Sensation Exam 3 Review 04/09/2013
Chapter 9: Hearing: Physiology and Psychoacoustics
Sounds are created when objects vibrate.
Vibrations of an object cause molecules in the object’s surrounding medium to vibrate as well, which
causes pressure changes in the medium.
Sound waves travel at a particular speed
Depends on medium
Example: Speed of sound through air is about 340 meters/second, but speed of sound through water is
5 times faster in water because water is more dense than air; more particles to vibrate
Light travels a million times faster than sound.
You see lightning before you hear thunder.
Physical qualities of sound waves:
Amplitude: The magnitude of displacement of a sound pressure wave.
Intensity: The amount of sound energy falling on a unit area.
Frequency: For sound, the number of times per second that a pattern of pressure repeats.
Units for measuring sound:
Hertz (Hz): A unit of measure for frequency. One Hz equals one cycle per second.
Decibel (dB): A unit of measure for the physical intensity of sound.
Psychological qualities of sound waves:
Loudness: The psychological aspect of sound related to perceived intensity or magnitude.
Pitch: The psychological aspect of sound related mainly to the fundamental frequency.
Timbre: Psychological sensation by which listener can judge that two sounds that have same loudness
and pitch are dissimilar, determined by the harmonic structure of the sounds. 1 Hz = 1 cycle/second
Human hearing uses a limited range of sound energy:
From about 20 to 20,000 Hz
ELEPHANTS can hear lower frequencies. (to hear other elephants around); WHALES can too.
DOGS: high frequencies we can’t hear.
BATS: frequencies above 60000hz
Decibel (dB): A unit of measure for the physical intensity of sound:
Named after the inventor of the telephone, Alexander Graham Bell.
Decibels define the difference between two sounds as the ratio between two sound pressures:
dB = 20 log10(p1/p0)
Each 10:1 sound pressure ratio equals 20 dB, and a 100:1 ratio equals 40 dB
Doubling in sound pressure corresponds to 6 dB
Ratio between faintest and loudest sounds is more than 1:1,000,000
One of simplest kinds of sounds: Sine wave, or pure tone
Sine wave: Waveform for which variation as a function of time is a sine function
Time taken for one complete cycle of sine wave: Period
There are 360 degrees of phase across one period
Sine waves: Not common everyday sounds because not many vibrations in the world are so pure
Most sounds in world: Complex sounds, (e.g., human voices, birds, cars, etc.)
All sound waves can be described as some combination of sine waves
Complex sounds can be described by Fourier analysis
A mathematical theorem by which any sound can be divided into a set of sine waves. Combining these sine
waves will reproduce the original sound Results can be summarized by a spectrum
Harmonic spectra: Typically caused by simple vibrating source, (e.g., string of guitar, or reed of saxophone).
First harmonic: Fundamental frequency
lowest frequency component of the sound.
Timbre: Psychological sensation by which listener can judge that two sounds that have same loudness
and pitch are dissimilar – defined by the shape of the harmonic spectrum.
quality of the sound the depends upon the relative energy of the harmonic components
How are sounds detected and recognized by the auditory system?
Sense of hearing evolved over millions of years
Sounds are first collected from environment by the pinnae
Sound waves are funneled by the pinnae into ear canal
Length and shape of ear canal enhance sound frequencies
Main purpose of canal is to insulate structure at its end:
Tympanic membrane: Eardrum; a thin sheet of skin at end of outer ear canal; it vibrates in response
Common myth: Puncturing your eardrum will leave you deaf.
In most cases it will heal itself.
It is possible to damage it beyond repair.
Tympanic membrane is border between outer ear (ear canal) and middle ear.
Consists of three tiny bones, ossicles, that amplify sounds. Ossicles:
Malleus, incus, stapes
smallest bones in body
Amplification provided by the ossicles is essential to our ability to hear faint sounds.
Ossicles have hinged joints that work like levers to amplify sounds.
Stapes transmits vibrations of sound waves to oval window, another membrane which represents border
between middle ear and inner ear.
The stapes has a smaller surface than the malleus, so sound energy is concentrated.
The inner ear consists of fluidfilled chamber. It takes more energy to move liquid than air
Middle ear: Two muscles:
tensor tympani and stapedius
Purpose: To tense when sounds are very loud, muffling pressure changes.
