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
Pathways to the auditory cortex → LONG
Inner hair cells synapse on bipolar cells that form the auditory nerve (part of the 8th cranial nerve, which governs nearing and balance) → all connects to brainstem
↳ cochlear nerve axons enter the brainstem at the level of the medulla and synapse in the cochlear nucleus
The cochlear nucleus projects to the superior olive (a nucleus in the olivary complex) and the trapezoid body
↳ projections from the cochlear nucleus connect with cells on BOTH hemispheres of the brain » mixes input from the 2 ears for single sound perception
The cochlear nucleus and the superior olive send projections to the inferior colliculus in the dorsal midbrain
The inferior colliculus goes to the medial geniculate nucleus (thalamus)
↳ ventral region of the medial geniculate nucleus projects to the primary auditory cortex, area A1
↳ dorsal region projects to the auditory cortical regions adjacent to area A1
Analogous to the visual system are two distinct pathways in the auditory system
One identifies objects by their sound characteristics (temporal lobe/ventral stream → WHAT)
Other directs movements by the sounds heard (posterior parietal lobe/dorsal stream → HOW)
Auditory cortex
Primary auditory cortex, A1, lies within heschl’s gurus, surrounded by secondary cortical areas A2 → secondary cortex behind heschls gurus is called Planum Temporale
The cortex of the left planum forms a speech zone: Wernicke’s area
The cortex of the larger, right hemisphere heschls gyrus has a special role in analyzing music
Right handed → Heschl is larger in left hemisphere
Lateralization
Process whereby functions become localized primarily on one side of the brain
Analysis of speech takes place largely in the left hemisphere (Broca’s and Wernicke’s area)
Analysis of musical sounds takes place largely in the right hemisphere
Insula → language and taste
Located within the lateral fissure; multifunctional cortical tissue containing regions related to language, to the perception of taste, and to the neural structures underlying social cognition
Injury to the insula can produce disturbances of both language and taste
Left-handed people and audition
About 70% are similar to right-handers → having language in the left hemisphere
In remaining 30% speech is represented either in right hemisphere or bilaterally
Does not change perception of sound and music
Unsure of what drives difference
Neural activity and hearing → hearing pitch
Hair cells in the cochlea code frequency as a function of their location on the basilar membrane → largest level of displacement
The tonotopic representation of the basilar membrane is reproduced in the cochlear nucleus
This systematic representation is maintained throughout the auditory pathways and into the primary auditory cortex
Similar tonotopic maps can be constructed for each level of the auditory system
Tonotopic representation
Structural organization for processing of sound waves from lower to higher frequencies → matches mapping found in cochlea
Tonotopic representation of A1
Low Frequency (corresponds to apex of cochlea) → High Frequency (corresponds to base of cochlea)
Detecting loudness
The greater the amplitude of the incoming sound waves, the higher the firing rate of bipolar cells in the cochlea
More intense sound waves trigger more intense movements of the basilar membrane
Causes more shearing action of the hair cells
Leads to more neurotransmitter release onto bipolar cells
Detecting location
Estimate location of a sound both by taking cues derived from one ear and by comparing cues received at both ears
↳each cochlear nerve synapses on both sides of the brain to locate each sound source
Neurons in the brainstem compute the difference in a sound waves arrival time at each ear → the interaural time difference (ITD)
Another mechanism for source detection is relative loudness on the Left and right → the interaural intensity difference (IID)
Detecting location → medial superior Olivary Complex
Cells in each hemisphere receive inputs from both ears and calculate the difference in arrival times between the two ears
More difficult to compare the inputs when sounds move from the side of the head toward the middle → the difference in arrival times is smaller
When we detect no difference in arrival times, we infer that the sound is coming from directly in front or behind us
Detecting location → lateral superior olive and trapezoid body
Source of sound is detected by the relative loudness on the left or right side of the head
Since high-frequency sound waves do not easily bend around the head, the head acts as an obstacle
As a result, higher-frequency sound waves on one side of the head are louder than on the other
Locating a sound
Compression waves originating on the left side of the body reach the left ear slightly before reaching the right
The ITD is small but the auditory system can discriminate it and fuse the dual stimuli
So that we perceive a single, clear sound coming from the left
Detecting patterns in sound
Music and language are perhaps the primary sound wave patterns that humans recognize
Ventral pathway decodes spectrally complex sounds (auditory object recognition) including the meaning of speech sounds for people
Dorsal auditory stream integrates auditory and somatosensory information to control speech production (audition for action)
