get addicted; effectiveness of these drugs diminishes w/
continual use thus ppl need to consume higher & higher doses which is dangerous b/c opiates can suppress
breathing until it stops & leads to death. In extreme cases of chronic pain, physicians turn to neurosurgery;
destroying the pathways that carry the pain info to the brain can be effective for some ppl. Cingulotomy is
an extreme form of neurosurgery to relieve intractable chronic pain destroying the cingulate cortex
-*Quick ways to reduce acute pain: Gate control theory suggests that touch sensations, which freq
travel along fast fibers, can help prevent some pain sensations from travelling on the slow pathways form
reaching areas of the brain where pain is perceived. According to the theory, the brain holds only so much
input so touch can help set up a gate that stops the pain. To reduce acute pain, we can rub the areas that
have been injured, focus on breathing (Lamaze method for childbirth), active distraction, stress & sexual
-No pain: some are incapable of detecting painful stimuli; though sounds promising at 1 , ourt