However, acoustic reflex follows onset of loud sounds by about 200 ms, so cannot protect against abrupt
sounds, (e.g., gun shot).
Muscles are also tensed during swallowing, talking and general body movment (inhibiting sounds from your
Inner ear: Fine changes in sound pressure are translated into neural signals.
Function is roughly analogous to that of retina.
THREE CANALS inside of cochlea.
Separated by TWO membranes.
The basilar membrane is a plate of stiff fibers that form the base of the COCHLEAR partition (where sound
waves are TRANSDUCED into neural signals).
TRAVELING WAVES go from base of the cochlea to its APEX (dispersment dissipated at the apex) and
travel through the VESTIBULAR canal.
If the pressure is too intense, it is dissipated through the helicotrema back to the base through the tympanic
canal, where it is absorbed by the round window (exit of pressure).
Oval = entrance of pressure circle = exit of pressure
As pressure travels through the vestibular canal, it causes a "bulge" which pushes into the middle canal and
then displaces the COCHLEAR PARTITION.
Cochlea: Spiral structure of the inner ear containing the Organ of Corti.
Cochlea is filled with watery fluids in three parallel canals:
Tympanic canal: Extends from round window at base of cochlea to helicotrema at the apex.
Vestibular canal: Extends from oval window at base of cochlea to helicotrema at the apex.
Middle canal: Sandwiched between the tympanic and vestibular canals and contains the cochlear
Three cochlear canals are separated by membranes:
Reissner’s membrane: Thin sheath of tissue separating the vestibular and middle canals in the
Basilar membrane: Plate of fibers that forms the base of the cochlear partition and separates the
middle and tympanic canals in the cochlea
Vibrations transmitted through tympanic membranes and middleear bones cause the stapes to push and
pull the flexible oval window in and out of the vestibular canal at the base of the cochlea.
Any remaining pressure from extremely intense sounds is transmitted through the helicotrema and back to
the cochlear base through the tympanic canal, where it is absorbed by another membrane, the round
Organ of Corti: A structure on the basilar membrane of the cochlea that is composed of hair cells and
dendrites of auditory nerve fibers.
Movements of the cochlear partition are translated into neural signals by structures in the organ of Corti.
ORGAN of CORTI (like the RETINA for the eye).
COMPOSED OF HAIR CELLS and DENDRITES of auditory nerve fibers. (and a "scaffolding" of supporting
STEREOCILIA: hairlike extensions on tips of hair cells that initiate the release of neurotransmitters when
Tectorial membrane: A gelatinous structure, attached on one end, that extends into the middle canal
of the ear, floating above inner hair cells and touching outer hair cells.
Vibrations cause displacement of the tectorial membrane, which bends stereocilia attached to hair cells
and causes the release of neurotransmitters.
Stereocilia: Hairlike extensions on the tips of hair cells in the cochlea that initiate the release of
neurotransmitters when they are flexed.
The tip of each stereocilium is connected to the side of its neighbor by a tiny filament called a tip link. Neural transduction of sound energy at the stereocilia:
1.Tip link mechanically opens potassium channels.
2.Potassium enters cell, leading to depolarization of the cell membrane.
3.Depolarization opens calcium channels, causing vesicles to fuse with the cell membrane and release the
neurotransmitter into the synaptic cleft.
4.Postsynaptic button of the afferent auditory nerve gets activated by the neurotransmitter.
5.Firing of auditory nerve fibers into patterns of neural activity finally completes process of translating sound
waves into patterns of neural activity (sensation).
Inner and outer hair cells
Inner hair cells: Convey almost all information about sound waves to brain.
Outer hair cells: Convey information from brain (use of efferent fibers). They are involved in elaborate
when stiffer, can suppress noise
when less stiff, can tune to a given frequency
Otoacoustic emission (OAE): Active amplification of sounds by the outer hair cells leads to
production of sounds by the ear.
Evoked OAE: OAE stimulated by pure tone frequency, computed as difference between stimulation and
sound measured at the hear.
Evoked OAE is used as a noninvasive hearing test, which tests the function of all elements of
Coding of amplitude and frequency in the cochlea
Amplitude: The larger the amplitude, the bigger the shear of tectorial membrane.
