Chapter 10 Flashcards
Sound waves
Mechanical displacement of molecules caused by changing pressure → waves of pressure changes in air molecules are sound waves
Visualizing a sound wave
Air molecule density is plotted against time at a single point relative to the tuning forks right prong
Cycle: complete peak/valley → change from min/max air pressure to next min/max max air level, respectively
Resulting cyclical waves sine waves → every sound signal can be decomposed into waves
Frequency and pitch perception
The rate at which sound waves vibrate is measured as cycles per second, or hertz (Hz)→the number of cycles that a wave completes in a given amount of time
Low frequency → low pitched sound
High frequency → high pitched sound
Each note in a musical scale has a different frequency
Amplitude and perception of loudness
Intensity of sound is usually measured in decibels (dB)
High amplitude → loud sound
Low amplitude → soft sound
The magnitude of change in air molecule density
Normal human speech → 40 dB
Complexity and timbre (perception of sound quality)
Most sounds are a mixture of frequencies
A sounds complexity determines its timbre, allowing us to distinguish → for example: a trombone from a violin playing the same note
Simple → pure tone
Complex → mix of frequencies (most waves)
Hearing ranges among animals
Humans → 20 - 20,000 Hz
Whales and dolphins and dogs → 0 - 100,000 Hz
Rodents, bats, Birds, etc. → much less variation
Breaking down complex tones
A number of pure tones give rise to complex waves which results in complex sound (think like colour theory)
Fundamental frequency → the rate at which the complex waveform pattern repeats (at regular intervals)
Overtones → set of higher-frequency sound waves that vibrate at whole-number multiples of the fundamental frequency
Periodicity → the fundamental frequency repeats at regular intervals: sounds that are aperiodic, or random, we call noise
Perception of sound → basic
Auditory system converts the physical properties of sound wave energy to electrochemical activity → through transcluction so CNS can interpret →then processed by neurons in auditory system
Mechanical → electrochemical
Properties of language and music as sounds
Left temporal lobe analyzes speech for meaning
Right temporal lobe analyzes musical sounds for meaning
Non speech and nonmusical noise produced at a rate of about 5 segments per second is perceived as a buzz → normal speed of speech is on the order of 8 to 10 segments per second (up to 30)
Segmentation: implicit mechanism that helps us know when a word begins and ends→ essential for understanding accents etc.
Properties of language
Experience with a language helps with rapid speech
We hear variations of a sound as if they were identical → allows us to understand accents
The auditory system has a mechanism for categorizing sounds as the same despite small differences in pronunciation
→ makes learning foreign languages later in life more difficult because we are hardwired to understand language in a certain way
Properties of music
Loudness, or amplitude, of a sound wave: subjective → what is loud to some is only moderately loud to others
Pitch: position of each tone on a musical scale; frequency of the sound wave → any pure note is perceived as the same regardless of the instrument
Quality: The timbre of a sound, regardless of pitch → you can distinguish between a trumpet and a piano sound » quality of these sounds differs
Functional anatomy of the auditory system
The ear collects sound waves from the surrounding air
Converts mechanical energy to electrochemical neural energy
Routed through the brainstem to the auditory cortex
Auditory system is structured to decode frequency, amplitude, and complexity → some mechanism must locate sound waves in space
Neural systems for sound production and analysis must be closely related
Anatomy of human ear
Outer ear → Pinna and ear canal
Middle ear → eardrum and malleus, incus, stapes (ossicles)
Inner ear → semicircular canals, cochlea, and auditory nerve
Pinna
Funnel-like external structure designed to catch sound waves in the environment and deflect them into the ear canal
External ear canal
Amplifies sound waves and directs them to the eardrum, which vibrates in accordance with the frequency of the sound wave
Middle ear → ossicles
Air filled chamber that comprises the ossicles
Bones in the middle ear
↳ Hammer (malleus)
↳ Anvil (incus)
↳ Stirrup (stapes)
Connects the eardrum to the oval window of the cochlea, located in the inner ear
Inner ear → cochlea
Fluid-filled structure that contains the auditory receptor cells
Organ of Corti: receptor hair cells and the cells that support these
Inner ear → basilar membrane
Receptor surface in the cochlea that transduces sound waves to neural activity → scratches hair cells to produce reaction
Inner ear → hair cells + tectorial membrane
Hair cells → specialized neurons in the cochlea tipped by cilia
Tectorial membrane → membrane overlying hair cells
Sound waves bend basilar membrane → cilia fire
Basilar membrane → transducing sound waves into neural impulses
Sound waves produce a travelling wave that moves all along the basilar membrane → all parts of basilar membrane bend in response to incoming waves of any frequency
Basilar membrane is maximally responsive to frequencies mapped as the cochlea uncoils
↳high frequencies caused maximum displacement near the base of the membrane
↳ low frequencies caused maximum displacement near the membranes apex
Base → 20,000 Hz Apex → 100 Hz
When wave travels down basilar membrane, hair cells at the point of peak displacement are stimulated → maximal neural response in those cells
After incoming signal composed of many frequencies causes several points along the basilar membrane to vibrate → excites hair cells at all these points
Auditory receptors
Transduction of sound waves to neural activity takes place in the hair cells
3500 inner hair cells (auditory receptors) → fixed #
12,000 outer hair cells (alter stiffness of tectorial membrane)
Movement of the basilar membrane stimulates the hair cells via bending and shearing action → causes AP and neural activity
Movement of cilia on hair cells changes membrane potential and alters neurotransmitter release
Outer hair cells
Outer hair cells function by sharpening the cochlea’s resolving power, contracting or relaxing and thereby changing tectorial membrane stiffness → cilia attached to tectorial membrane
Outer hair cells amplify sound waves, providing an energy source that enhances cochlear sensitivity and frequency selectivity → creating mechanical changes in cochlea and cochlear fluid
Inner hair cells
Animals with no inner hair cells are effectively deaf→ can only perceive very loud, low frequency sounds
Inner hair cells act as auditory receptors
Embedded in the basilar membrane (the organ of Corti) are tipped by cilia
↳movements of the basilar and tectorial membranes displace the cilia, leading to changes in the inner hair cells membrane potentials
Neurons in the auditory nerve have a baseline rate of firing
↳ rate is changed by neurotransmitters released by inner hair cell
No stimulation → still working at a baseline
Movement of cilia
Movement of cilia in one direction depolarizes the cell, causing calcium influx and release of neurotransmitter → stimulates cells that form the auditory nerve » nerve impulses increase
Movement of cilia towards the opposite direction hyperpolarizes the cell, resulting in less neurotransmitter release → activity in auditory neurons decreases
