Sensation is the simple stimulation of a sense organ. It is the basic registration of light, sound, pressure, odor, or taste. For our purposes it stops at the thalamus, either of two masses of gray matter lying between the cerebral hemispheres on either side of the third ventricle, relaying sensory information and acting as a center for pain perception.
Perception occurs in your brain as sensation is registered there. It is the organization, identification, and interpretation of a sensation in order to form a mental representation.
Systems approach: remember that the key element of our eyes--the retina--is an extrusion of brain matter, this lecture looks at how physical energy in the world around us is encoded by our senses, sent to the brain, and enters conscious awareness.
Transduction occurs when sense receptors convert physical signals from the environment into neural signals that are sent to the central nervous system. An example of transduction: vibrations from a guitar string cause changes in air pressure that propagates through space to the listener's ears. Generally, in touch, the pressure of a surface against the skin signals its shape, texture and temperature.
Sensory adaptation is the process whereby sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current and unchanging conditions. Our sensory systems respond more strongly to changes in stimulation than to constant stimulation; this is another example of how our nervous system perceives only a subset of what is 'out there'.
Psychophysics methods that systematically relate the physical characteristics of a stimulus with an observer's perception. In psychophysics, The simplest quantitative measurement is the absolute threshold (or boundary), the minimum intensity needed to just barely detect a stimulus in 50% of the trials. That 50% value is not arbitrary; it comes from the statistics of the normal curve.
The shape of the curve from hearing to not hearing is gradual rather than abrupt. The just noticeable difference (jnd) is the minimum change in a stimulus that just barely be perceived (note the lack of mechanical exactitude). The jnd is not a fixed quantity, rather it depends on how intense the stimuli being measured are and on the particular sense being measured. If I want to go from a 100 Kg bench press to a 100.01 Kg bench press (a twoney on each end of the bar, perhaps), I won't notice the difference. This is an example of Weber's Law: for every sense domain, the change in stimulus that is just noticeable is a constant proportion, despite variations in intensities.
Signal detection theory is a way of analyzing data from psychophysics that measure the individual's perceptual sensitivity. If the sensory evidence exceeds the decision criterion, the person responds. Signal detecton theory offers a practical way to choose among criteria that permit decisions makers to take into account the consequences of hits, misses, false alarms, and correct rejections.
Let's apply this theory in the real world, but looking at multitasking and selective attention.
By one estimate, using a cell phone will driving makes having an accident four times more likely. (McEvoy et al., 2005).
Why is driving and using a smartphone at the same time so dangerous? Shomstein & Yantis,
(2004) have found that when attention is directed to hearing, activity in visual areas
decreases. Strayer, Drews & Johnson, (2003) found that even experienced drivers react more slowly
during phone conversations than during other tasks. Why? Because the conversation
requires memory retrieval, deliberation and planning. Slower breaking response translates
into increased stopping distance. Whether the phone is hand-held or free makes little
difference. The problem is that while attention is directed to hearing, activity in the visual areas decreases. (Shomstein & Yantis, 2004).
Text-messaging is even worse. Hosking, Young & Regan (2009) report that compared with a
no-texting control condition, when either sending or receiving a text message in the
simulator, drivers spent dramatically less time looking at the road, had a much harder time
staying in their lane, missed numerous lane changes, and had greater difficulty maintaining
an appropriate distance behind the car ahead of them.
Pascal-Ferra, Liu & Beatty (2012) concluded that the impaired effect of texting while driving is
comparable to that of alcohol consumption and greater than that of smoking marijuana.
All this is because of our limited selective attention, focusing on only what it relevant to a
particular person at a particular moment.
Our technology isn't all bad. Video game players not only exhibit better selective attention over space, they also exhibit enhanced selective attention to objects. For example, VGPs can track a greater
number of dynamic, moving objects as compared to NVGPs
https://www.sciencedirect.com/science/article/pii/S004269891100287
The fovea is the area of greatest visual acuity, where most colour-sensitive cones are located.
Fine detail is correlated to colour. Note that each cone has its own pathway from bipolar to cone cell
to the retinal ganglion cells, while rods are clustered together .
Six million cones detail colour, operate under daylight conditions, and allow us to see fine
details; rods become active under low-light conditions for night vision.
Because rods contain the same photopigment, they provide no information about colour and
only respond to shades of gray.
There are no rod cells in the central area of the human fovea.