Place code: Tuning of different parts of cochlea to different frequencies, in which information about the
particular frequency of incoming sound wave is coded by place along cochlear partition with greatest
THE LARGER the amplitude of sound, THE larger the displacement of the tectorial membrane, the more
neurotransmitters are released.
Mostly, place coding is due to the basilar membrane. wider towards the apex and thinner.
So, high frequencies can bend the stiffer regions of the membrane near the base and low frequencies
cause greater displacement in the more felxible regions near the apex.
The auditory nerve Responses of individual Auditory Nerve fibers to different frequencies are related to their place along the
Frequency selectivity: Clearest when sounds are very faint
Threshold tuning curve: Map plotting thresholds of a neuron or fiber in response to sine waves with
varying frequencies at lowest intensity that will give rise to a response
Threshold Tuning Curve
CHARACTERISTIC frequency: frequency at which the lowest intensity sound excites AN neuron.
BOTTOMEST point of threshold tuning curve.
Twotone suppression: Decrease in firing rate of one auditory nerve fiber due to one tone, when a
second tone is presented at a similar frequency the same time
Are AN fibers as selective for their characteristic frequencies at levels well above threshold as they are for
the barely audible sounds?
To answer this, look at isointensity curves: Chart by measuring an AN fiber’s firing rate to wide range
of frequencies, all presented at same intensity level.
NEURON is VERY selective for quite sounds. Not SO MUCH FOR LOUDER SOUNDS!!!
Rate saturation: Point at which a nerve fiber is firing as rapidly as possible and further stimulation is
incapable of increasing the firing rate
Rate saturation means:
We can NOT use a direct decoding rule like:
if a 2000 Hz AN fiber is firing very fast, the sound must be 2000 Hz
Rate intensity function:
A map plotting firing rate of an auditory nerve fiber in response to a sound of constant frequency at
Firing Rate v. Sound Intensity
Red: low spontaneous firing rate, sensitive to high intensities, selective over wide range, compare
to cones; like cones: increase firing rate only for "loud" sounds, don't saturate as easily. Remain selective
over a broad range of intensities.
Blue: high spontaneous firing rate, sensitive to low intensities, saturate quickly, compare to rods;
LIKE RODS: increase their firing rate with low intensity sounds, but saturate quickly
The brain uses the PATTERN of firing rates across fibers to determine frequency. About 14000 fibers in each ear to describe each pattern
Just like in vision: the pattern of all three cones is important for color, here the pattern of all AN fibers is
important to decode tone.
Temporal code for sound frequency:
Auditory system has another way to encode frequency aside from the cochlear place code.
Phase locking: Firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a
Existence of phase locking: Firing pattern of an AN fiber carries a temporal code.
Temporal code: Tuning of different parts of the cochlea to different frequencies, in which information
about the particular frequency of an incoming sound wave is coded by the timing of the neural firing as it
relates to the period of the sound:
–firing every 0.01 seconds (phase) => frequency of sound is: 1/0.01= 100 Hz
The volley principle:
An idea stating that multiple neurons can provide a temporal code for frequency if each neuron fires at a
distinct point in the period of a sound wave but does not fire on every period
Auditory brain structures:
Cochlear nucleus: The first brain stem nucleus at which afferent auditory nerve fibers synapse.
Superior olive: An early brain stem region in the auditory pathway where inputs from both ears
Inferior colliculus: A midbrain nucleus in the auditory pathway.
Medial geniculate nucleus: The part of the thalamus that relays auditory signals to the temporal
cortex and receives input from the auditory cortex.
Primary auditory cortex (A1): The first area within the temporal lobes of the brain responsible for
processing acoustic organization.
Belt area: A region of cortex, directly adjacent to A1, with inputs from A1, where neurons respond to
more complex characteristics of sounds.
Parabelt area: A region of cortex, lateral and adjacent to the belt area, where neurons respond to more
complex characteristics of sounds, as well as to input from other senses.
Tonotopic organization: An arrangement in which neurons that respond to different frequencies are
organized anatomically in order of frequency.
