Chapter 9

Question 1

sound: definition and compartments

Answer 1

• definition: vibrations that travel through the air or another medium
• AMPLITUDE: intensity, perceived as LOUDNESS, measured by DECIBEL (dB)
• FREQUENCY: number of cycles per second of vibration, perceived as PITCH, measure in HERTZ (Hz)
  + higher frequency = more cycles per second of vibration, travel shorter distances because they are easy to degrade in environment
  + low frequency = bigger wavelengths

Question 2

sound perception across species

Answer 2

• sensitivity varies based on species
• classifications of sounds:
  + infra sound: 0.1 - 25 Hz, short frequency, can travel well (>3km) -> long-distance communications (e.g. elephants)
  + audible sound: 20 Hz - 20 kHz, medium frequency, human hearing
  + ULTRASOUND: > 20 kHz, high frequency, travel only short distance -> target detection within 10m (bats)
  + ECHOLOCATION: bats release sonar wave, which meets and bounces off different objects -> bats listen to echoes to locate + identify objects and forage
  + ULTRASONIC: 14,000 - 200,000+ Hz, 110-120 dB

Question 3

hearing = transmit sounds from air pressure (vibration or sound wave) to neural activity (electrical-chemical signals)

Answer 3

• EXTERNAL EAR, PINNA, and ear canal collect sound waves
  + shape of external ear transforms sound energy
• MIDDLE EAR concentrates sound energy by increasing vibrations so waves are not stopped from traveling through inner ear’s liquid medium
  + sound waves (air pressure) pass to eardrum
  + 3 ear bones/ossicles - MALLEUS, INCUS, and STAPES - connect TYMPANIC MEMBRANE (eardrum) to OVAL WINDOW
• INNER EAR structures convert mechanical vibration into neural activity
  + COCHLEA, a snail-looking, liquid-filled coiled organ containing receptors, converts vibrations to nerve impulses

Question 4

organ of Corti

Answer 4

• inside the cochlea, innervates auditory vibrations
• main structures:
  + sensory cells, or HAIR CELLS: with hair (STEREOCILIA) protruding out; send nerve impulses
  
  1. inner hair cells (IHCs)
  2. outer hair cells (OHCs)
     + framework of supporting cells
     + BASILAR MEMBRANE: oscillates in response to sound vibrations
     + allows cochlea to localize + organize functions:
  3. BASE of membrane: processes HIGH FREQUENCY
  4. APEX: processes LOW FREQUENCY

Question 5

process of sound transmission

Answer 5

• sound vibrations are present
• tectorial membrane move + touch hair cells
• hairs send neural signals
• mechanical sequence creates action potential

Question 6

auditory stimulation sensing by stereocilia

Answer 6

• TIP LINKS, thin fibers, run across each stereocilia
  + these link all hairs together -> move 1, move all -> induce action to create action potential
• vibrations sway stereocilia, causing ion channels to open
• hair cell depolarizes
• calcium influx at base of cell -> release neurotransmitters

Question 7

how is sound encoded?

Answer 7

• pitch difference

- sound location

Question 8

auditory system pathways: brainstem to cortex

Answer 8

• cochlear nuclei
• SUPERIOR OLIVARY NUCLEI: receives bilateral input in the brainstem
• INFERIOR COLLICULI (in midbrain)
• MEDIAL GENICULATE NUCLEI (in thalamus)
• AUDITORY CORTEX

Question 9

responses in auditory cortex: random sounds vs. speech

Answer 9

• learning is an advanced function -> utilizes complex system
• brain shows less neural activity if you don’t know the language (bc you can’t distinguish familiar vs. strange sounds)

Question 10

pitch coding: 2 ways

Answer 10

1. place coding, or TONOTOPIC REPRESENTATION (spatial)
   - present in all levels of auditory pathway
   - pitch is encoded in receptors located on basilar membrane
   + these are arrange in a map in the nucleus of brainstem or in the cortex, based on what frequencies to which they respond (e.g. high -> base, low -> apex)
2. TEMPORAL PATTERN OF FIRING OF CELLS (temporal)
   - firing rate of auditory neurons encodes the frequency of the auditory stimulus (high frequency -> high firing rate)

Question 11

sound localization

Answer 11

• BINAURAL cues signal sound location
  + INTENSITY DIFFERENCES: differences in loudness at the two ears (head can cause a sound shadow, which reduces the volume because the sound is blocked by the head)
  + LATENCY DIFFERENCES: difference between the two ears in the time of arrival of sounds
• DUPLEX THEORY: sound localization requires processing both intensity and latency differences