Rods are completely absent in the central 300-350 micrometers of the fovea but are highly concentrated in a ring outside of this area. The distribution of cones directly affects visual acuity and why peripheral objects are not so clear.
The bipolar cells collect neural signals from the rods & cones,transmitting them to the
outermost layers of the retina, where retinal ganglion cells organize the signals and send them
to the brain. These bundles form the optic nerve; the area of exit is called the blind spot, the location in the visual field that produces no sensation on the retina. With both eyes open, the blind spots are not perceived because the visual fields of the two eyes overlap.
Cones are responsible for colour vision, and come in three types, red, green and blue.
Colour perception arises from the combination of these three basic elements in the retina.
Note that in mixing light, not pigments, all three together produce white, not black.
A genetic disorder where one of the cone types is missing results in a colour deficiency.
Cones tire out after firing, and this creates an afterimage of the opposed colour. Blue-
sensitive cells work against yellow-sensitive; green-sensitive against red-sensitive.
Half of the axons in the optic nerve code information to the right visual field, whereas half
code to the left visual field.
The optic nerve travels from each eye to the lateral geniculate nucleus located in the
thalamus. From there the signal travels to the V1 area of the occipital lobe, the primary
visual cortex.
Area V1 is specialized for encoding edge orientation. It contains populations of neurons
each tuned to respond to edges oriented at each position of the visual field.
Visual Streaming
The ventral stream travels across the occipital lobe into the lower levels of the temporal
lobe, including brain areas that represent an object's shape and identity.
The dorsal stream travels up from the occipital lobe to the parietal lobe (including some of
the middle and upper levels of the temporal lobes) connecting with brain areas that identify the location and motion of an object. Because the dorsal stream is crucial for guiding actions, such as aiming, reaching, or tracking with the eyes, the 'where' pathway might be more appropriately called the 'perception for action' pathway.
The two pathways were discovered when studying visual form agnosia. When
researches asked patient D.F. to orient her hand to match the angle of the slot in the
testing apparatus, she was unable to comply. However, when asked to insert a card into
the slot at various angles, she accomplished the task perfectly.
Conversely, damage to the parietal lobe (a section of the dorsal stream) causes difficulty
using vision to guide reaching and grasping, though they can recognize what objects are.
recent fMRI research indicates that some regions within the dorsal stream are sensitive
to properties of an object's identity, responding differently to line drawings of the same
object in different sizes or viewed from different perspectives. This may be how the
ventral and dorsal streams exchange information about what and where.
The question of the binding problem is still open. Why do we see unified, whole objects in
stead of disconnected details? Eguchi et. al (2018) suggests that there are binding
neurons somewhere in our visual corteces.
The neurons in the ventral visual stream that receive simultaneous input from other neurons involved in representing different features of an object. They participate in a larger array of neurons that signal a whole object by engaging in an intricate and repeatable dance of action potentials.
Perceptual Mistakes
The binding problem: how features are linked together so that we see unified objects in
our visual experience rather than free-floating or uncombined features.
Illusory Conjunctions occur whenever features such as colour or shape from
multiple objects are combined incorrectly. These illusions look real to the participants,
who were just as confident they had seen them as they were about the actual coloured
letters they perceived correctly.
Illusory conjunctions are theorized to occur because of feature integration. Focused
attention is not required to detect the individual features that comprise a stimulus, such
as colour, shape, size & location, but is required to bind those individual features
together.
In the experiment being considered, participants were required to process the digits that
flank the coloured letters, thereby reducing attention to the letters and allowing illusory
conjunctions to occur.
Because binding involves linking together features processed in distinct parts of the
ventral stream at a particular location, it also depends critically on the parietal lobe in the
dorsal stream.
Recent studies suggest that damage to the upper and posterior portions of the parietal
lobe is likely to produce severe problems attending to spatially distinct objects.
Recognizing Objects by Sight
Why is it that we can recognize the letter 'G' in hundreds of different fonts?
In general object recognition proceeds fairly smoothly, largely due to feature detectors.
Modular view: specialized brain areas detect and represents faces, houses and body
parts. Using fMRI in healthy young patients, a subregion in the temporal lobe was found that
responds most strongly to faces, whereas a nearby area to buildings and landscapes.
Distributed view: a pattern of activity across multiple brain regions identifies a viewed
object. Both views are necessary for a full understanding.
