Perceptual processes Chapter 9

• Final exam is on April 12 at 2:00 pm in the Education Gym.
• Exam seating is in rows 1 and 2-14 (even rows only).
• Multiple exams are being written in the same location, so ensure you have the correct one.

Chapter 9: Hearing Physiology and Psychoacoustics

• Sounds are created when objects vibrate.
• Object vibrations cause molecules in the surrounding medium (air, water, etc.) to vibrate, creating pressure changes in the medium.
  • These pressure changes travel away from the source of the object vibration, pictured as a wave.
• The pattern of pressure fluctuations of a sound stays the same as the sound wave moves away from the source.
• The amount of pressure change, the height of peaks relative to the depth of valleys, decreases with increasing distance.
  • Waves squish as they move away but maintain their proportions.
• Sound waves travel at a particular speed dependent on the medium.
  • The speed of sound through air is about 340 meters/second.
  • The speed of sound through water is 1,500 meters/second.
• Sound travels fastest through solids because the molecules are more tightly packed, allowing for more efficient signal transmission.
• Liquids are less densely packed, and therefore transmit the signal slower.
• Gases are even less densely packed, and therefore transmit the signal even slower.

Physical and Psychological Qualities of Soundwaves

• Amplitude or Intensity refers to the magnitude of displacement (increase or decrease) of a sound pressure wave.
• Perceived as loudness- this is psychological amplitude.
• Low amplitude sounds are quiet.
• High amplitude sounds are loud.
• Frequency refers to the number of times per second that a pattern of pressure change repeats for sound.
  • Perceived as pitch - Frequency is a psychological experience.
  • Low-frequency sounds are low pitch.
  • High-frequency sounds are high pitch.

How We Measure Sound

• Hertz (Hz) is a unit of measure for frequency
  • 1 Hz equals 1 cycle per second.
• Decibel (dB) is a unit of measure for the physical intensity of sound.
  • It uses a logarithmic scale.
  • Decibels define the difference between two sounds as the ratio between two sound pressures.
  • Relatively small decibel changes correspond to large physical changes.
  • Each 10:1 sound pressure ratio equals 20 dB, and a 100:1 ratio equals 40 dB.

Range of Human Hearing

• Humans can hear across a wide range of sound intensities.
  • The ratio between faintest and loudest sounds is more than 1:1,000,000

Noise Levels

• Sounds at or below 70 dB are safe.
• Sounds above 70 dB are harmful.
• Leaves rustling/whispering = 20 decibels.
• Ticking watch sounds are 20dB.
• Background music = 30 dB.
• Average room noise = 30dB.
• Average office noise = 50 - 60 db
• Landscaping equipment = 70 dB.
• City traffic from inside a car or a noisy restaurant = 75dB to 80db.
• Inside an airplane or electric vacuum = 80 dB
• Food processor or DJ'd School dance or crowing rooster = 90 to 95 dB
• Hairdryer = 85 to 95 dB.
• Motorcycle or Automatic Hand Drier = 100 to110 dB
• Approaching Subway train / Car horn at 16 feet and Pro sports games = 90 to 100 dB.
• Nightclubs and bars / Gas-powered Leaf Blower / Ice cream truck = 105 to 110 dB.
• Dog barking in ear / Ice Cream truck = 110 dB
• Rock concert / siren = 110 to 120 dB.
• Trombone, Jackhammer = 130 to 135 dB.
• Jet engine (from 100 yards) / Gunshot = 140+ dB.

Simple Soundwaves

• The simplest kinds of sounds are sine waves, or pure tones.
  • Sine wave: A waveform for which variation as a function of time is a sine function.
  • Sine waves are not common in everyday sounds because vibrations worldwide are not so pure.

Simple vs. Complex Soundwaves

• All complex sounds (music, speech, and ambient noise) are made up of multiple sine waves.
• Complex sounds can be broken down into the component sine waves through Fourier Analysis.

Spectra

• Spectrum is a visual display of how much energy (amplitude) is present at multiple frequencies in a complex sound.
  • A cat's spectra shows how most of the sound energy in a cat's meow is 122 Hz (1.22kHz)

Harmonic Spectrum

• The harmonic spectrum is sound in which energy is at integer multiples of the fundamental frequency.
  • Typically caused by a simple vibrating source (e.g., string of a guitar or reed of a saxophone)
• The fundamental frequency is the lowest-frequency component of a complex periodic sound.
• Timbre is the psychological sensation that allows a listener to determine that two sounds with the same pitch and loudness are dissimilar.

A Brief History of the Auditory System

• Evolved over millions of years.
• Mammalian hearing is diverse and adapted to each species' needs.
  • The human hearing range is 20–20,000 Hz.
  • Elephants can hear lower-frequency sounds than humans.
  • Cats can hear higher-frequency sounds than humans.
  • Sound travels better at lower frequencies, so it makes it easier to communicate across greater distances..
  • Sounds travel to hunt their small, squeaky prey.

Outer Ear

• Sounds are first collected from the environment by the pinnae.
• Sound waves are funneled by the pinnae into the ear canal.
• The length and shape of the ear canal enhances certain sound frequencies.
• The purpose of the ear canal is to collect/funnel sound waves to the tympanic membrane, insulate, and protect the tympanic membrane.
• Humans have vestigial muscles in the ear.
  • These muscles were once used to move the ears to source sounds in evolutionary ancestors.
  • These muscles currently have no function (besides slightly wiggling our ears).

Tympanic Membrane

• The tympanic membrane is named for the skin on the tympani and functions just like a drum.
• It is a thin sheet of skin at the end of the outer ear canal.
• It vibrates in response to pressure changes in the ear canal.
  • This transfers the physical soundwave signal to the middle ear.