Question 12

symmetry vs. asymmetry in sound perception

Answer 12

• ear asymmetry (e.g. owls) allows for sound originating from BELOW eye level to sound LOUDER IN LEFT ear and sound ABOVE eye level to sound LOUDER IN RIGHT ear + helps localize high frequency -> each ear can detect and process different sounds from different sources
• owls with symmetrical ears cannot be trained to locate prey in total darkness, those with asymmetrical ears can

Question 13

classic model of sound localization in brainstem of birds

Answer 13

• in birds, a group of neurons form the nucleus laminaris
  + each neuron functions as a coincidence detector
  + a map is formed as neurons respond maximally to sounds from a particular space; sound from center comes to 2 ears equally

Question 14

external ear and sound perpeption

Answer 14

• external ear’s shape and orientation vary by species (e.g. rabbits = long, tall ear, elephants = large, soft, fan-like)
• external ear structure selectively reinforces some frequencies, called SPECTRAL FILTERING (can filter out certain frequencies)
• unlike binaural intensity and latency cues that localize a sound in azimuth (horizontal location), spectral cues provide critical information about elevation (vertical location)

Question 15

auditory cortex

Answer 15

• parts:
  + PRIMARY AUDITORY CORTEX: part of parietal lobe
  + includes basilar membrane -> tonotopic organization: apex of cochlea processes low pitch and base processes high
  + SECONDARY: part of temporal lobe
• main function: process complex information, especially related to associative learning and memory (e.g. hearing someone’s voice and encoding it with their face to distinguish who is speaking even when you close your eyes)
• 2 streams for sound analysis:
  + DORSAL stream: in parietal lobe, involved in SPATIAL location
  + VENTRAL stream: in temporal lobe, analyzes COMPONENTS of sound (e.g. pronunciation perception)

Question 16

auditory learning: experience

Answer 16

• EXPERIENCE affects and modulates perception + pathway via learning
• learning/tuning via auditory experience in mice:
  + training: exposure to new experience (high pitch sound) paired with reward (food)
  + neurons begin to respond to higher-pitched sounds
  + change in preference - new best cellular response = to a different, higher pitch
• relation to music:
  + many musicians have a larger Heschl’s gyrus, which is the portion that processes music in the auditory cortex -> test whether this is born or trained by:
  + compare fMRI between babies before auditory exposure -> maybe development is even prenatal?
  + genetic analysis - several gene mutations are strongly associated with musicians (e.g. those with perfect pitch)
• auditory experience contributes to development of sound localization and plasticity can last into adulthood

Question 17

auditory processing: musicians vs. non-musicians (NM)

Answer 17

• while listening to piano tunes alone: musicians show more brain activation compared to NM
• while paired with playing on a soundless keyboard with right hand:
  + musicians show a dramatic difference in brain activation upon hearing music, especially in motor cortex
  + motor cortex is active due to mirror neurons, thanks to previous exposure and practice with music
  + musicians show activation in auditory + motor + speech learning cortexes -> integrate music and language

Question 18

cause of hearing disorder

Answer 18

• loud noise (high amplitude) causes cochlear damage
  + damaged cochlea shows a “white out” are where hair cells are missing
  + stereocilia becomes disorganized and deformed -> do not function well and will die off
• genetics

Question 19

hearing loss vs. deafness

Answer 19

• HEARING LOSS: moderate to severe decreased sensitivity to sound
• DEAFNESS: profound loss of hearing in which speech cannot be perceived even wth the use of hearing aids
  + causes:
  
  1. CONDUCTION deafness: disorders of OUTER or MIDDLE ear that prevent sounds from reaching cochlea
  2. SENSORINEURAL deafness: originates from COCHLEA or auditory nerve lesions
  3. CENTRAL deafness: caused by BRAIN lesions (e.g. stroke) with complex results
     -> if from birth, then cause can be any of those 3 or genetic mutation
• treatments:
  + COCHLEAR IMPLANTS treat deafness due to hair cell loss
  + normal hair cells convert sound wave to nerve impulses
  + electrical currents stimulate auditory nerve in cochlea -> amplify sounds and convert them to electrical signals
  + stem cell therapy: develop new ears (internal and external)