Maintained in primary auditory cortex (A1)
Neurons from A1 project to belt area, then to parabelt area Comparing overall structure of auditory and visual systems
Auditory system: Large proportion of processing is done before A1.
Visual system: Large proportion of processing occurs beyond V1.
Differences may be due to evolutionary reasons:
hearing is probably an older sense than seeing.
speech (recent in evolution) is in the cortex (the newer structure).
Psychoacoustics: The study of the psychological correlates of the physical dimensions of acoustics; a
branch of psychophysics
–frequency ~> pitch
–intensity ~> loudness
2 sounds of same intensity can be hear as having different loudness
Audibility threshold: A map of just barely audible tones of varying frequencies
Temporal integration: Process by which a sound at a constant level is perceived as being louder
when it is of greater duration.
LOUDNESS ALSO DEPENDS ON LENGTH OF SOUND. LONGER = LOUDER.
TEMPORAL INTEGRATION is over a range between 100200 ms.
Not true for sounds longer than say 300 ms.
We are sensitive to changes as little as 1 dB. IMPORTANT for knowing WHERE sounds comes from.
Sensitivity is achieved because of different sensistivities of different AN fibers (025, 2040, 4065, etc…).
Tonotopic organization of auditory system suggests that frequency composition is determinant of how we
People are more sensitive to changes in pitch at lower frequencies than at higher frequencies:
500 more different from 1000 Hz than
5000Hz is from 5500Hz. Psychoacousticians: Study how listeners perceive pitch
Research done using pure tones suggests that humans are good at detecting small differences in
frequency (as little as 1Hz between 999 Hz and 1kHz).
Masking: Using a second sound, frequency noise, to make the detection of another sound more difficult;
used to investigate frequency selectivity.
White noise: Consists of all audible frequencies in equal amounts; used in masking.
Critical bandwidth: Range of frequencies that are conveyed within channel in auditory system
The filtering property of the auditory system can also be demonstrated psychophysically.
This is achieved by measuring the ability of listeners to detect tones in the presence of bandpass noise.
Hearing can be impaired by damage to any of structures along chain of auditory processing
Obstructing the ear canal results in temporary hearing loss (e.g., earplugs).
Excessive buildup of ear wax (cerumen) in ear canal.
Conductive hearing loss: Caused by problems with the bones of the middle ear, (e.g., during ear
infections, otitis media).
Otosclerosis: More serious type of conductive loss. Caused by abnormal growth of middle ear bones;
can be remedied by surgery.
However, hearing loss also means to have an inability to interpret spectral and
temporal differences in signals (to use the signlas) and that can happen even with
sounds you can hear.
Sensorineural hearing loss: Most common, most serious auditory impairment. Due to defects in
cochlea or auditory nerve; when hair cells are injured, (e.g., as result of antibiotics or cancer drugs,
Common hearing loss: Damage to hair cells due to excessive exposure to noise.
Hearing loss: Natural consequence of aging
Young people: Range of 20–20,000 Hz
By college age: 20–15,000 Hz
HORNS were better than hearing aids as they allowed people to focus on a given frequency more easily.
Harder to focus on the aspect of the sound you're most interested, because of compression., Distracting
Noise harder to filter out!
Cochlear implants: Tiny flexible coils with miniature
Surgeons thread implants through
round window toward cochlea apex.
Tiny microphone transmits radio signals to a receiver in the scalp.
Signals activate miniature electrodes at appropriate positions along the cochlear implant.
A speech processor
A transmitter and
Electrodes inserted into
the cochlear stimulate
the auditory nerves.
This is NOT the same perception
as normal hearing!
Chapter 10: Hearing in the Environment
You can locate very precisely a cricket way before you are able to SEE it. Cricket example helps with sound
SHOWCASE DIFFERENCE: in vision: the STIMULUS enters at different places if they are to the right or to
the left or your body. You can use that information to understand WHERE they are.
Not so with SOUNDS. Regardless of their location, sounds enters your system at the same location.
What is similar is that there is a COMPARISON process in hearing and vision. In vision, we used
comparisons to determine the disparity and with disparity, relative depth of two stimuli.