Middle Ear

• An air-filled chamber contains ossicles that amplify and convey vibrations from the tympanic membrane to the oval window.
• The middle ear consists of three tiny bones – ossicles – that amplify and transmit sounds to the inner ear.
  • Malleus (hammer): The most exterior ossicle is connected to the tympanic membrane. It receives vibration from the tympanic membrane and transmits it to the incus.
  • Incus (anvil): Middle ossicle that transmits vibration from the malleus to the stapes.
• Stapes (stirrup): The most interior ossicle receives vibrations from the incus and transfers them to the oval window.

Amplification

• 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.
• The stapes has a smaller surface than the malleus, so sound energy is concentrated, and it moves more, amplifying the vibration.
• The inner ear consists of fluid-filled chambers.
• It takes more energy to move liquid than air, which requires amplification.

Middle Ear Dampening

• The tensor tympani and stapedius are two muscles in the middle ear that decrease ossicle vibrations when tensed.
• These muffle loud sounds and protect the inner ear.
• The acoustic reflex follows the onset of loud sounds by 200 ms, so it cannot protect against abrupt sounds (e.g., gun shot).
• Approximately 5% of the population can activate these muscles on demand.

Inner Ear

• A hollow cavity in the temporal bone of the skull houses the innermost structures of the ear.
• The inner ear transforms physical soundwaves into electrochemical signals.
• This contains:
  • Cochlea
  • Semicircular canals

Cochlea

• A spiral structure of the inner ear containing the organ of Corti.
  • Its function allows you to perceive sound.
• The cochlea is filled with watery fluids in three parallel canals.
• Organ of Corti: Structure on the basilar membrane of the cochlea composed of hair cell layers and dendrites of auditory nerve cells
• Helicotrema: The opening connects the tympanic and vestibular canals to the apex of the cochlea
• Cochlear partition: Flexible structure that divides the cochlea into two sections – formed in part by the basilar membrane

Cochlear Canals

• Vestibular canal: Extends from the oval window at the base of the cochlea to the helicotrema at the apex.
  • The canal is closest to the ossicles and through which pressure waves first move.
• Tympanic canal: Extends from the helicotrema at the apex to the round window at the base of the cochlea
• Middle canal: Sandwiched between the vestibular and tympanic canals and contains the cochlear partition

Sending Sounds Through the Oval Window

• Vibrations transmitted through tympanic membranes and middle-ear 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 released through another membrane—the round window.
• The round window is the air escape hatch, not to be confused with the oval window.
• Movements of the cochlear partition are translated into neural signals by structures in the organ of Corti.

Inner Ear Cells

• Hair cells: Cells that support the stereocilia
• Arranged in four rows that run down the length of the basilar membrane
  • Contains one inner row and three outer rows.
• Stereocilia: Hair-like extensions on the tips of the hair cells
• The tip of each stereocilium is connected to the side of its neighbor by a tiny filament called a tip link.
• Tectorial membrane: A gelatinous structure, attached at 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.

Activating Hair Cells

• Sound-induced vibration on the inner hair cell gives/causes shear force.
• Sound-induced vibration on the outer hair cell results in upward phase/ downward phase.

Sound Transformations in the Cochlea

• The coding of amplitude and frequency occur in the cochlea.
• Place code: Each area along the cochlea is tuned to different frequencies when a specific sound frequency is experienced where, only ear cells in a specific location along the cochlea are activated.
• Frequencies are laid out in order along the cochlear surface.
• Lower frequencies are activated at the cochlear apex (closest to the inside of the spiral).
• Higher frequencies are activated at the cochlear base (closest to the oval window).
• Inner hair cells convey almost all information about sound waves to the brain using afferent fibers.
  • Afferent fibers are nerve cells sending signals to the brain.
• Outer hair cells receive information from the brain using efferent fibers as part of an elaborate feedback system.
  • Efferent fibers are nerve cells sending signals from the brain.
• The inner and outer hair cells work together to adjust how sensitive we are to certain frequencies
• Each place in the cochlea determines its frequency sensitivity for individual AN fibers responses to different frequencies relate
• Frequency selectivity is clearest when sounds are very faint (low amplitude = lower pressure).
• When sound amplitude is low, very few selective cells respond to a specific frequency (lower pressure = less movement along the basilar membrane).
• As sound amplitude increases, more cells will respond to a specific frequency, creating more overlap and less selectivity (higher pressure = greater movement along the basilar membrane).
• Outer hair cells receive feedback from the brain and can make parts of the cochlear partition stiffer.
• This makes the responses of inner hair cells more sensitive and more sharply tuned to specific frequencies.
• Threshold tuning curve: A graph plots the thresholds of a neuron or fiber in response to sine waves at varying frequencies giving rise to responses.
• The characteristic Frequency is the frequency to which as a specific cell is most sensitive.

From Ear to Brain

• Cochlear nucleus: The first brainstem nucleus at which afferent auditory nerve fibers synapse.

• Superior olive: An early brain stem region in the auditory pathway where inputs from both ears converge.

• 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 the primary auditory cortex 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. Auditory processing is actually dependent on visual input for accuracy.

• Tonotopic organization: Arrangement of neurons that respond to specific frequencies are organized anatomically in order of frequency.

• Starts in the cochlea and is maintained all the way through the primary auditory cortex

• Auditory vs. visual processing, the auditory system is mainly completed before, and has multiple mid-brain gateways before arriving at Al, whereas visual is majorly completed later and has few mid-brain gateways before arriving at V1.

• This gives the auditory signals much more integration of sensory information for processing than visuals.

• Chapter 10 discusses hearing in the environment.

• Where our brain takes wave characteristics and converts them into complex auditory soundscapes.