↳ less known about neurons in the dorsal stream
Uniformity of language structure
All languages have common structural characteristics stemming from a genetically determined constraint
Language is universal in human populations
Humans learn language early in life, seemingly without effort
→ likely a sensitive period for language acquisition that runs from about 1-6 yrs of age
Languages have many structural elements in common
→ example: syntax and grammar
Brocas Area → localizing language
Anterior speech area in the left hemisphere that functions with the motor cortex to produce movements needed for speaking
Thought → wernickes area → brocas area → facial area of motor cortex → cranial nerves → speak
Wernicke’s Area → localizing language
Posterior speech area at the rear of the left temporal lobe that regulates language comprehension
Also called the posterior speech zone
Spoken word → A1 → wernicke’s area → comprehension of word
Brocas aphasia
Inability to speak fluently despite having normal comprehension and intact vocal mechanisms
Disconnect between language comprehension and production
Wernickes aphasia
Inability to understand or produce meaningful language even though the production of words is still intact
Speaks fluently but makes no logical sense → word salad
Brain stimulation → auditory cortex
Penfield used weak electrical current to stimulate the brains surface
Patients reported hearing various sounds (e.g., ringing that sounded like a doorbell, buzzing noise, birds chirping)
Brain stimulation → A1
Produced simple tones → ex. ringing sounds
Brain stimulation → wernickes area
Apt to cause some interpretation of a sound
Ex: buzzing sound to a familiar source such as a cricket
Disrupting speech → Brain stimulation mapping
Supplementary speech area on the dorsal surface of the frontal lobes stops ongoing speech completely → speech arrest
Eliciting speech → Brain stimulation mapping
Stimulation of the facial areas in the motor cortex and the somatosensory cortex produces some localization related to movements of the mouth and tongue
Auditory cortex mapped by PET → Hypothesis
Zatorre and colleagues → hypothesized that simple auditory stimulation, such as bursts of noise, are analyzed by area A1, whereas more complex auditory stimulation, such as speech syllables, are analyzed in adjacent secondary auditory areas
↳ Wernicke and Brocas area
Also hypothesized that performing a discrimination task for speech sounds would selectively activate left-hemisphere regions
Auditory cortex mapped by PET
Passively listening to noise bursts activates the primary auditory cortex → A1
Listening to words activates the posterior speech area, including Wernicke’s area
Making a phonetic discrimination activates the frontal region, including Broca’s area
Both types of stimuli produced responses in both hemispheres but with greater activation in the left hemisphere for the speech syllables
A1 analyzes all incoming auditory signals, speech and nonspeech, whereas the secondary auditory areas are responsible for some higher-order signal processing required for analyzing language sound patterns
Processing music
Largely a right-hemisphere specialization
The left hemisphere plays some role in certain aspects of music processing, such as those involved in making music
→ recognizing written music, playing instruments, and composing: LH
Localizing music → Zatorre and colleagues: PET study
Passively listening to noise bursts activates Heschl’s gyrus
Perception of melody triggers major activation in the right-hemisphere auditory cortex → A2
Making relative pitch judgments about two notes of each melody activates a right frontal lobe area
Music as therapy
Music is used as a treatment for mood disorders such as depression
The best evidence of its effectiveness lies in studies of motor disorders → such as stroke and Parkinson disease
Listening to rhythm activates the motor and premotor cortex and can improve gait and arm training after stroke
Parkinson patients who step to the beat of music can improve their gait length and walking speed
Birdsong
Many nonhuman animals communicate with other members of their species by using sound
Birdsong functions → attracting mates, demarcating territories, and announcing locations
Parallels between birdsong and language
Song development in young birds is influenced by both genes and early experience/learning
Gene-experience interactions are epigenetic mechanisms
Brain areas that control singing in adult sparrows show altered gene expression in spring as the breeding and singing season begins
Both appear to be innate yet are shaped by experience
Humans seem to have a template for language that is programmed into the brain, and experience adds a variety of specific structural forms to this template
If a young bird is not exposed to song until is is a juvenile and then listens to recordings of birdsongs of various species, the young bird shows a general preference for its own species song
Whale songs
Cetaceans (whales, dolphins, and porpoises) have evolved to use a variety of AM and FM sounds for several different communication purposes → LOW frequency sounds
The humpbacked whales songs are composed of a set of predictable and regular sounds that bear striking similarities to human musical traditions