In hearing, we use as a comparison the ARRIVAL times of sound into our brains!!! and the intensity of
sound in both ears
LET's first talk about ARRIVAL TIMES
Interaural time difference (ITD): The difference in time between a sound arriving at one ear versus
FROM frequencies higher than 1000Hz, the HEAD itself blocks some of the energy in the acoustic wave.
Azimuth: Used to describe locations on imaginary circle that extends around us, in a horizontal plane
STUDIES SHOW we can detect IDT s of as little as 10 MICRO SECONDS > direction accurate within one
degree! Physiology of ITD
Medial superior olives (MSOs): First place where input converges from two ears.
ITD detectors form connections from inputs coming from two ears during first few months of life.
THE MORE SYNAPSES, the less precise is the temporal coding of the ITD.
Auditory Information Pathway
LSO = lateral superior olive
MSO = medial superior olive
MNTB = medial nucleus of the trapezoid body
Interaural level difference (ILD): The difference in level (intensity) between a sound arriving at one
ear versus the other.
Sounds are more intense at the ear closer to sound source
ILD is largest at 90 degrees and –90 degrees, nonexistent for 0 degrees and 180 degrees
ILD generally correlates with angle of sound source, but correlation is not quite as great as it is with ITDs
Physiology of ILDs
Lateral superior olives (LSOs): Neurons that are sensitive to intensity differences between two ears
Excitatory connections to LSO come from ipsilateral (same side) ear
Inhibitory connections to LSO come from contralateral (opposite side) ear
ILD works best for high frequencies.
ITD and ILD compared:
Low frequencies are diffracted by the head (like an ocean wave around a pylon), high frequencies are
ITD works best for low frequencies. ILD works best for high frequencies. ILD is almost nonexistent below 1000 Hz.
Potential problem with using ITDs and ILDs for sound localization
Cone of confusion: Regions of positions in space where all sounds produce the same time and level
(intensity) differences (ITDs and ILDs)
Experiments by Wallach (1940) demonstrated this problem
Shape and form of pinnae helps determine localization of sound
Headrelated transfer function: Describes how pinnae, ear canal, head, and torso change
intensity of sounds with different frequencies that arrive at each ear from different locations in space
(azimuth and elevation)
IPOD plugs: SOUNDS come from inside your head. One can simulate ITDs and ILDs but not HRTFs for
everyone, so we lose that piece of localization information when we heard music through earplugs instead
of LIVE at the concert!.
BINAURAL RECORDINGS: recording through microphones inside your head, near the eardums: Head
related transfer function included. Then you feel sound as coming from outside of your HEAD!! But
recordings are different for each person.
HRTF are learned through experience (age).
Experiments have been done with artificial pinnae (attaching different pinnae to your own). Your ability to
localize sound deteriorates tremendously, for about 6 weeks. Then, you are good as new. Both with your
new ears and with your old.
So, I guess, they could design one set of universal pinnae, force everyone to use them, and then we would
really enjoy movies and recorded music.
How do listeners know how far a sound is?
Simplest cue: Relative intensity of sound
Inversesquare law: Sound intensity decreases with 1/d with increasing distance d in 3D space.
In 2D: intensity decreases with 1/d
In 1D: intensity stays constant over distance (ignoring absorption)
Spectral composition of sounds: Higher frequencies decrease in energy more than lower frequencies as
sound waves travel from source to one ear ( low frequencies travel farther )
RELATIVE INTENSITY can lead to illusions: soft is not always farther away, local environment might be
muffling the sound…
INVERSE square law: for example: a sound that is 1 meter away is 6dB more intense than one 2 meters
But the same 1 meter difference for sounds located 39 and 40 meters away produces a much smaller
(fraction ) of a dB in intensity difference.
So: Intensity is a good cue for depth, but only for objects within our reach (not so valuable). We always
underestimate the distance of farther away sounds (think it is closer than it really is). Intensity works BEST when the sound is MOVING ( towards YOU for example, or as you mvoe around in
Spectral composition differences: "CRACK" vs BOOM of (near vs far) thunder.
Relative amounts of direct vs. reverberant energy
also help evaluate distance
–Lowest frequency of harmonic spectrum: Fundamental frequency
–Auditory system is acutely sensitive to natural relationships between harmonics
VOICES are HARMONIC SOUNDS!