Question 20

smell: general

Answer 20

• humans have very limited olfactory function -> better animal models are mice, cats, and dogs
  + olfactory bulb size is different depending on species -> depends on whether smell is used in social communication
  + dogs can identify diabetic patients via smelling urine -> detect subtle differences in odor caused by increased level of glucose
• humans:
  + have turbinates, curved surfaces in the nasal cavity, to direct air flow
  + chemical enters nose, binds with receptors connected to olfactory bulb
  + olfactory bulb transfer signal to cortex

Question 21

chemicals and odors

Answer 21

• chemicals in the air elicit odor sensations
• different parts (mucosa, receptor cells, cilia, dendritic knob) interact with different type of chemicals/odorants, similar to the highly organized processing of sounds
  + apical dendrite extend from receptor cell to mucosal surface
  + CILIA emerge from DENDRITIC KNOB
  + axon extends from other end of receptor cell to olfactory bulb
  + ODORANTS interact with receptors on cilia + dendritic knobs
  + G(olf) protein is activated -> production of second messengers

Question 22

steps in olfactory sensory transduction

how action potential is conducted

Answer 22

1. odorant molecule binds to specific receptor protein
2. receptor-odorant complex activates G protein, which combines with molecule of GTP + displaces GDP
3. G protein alpha subunit dissociates and activates adenyl cyclase, which produces cAMP
4. cyclic AMP (second messenger) activates gated cation channel -> calcium and sodium ions enter the cell -> polarize cell + induce gated channels to open -> further depolarization
5. receptor protein turns to unbound state

Question 23

odor differentiation

Answer 23

• mice have about 2 million receptor cells, each of which only expresses 1 of ~1000 receptor proteins -> highly organized and specialized -> very flexible, can detect many scents
• receptor proteins can be divided into 4 subfamilies, 250 receptors/each
  + each subfamily is synthesized in a separate band of the epithelium

Question 24

odor processing

Answer 24

• olfactory receptor cell axons end in OLFACTORY BULB
• GLOMERULI: spherical units containing a complex arbor of dendrites from a group of olfactory cells
• olfactory receptor axons synapse onto dendrites of MITRAL cells and conduct smell information directly to cortex + other brain regions

Question 25

odor projections organization in the brain

Answer 25

• located in the GLOMERULI, a specialized cluster of cells aggregated with dendrites and axons
  + forms a map in the cortex with related substances activating adjacent brain regions
  + all olfactory neurons expressing one type of receptor terminate on the same glomerulus -> each category of stimuli correspond to a certain location
  + smells interact with receptor cells -> olfactory bulb -> reach out to different cortical regions
• e.g. smell different types of apples -> register all under the category “apple” -> smells are processed by particular groups of neurons in epithelium to olfactory bulb

Question 26

adult neurogenesis: olfactory

Answer 26

• olfactory receptor neurons can be replaced in adulthood (regrowth)
• if epithelium is damaged, it can be regenerated and will properly reconnect to the olfactory bulb
  => cause/adaptive function is unknown, but very beneficial for mammals who depend on smell to communicate

Question 27

smell vs. pheromones

Answer 27

• smell: any type of chemical the olfactory system can detect
• pheromones: chemicals used to communicate between individuals of the same species
  + must be released by an organism
  + function: identification and mate choice (e.g. urine - different species have different chemical make-ups, dogs use to mark territory)
  + many animals have vomenorasal organ (second nose) to detect pheromones (e.g. rats, dogs, cats, snakes)
  + humans do have this but does not seem functional -> degraded by evolution

Question 28

pheromones example: moths

Answer 28

• moths use pheromones for mating
  + females ready to mate release pheromones -> spread in air
  + males detect with a special antenna -> excited wing-fluttering response
• experiment to test function:
  + extract female moth pheromones from glands
  + trap female moths in transparent airtight box, from which pheromones cannot be released in air
  + spread pheromones (bombykol) on filter paper
  + males show no interest in physical females but become attracted + attempt to mate with filter paper

Question 29

pheromones example: humans

Answer 29

• human infants a few weeks old can identify and are attracted to both the axillary and breast odors of their own mothers but NOT other mothers
• mothers can also recognize odor of their own baby -> pheromones = means of communication between mom and infant
• many mammals have a pheromone located at the areola that attract newborns to the mother’s nipple to suckle
  + humans do have areola gland but pheromone is not yet identified
• sexual selection:
  + females synchronize menstrual cycles with co-habiting friends (Wellesley -McClintock- effect)
  + scent of ovulating women could cause testosterone increase in men
• human urine also has pheromones -> mice can identify individual person, dogs can identify cancer + identical twins based on urine