Quiroga et. al. (2005): electrodes were placed in the temporal lobes of epileptics. These
volunteers were then shown pictures of faces and objects, and their neural responses
were recorded.
Neurons in the temporal lobe responded to specific objects viewed from multiple angles,
and people wearing different clothing and facial expressions.
Neurons also responded to words corresponding to specific objects. This led to the
conclusion of perceptual constancy: even as aspects of sensory signals change,
perception remains constant.
Biological motion perception is our ability to perceive biological motion critical for identifying individuals and various socially relevant features, such as their emotional state, personality characteristics, whether they are vulnerable to attack, or engaging in deceptive actions. fMRI studies have revealed that a region in the back of the temporal lobe the posterior superior temporal sulcus, is strongly linked to biological motion perception.
Psy 120.3 Lecture 2025/10/01: Audition and Other Senses
Audition
Different forms of energy require different means of transduction. The human ear is divided into three
distinct parts.
The outer ear consists of the visible ear (the pinna), the auditory canal, and the eardrum, which
responds to sound waves gathered by the pinna and channeled into the canal.
The middle ear is a tiny air-filled chamber behind the eardrum, containing the ossicles–the hammer,
anvil and stirrup bones–all of which create a lever that mechanically transmits and intensifies vibrations
from the eardrum to the inner ear.
The inner ear contains the spiral-shaped cochlea, a fluid-filled tube that is the organ of auditory
transduction.
The basilar membrane of the inner ear is the structure that undulates when vibrations from the
ossicles reach the cochlear fluid. Its wave-like movement stimulates thousands of tiny hair cells,
specialized auditory receptor neurons embedded in this membrane.
The hair cells release neurotransmitter molecules, initiating a neural signal in the auditory nerve that
travels to the brain.
There are three physical dimensions of sound waves that correspond to auditory
perception: frequency, amplitude & complexity.
Frequency: perception of pitch, measured in cycles per second or Hertz
Amplitude: perception of loudness, measured in decibels
Complexity: perception of timbre, which allows us to distinguish two sources
with the same pitch and loudness
Sound is converted to neural impulses in the inner ear.
Cochlea: fluid-filled tube containing cells that transduce sound vibrations into
neural impulses
Basilar membrane: structure in the inner ear that moves up and down in time
with vibrations relayed from the ossicles, transmitted through the oval window
Sound causes the basilar membrane to move up and down in a traveling wave; the frequency of the sound determines where on the basilar membrane the wave is at its highest. When the frequency is low, the wide floppy tip of the membrane--the apex--moves the most. When the frequency is high, the narrow stiff end closest to the oval window--the base--moves the most. This action gives rise to the theory of place code: different frequencies stimulate neural signals at specific places along the basilar membrane. The movement of the basilar membrane causes inner hair cells to bend, initiating a neural signal in the auditory nerve. Axons fire the most in the hair cells along the area of the basilar membrane with the most motion. The brain processes the information about which axons are the most active.
How Do We Perceive Pitch?
From the inner ear, action potentials in the auditory nerve to the thalamus and ultimately to an area of the cerebral cortex called area A1, a portion of the temporal lobe that contains the primary auditory cortex.
Auditory areas in the left hemisphere analyze sounds related to language and in the right hemisphere rhythms and music.
There is also evidence that the auditory cortex is composed of two distinct streams, roughly
analogous to the dorsal and ventral streams of the visual system.
Spatial (where) auditory features allow the location of the source of a sound in space, and
are processed by areas toward the caudal (back) part of the auditory cortex.
Non-spatial (what) features, which allow identification of a sound are processed in the
ventral (lower) part of the auditory cortex.
All of hearing, however, depends on temporal aspects.
Neurons in the area A1 respond to simple tones, and successive auditory areas in the brain process sounds of increasing complexity.
The human ear is most sensitive to frequencies around 1000 to 3500 Hz.
The human ear has two mechanisms to encode sound waves, one for high frequencies and one for low frequencies. The second, temporal code, occurs when the brain uses the timing of the action potentials in the auditory nerve to determine the pitch you hear.
Timbre, or the complex texture of the sound you hear, depends on the relative amounts of different frequency components in sound, a mixture of the relative activity of the hair cells across the whole basilar membrane.
For example: Middle 'C' on a grand piano, and on a distorted electric guitar, are the same frequency, but with different timbres.