EXAMPLE OF VOWEL SOUND, female voice. GREATES ENERGY at 250 Hz. , less so at 500Hz, less still
at 750 (third harmonic), etc…
Richnessof sound comes from energy in the harmonics.
Peaks of energy at multiples of the fundamental frequency
Timbre: Psychological sensation by which a listener can judge that two sounds that have the same
loudness and pitch are dissimilar; conveyed by harmonics and other high frequencies
Perception of timbre depends on context in which sound is heard
Experiment by Summerfield et al. (1984)
“Timbre contrast” or “timbre aftereffect”
Sounds with holes in the spectrum, presented right before a uniform spectrum. The uniform spectrum is
heard as a vowel!!
Another characteristic of a sound:
Attack and decay
Attack: Part of a sound during which amplitude increases (onset)
Decay: Part of a sound during which amplitude decreases (offset) Categorical perception of consonants (onsets)
Researchers can manipulate sound stimuli to vary continuously from “bah” to “dah” to “gah”.
However, people do not perceive the sounds as continuously varying.
Instead, people perceive sharp categorical boundaries between the stimuli: Categorical perception.
McGurk effect: demonstrates the crossmodal combination of auditory and visual information for
speech perception. Perception of onset consonants is determined by a combination of visual and
auditory cues. Misbinding can lead to illusory perception of vowels.
A number of strategies to segregate sound sources
Spatial separation between sounds
Separation on basis of sounds’ spectral or temporal qualities
Auditory stream segregation: Perceptual organization of a complex acoustic signal into separate auditory
events for which each stream is heard as a separate event
Grouping by timbre
Tones that have increasing and decreasing frequencies, or tones that deviate from rising/falling pattern “pop
out” of sequence
Same timbre = same source!
Grouping by onset
Harmonics of speech sound or music
Grouping different harmonics into a single complex tone (like a vowel sound)
Rasch (1987) showed that it is much easier to distinguish two notes from one another when onset of one
precedes onset of other by very short time
Gestalt law of common fate
RASCH: Musicians do this and it helps us perceive all the different notes and instruments in the sound
COMMON FATE: grouping of sounds by common onset.
How do we know that listeners really hear a sound as continuous?
Principle of good continuation: In spite of interruptions, one can still “hear” sound. The missing
part is filledin by the auditory system. How do we know that listeners really hear a sound as continuous?
Experiments that use signal detection task (e.g., Kluender and Jenison) suggest that at some point restored
missing sounds are encoded in brain as if they were actually present!
Restoration of complex sound, (e.g., music, speech)
“Higherorder” sources of information, not just auditory information
"The state governors met with their respective legi*latures convening in the capital city".
"The *eel fell off the car"
“The *eel fell off the table".
Gestalt laws for auditory grouping:
Good continuation: Filling in of missing parts of sounds.
Common fate: Onset and offset at the same time or with a very short delay, e.g. in harmonics.
Similarity: Grouping by timbre or by similar pitch.
Chapter 12: Spatial Orientation and the Vestibular System
Vestibular organs: The set of five organs located in each inner ear that sense head motion and head
orientation with respect to gravity:
three semicircular canals
two otolith organs
Also called the “vestibular labyrinth” or the “vestibular system.”
An often overlooked sense: evolutionarily very old.
The vestibular organs help us in many ways, for instance:
Provide a sense of spatial orientation, consisting of
Angular motion Tilt
Allow for the vestibuloocular reflex
Stabilizes visual input by counter rotating the eyes to compensate for head movement
Problems with the vestibular system can lead to peculiar sensations:
Spatial Disorientation: Any impairment of spatial orientation (i.e., our sense of linear motion, angular
motion, or tilt)
Dizziness: Nonspecific spatial disorientation
Vertigo: A sensation of rotation or spinning
Spatial orientation: A sense comprised of three interacting sensory modalities:
Our senses of linear motion, angular motion, and tilt:
1. Angular motion: Can be sensed when rotating head from side to side as if to say “no”.
2. Linear motion: Sensed when accelerating or decelerating in a car.
3. Tilt: Can be sensed when nodding head up and down as if to say “yes”.
Why considered different “modalities”?
Sensing linear motion, angular motion, and tilt involves different receptors and/or
different stimulation energy.
Semicircular canals: The three toroidal tubes in the vestibular system that sense angular
acceleration, a change in angular velocity.
Source of our sense of angular motion.
Otolith organs: The mechanical structures in the vestibular system that sense both linear acceleration
Source of our sense of linear velocity and gravity. Coordinate system for classifying direction:
xaxis: Points forward, in the direction the person is facing.
yaxis: Points laterally, out of the person’s left ear.
zaxis: Points vertically, out of the top of the head.
Axes are defined relative to the person, not relative to gravity.
Three directions for sense of rotation:
Roll: Rotation around xaxis
Pitch: Rotation around yaxis
Yaw: Rotation around zaxis
Each spatial orientation modality can change in terms of its amplitude and direction
Amplitude: The size (increase or decrease) of a head movement (e.g., angular velocity, linear acceleration,
Direction: The line along which one faces or moves, with reference to the point or region toward which one
is facing or moving.
Movements represented in terms of changes in the x, y, and zaxes.
Any arbitrary linear motion can be represented as a change along these three axes.
The vestibular organs do not respond to constant velocity.
They only respond to changes in velocity: acceleration.
According to Einstein’s General Relativity Theory, gravity and acceleration can be considered equivalent.
Each one is about 3/4 of a toroid (donut) shape, measuring 15 mm long and 1.5 mm in diameter.
Canals are filled with a fluid called perilymph. A second, smaller toroid is found inside the larger toroid, measuring 0.3 mm in diameter.
Formed by a membrane filled with fluid called endolymph.
Cross section of each canal swells substantially near where the canals join the vestibule: Ampulla.
Within the endolymph space of each ampulla is the crista.
Cristae: The specialized detectors of angular motion located in each semicircular canal in a swelling
called the ampulla.
Each crista has about 7000 hair cells, associated supporting cells, and nerve fibers.
Cilia of hair cells project into jellylike cupula which forms an elastic dam extending to the opposite
ampulla wall, with endolymph on both sides of dam.
When the head rotates, the inertia of the endolymph causes it to lag behind, leading to
tiny deflections of the hair cells.
Hair cells: Support the stereocilia that transduce mechanical movement in the vestibular labyrinth into
neural activity sent to the brain stem.
Like the hair cells involved in hearing, hair cells act as the mechanoreceptors in each of the five vestibular
Mechanoreceptors: Sensory receptors that are responsive to mechanical stimulation (pressure,
Head motion causes hair cell stereocilia to deflect, causing a change in hair cell voltage and altering
Hair cell responses:
In the absence of stimulation, hair cells release neurotransmitter at a constant rate.
When hair cell bundles bend, change in hair cell voltage is proportional to the amount of deflection.
Bending toward tallest stereocilia: Depolarization
Bending away from tallest stereocilia: Hyperpolarization
Hair cells increase firing to rotation in one direction and decrease firing to rotation in the
Coding of direction in the semicircular canals:
Three semicircular canals in each ear.
Each canal is oriented in a different plane. Each canal is maximally sensitive to rotations perpendicular to the canal plane.
Hair cells in opposite ears respond in a complementary fashion to each other.
When hair cells in the left ear depolarize, those in the analogous structure in the right ear hyperpolarize.
Coding of amplitude in the semicircular canals:
In the absence of any rotation, many afferent neurons from the semicircular canals have a resting firing rate
of about 100 spikes/s.
This firing rate is high relative to nerve fibers in other sensory systems.
High firing rate allows canal neurons to code amplitude by decreasing their firing rate, as well as increasing
Changes in firing rate are proportional to angular velocity of the head aligned with the
canal the neuron is in.
Semicircular canal dynamics
Neural activity in semicircular canals is sensitive to changes in rotation velocity.
Constant rotation leads to decreased responding from the canal neurons after a few seconds.
Canal afferent neurons are sensitive to back and forth rotations of the head, as well.
Greatest sensitivity to rotations at 1 Hz or less.
Faster rotations than 1 Hz would be dangerous.
Firing rate goes up and down as the head rotates back and forth.
The overall normalized amplitude of the canal neuron response scales with head rotation frequency.
Otolith organs sense acceleration and tilt.