How do we determine the location of a sound? By using binaural cues. Sounds arrive a little sooner at the closer ear than at the farther ear. The time difference is effective for locating the low-frequency sources of a sound, even when it is only a little off to one side. The high-frequency components are more intense in the closer ear than in the farther one because the listener's head blocks high frequencies. The farther a sound is off to the side, the greater the between-ear difference in the level of these high frequency components.
Hearing Loss
Conductive hearing loss arises because the eardrum and ossicles are damaged to the point that they cannot conduct sound waves effectively to the cochlea. The cochlea itself is normal, making this a mechanical problem with the moving parts of the ear (hammer, anvil, stirrup, eardrum).
Sensorineural hearing loss is caused by damage to the cochlea, the hair cells, or the auditory nerve. It has two main effects: sensitivity decreases as sounds must more intense to be heard; acuity decreases so sounds smear together on the basilar membrane, making voices harder to understand, especially if there are competing sounds in the environment.
Sensorineural hearing loss can be caused by genetic disorders, premature birth, infections, medications, and accumulated damage from exposure to intense sounds.
A cochlear implant may offer some help; it is a device that replaces the function of the hair cells. The external parts of the device include a microphone and a speech processor, worn behind the ear. The implanted parts include a receiver just inside the skull and a thin wire containing electrodes inserted into the cochlea to stimulate the auditory nerve.
What matters is age at implantation.
Sound picked up by the microphone is transformed into electronic signals by the speech
processor; the signal is then transmitted to the implanted receiver which activates the
electrodes in the cochlea.
Infants who have not yet learned to speak are especially vulnerable because they may miss
the critical period for language learning. Without auditory feedback at this time, normal
speech is nearly impossible to achieve. Early use of cochlear implants had been associated
with improved speech and language skills for hearing-impaired children.
Haptic Perception
The tactile receptive field, a small patch of skin that relates information about pain, pressure, texture, pattern, or vibration to a receptor.
Haptic perception is the active exploration of the environment by touching or grasping
objects with our hands. Four types of receptors located under the skin's surface enable us
to sense pressure, texture, pattern, or vibration.
Thermoreceptors are nerve fibers that sense cold and warmth. Each touch receptor responds to stimulation within its receptive field, and the long axons enter the brain via the spinal cord or cranial nerves.
Pain receptors populate all body tissues that feel pain, distributed around muscles, bones and internal organs, as well as the skin. Sensory signals on the body travel to the somatosensory cortex.
High acuity is defined as more of the tactile brain is devoted to parts of the skin surface
where sensitivity to fine spatial detail (acuity) is greatest, such as fingertips and lips.
There is mounting evidence (again from DTI) for a distinction between 'what' and 'where'
pathways in touch analogous to vision and sound. 'What' system provides information about
the properties of surfaces and objects; 'Where' system provides information about a
location in external space that is being touched or a location on the body that is being
stimulated.
FMRI evidence suggests that the 'what' and 'where' touch pathways involve areas in the lower and upper regions of the parietal lobe, respectively.
Tissue damage is also transduced by A-delta fibres, which register the initial sharp pain, and the C-fibres which transmit the longer-lasting duller pain.
Perceiving Pain
There are two pain pathways: one pathway sends signals to the somatosensory cortex, identifying where the pain is occurring and what sort of pain it is.
The second pain pathway sends signals to the emotional and motivational centres of the brain, such as the hippocampus and the amygdala, as well as to the frontal lobe.
fMRI evidence shows us that we respond to others' pain, particularly in our frontal lobes.
Referred pain occurs when sensory information from the internal and external areas converges on the same nerve cells in the spinal cord. An interesting side note: pain in the jaw, toothache or a headache: The agony of experiencing a heart attack can spread down to both arms, to the jaw, head or to the back. Some people have reported tooth pain or a headache as a major symptom of a heart attack.
Gate-Control Theory
Pain intensity cannot always be predicted from the extent of the injury; pull out a nose hair right now and see for yourself.
A more accurate predictor is the amount of neural 'real estate' in the somatosensory cortex dedicated to that body part. That is why damage to the torso and legs is not as painful as damage to the lips or nose.
Gate-control theory holds that signals arriving from pain receptors in the body can be stopped, (or gated)
by interneurons in the spinal cord via feedback from two directions. Pain can be gated, for example, by
rubbing the affected area, activating skin receptors.
Pain can also be gated from the brain by modulating the activity of pain-transmission neurons. This neural
feedback is elicited not by the pain itself, but rather by activity deep within the thalamus.
This neural feedback comes from a region in the midbrain called the periaqueductal grey (PAG). Under
extreme conditions such as high stress, naturally occuring endorphins can activate the PAG to send
inhibitory signals to neurons in the spinal cord that then suppress pain signals to the brain. PAG is also
activated through the use of opiate drugs such as morphine. There is also a pain-facilitation signal that increases the sensation of pain when we are ill (for example influenza).
Finally it should be noted the gate-control theory is challenged by the perception that pain is a two way
street: bottom-up (eg. skin surface) versus top-down (brain).
Kinethesis
One aspect of sensation and perception is knowing where parts of your body are at any given
moment, or proprioception. Receptors in the muscles, tendons, and joints signal the position
of the body in space, whereas information about balance and head movement originates in the
inner ear. Muscle, joint, and tendon feedback about how your arms are moving can be used to improve
performance through learning (primarily in the cerebellum).
Maintaining balance depends primarily on the vestibular system, the three fluid-filled semi-
circular canals and adjacent organs located next to the cochlea in each inner ear. The
semicircular canals are arranged in three perpendicular orientations and studded with hair
cells that detect movement when the head moves or accelerates. The bending of hair cells
generates activity in the vestibular nerve which is then conveyed to the brain.
Vision also helps us keep our balance. Bertenthal et. al. (1997) experimented with the visual
aspect of balance by placing people in rooms that can be tilted forward and backward. If the
room tilts enough, people will topple over as they try to compensate for what their visual
system is telling them.
When a mismatch between the information provided by visual cues and vestibular feedback occurs, motion sickness can result. But that is not nearly as interesting as the emerging phenomenon of cyberpuke. Check out: https://www.livescience.com/54478-why-vr-makes-you-sick.htmlPain
The Chemical Senses, Our Most Primitive
Olfaction is the least understood sense and the only one directly connected to the forebrain & amygdala.
Remember that the other senses connect through the thalamus. This indicates how ancient this neural
pathway must be. This mapping indicates that smell has a close relationship with areas involved in emotional and social behavior.
Terms to remember: olfactory receptor neurons (ORN); odorant molecules; olfactory epithelium;
glomerulus.
Groups of ORNs send their axons from the olfactory epithelium into the olfactory bulb, a brain structure
located above the nasal cavity beneath the frontal lobes. Humans possess 350 different ORN types,
allowing us to discriminate between 10,000 odorants, as each one has a unique pattern of neural activity.
Some dogs have 100 times more ORNs than do humans.
Humans can sense some smells in extremely small concentrations, such as mercaptan, at 0.0003 ppm.
Odour perception includes both information about the identity of the odour, as well as the emotional
response. The object-centred approach suggests that information about the identity of the 'odour object'
is quickly accessed from memory, triggering an emotional response.
The valence-centred approach suggests that the emotional response comes first, providing a basis for
determining the identity of the odour. Research presently suggests that odour perception is guided first
by memory and then by emotion.
Smell also exhibits sensory adaptation. Smells fade after a few minutes; reducing sensitivity allows us to
detect new smells after the initial evaluation.
Top down: fMRI evidence indicates that the orbitofrontal cortex responds more strongly to smells
labeled as 'pleasant' rather than 'unpleasant'.
Sense of Taste
The tongue is covered with thousands of papilla, within each are hundreds of taste buds.
Each taste bud contains 50 to 200 taste receptor cells.
Taste perception fades with age; half of the taste receptors are lost by age 20.
The test system contains only five main types of taste receptors: salt, sour, bitter, sweet,
savoury (high proteins like meat and cheese (Yamaguchi, 1998)).
Microvilli react to tastant molecules: salt to NaCL; sour to acids; bitter has 50 to 80 distinct
binding sites; sweet to sugars and others.
Savoury (umami) receptors respond to glutamate, an amino acid in protein foods. Glutamate
is a major excitatory neurotransmitter, hence monosodium glutamate (MSG) is often used to
flavour Asian foods.
Taste & smell collaborate to produce flavour. Taste experiences also vary widely across individuals. About 50% of people (tasters) report a mildly bitter taste caffeine (for example), whereas 25%(non-tasters) do not.
Bartoshuk, 2000 reported that the remaining 25% are super-tasters who find dark green
vegetables bitter to the point of being inedible. Super tasters also tend to avoid fatty,
creamy foods.