Two otolith organs in each ear:
Utricle: Contains about 30,000 hair cells
Saccule: Contains about 16,000 hair cells Each organ contains a macula: A specialized detector of linear acceleration and gravity.
Each macula is roughly planar and sensitive primarily to shear forces.
Hair cells are encased in a gelatinous structure that contains calcium carbonate crystals called otoconia
(“ear stones” in Greek).
Coding of amplitude in the otolith organs:
Larger accelerations (or larger gravitational shear forces) move the otolith organ’s otoconia more.
This leads to greater deflection of the hair cell bundles.
Change in receptor potential is proportional to magnitude of linear acceleration or gravitational shear.
Coding of direction in the otolith organs:
Arises in part from the anatomical orientation of the organs.
Utricular macula: horizontal plane
Sensitive to horizontal linear acceleration and gravity.
Saccular macula: vertical plane
Sensitive to vertical linear acceleration and gravity.
Three experimental paradigms are typically used to investigate spatial orientation perception:
Threshold estimation: What is the minimum motion needed to correctly perceive motion direction?
Magnitude estimation: Participants report how much (e.g., how many degrees) they think they tilted,
rotated, or translated.
Matching: Participants are tilted and then orient a line with the direction of gravity. This is done in a dark
room with only the line visible to avoid any visual cues to orientation.
At first, constant rotation (in the dark) is perceived accurately.
Soon, however, subjects feel as if they are slowing down.
After 30 seconds, they no longer feel as if they are rotating. Time course of habituation for perceived velocity is slower than time course of
habituation for velocity neurons: “Velocity storage”
When rotation stops, subjects feel as if they are rotating in opposite direction.
Yaw rotation thresholds
Humans are so sensitive to yaw rotation that we can detect movements of less than 1 degree per second.
At this rate, it would take 6 minutes to turn completely around.
As yaw rotation frequency decreases, it takes faster movement to be detected.
When people are passively translated in the dark, they are able to use a joystick to reproduce the distance
they traveled quite accurately.
Interestingly, they also reproduce the velocity of the passivemotion trajectory.
This implies that the brain remembers and replicates the velocity trajectory.
The otolith organs register acceleration, and our brains mathematically integrate the
acceleration and turn it into the perception of linear velocity.
We are very accurate when perceiving tilt for angles between 0 degrees (upright) and 90 degrees (lying
Illusion: If you roll tilt your head to the left or right while looking at a vertical streak of light, the light appears
to tilt in the opposite direction.
Sensory integration: The process of combining different sensory signals.
Sensory integration typically leads to more accurate information than can be obtained
from individual senses alone.
Vection: An illusory sense of self motion produced when you are not, in fact, moving.
Example: The feeling of flying while watching an IMAX movie.
Example: Being stopped in your car at a light next to a semi. The semi begins to roll forward and you press
on the brake because you feel as if you are rolling backwards.
Observers looking at a rotating display report rotational vection Su. ects have the illusion of tilt but
do not feel as if they turn upsidedown. Why don’t people feel as if they are turning upside down?
The vestibular system’s sense of gravity stops the illusion.
Astronauts without gravity feel as if they are tumbling under these circumstances.
Thus, vestibular information is combined with visual information to yield a “consensus”
about our sense of spatial orientation.
Vestibuloocular reflexes (VORs): Counterrotating the eyes to counteract head movements and
maintain fixation on a target.
Angular VOR: The most wellstudied VOR
Example: When the head turns to the left, the eyeballs are rotated to the right to partially counteract this
Torsional eye movements: When the head is rolled about the xaxis, the eyeballs can be rotated a
few degrees in the opposite direction to compensate.
VORs are accomplished by six oculomotor muscles that rotate the eyeball.
1.Vestibular afferent neurons
3.Efferent oculomotor neurons
Autonomic nervous system: The part of the nervous system innervating glands, heart, digestive
system, etc., and responsible for regulation of many involuntary actions.
Blood pressure is regulated by vestibuloautonomic responses.
Motion sickness: Results when there is a disagreement between the motion and orientation signals
provided by the semicircular canals, otolith organs, and vision.
Motion sickness could be an evolutionary response to being poisoned.
Vestibular influences on blood pressure in patients with lesioned vestibular sy