THE HEARING BRAIN Flashcards
Sound waves: What are they, physical and perceptual properties
(general)
The Auditory Stimulus: airborne sound waves
Traveling vibrations of air that alternate between compression (squeezing) and rarefaction (stretched apart) of air molecules
Sound waves have different physical qualities which are approximately related to how we perceive the sound
frequency is :
measured in ___
Physical quality ____
perceptual quality ____
physical qualities which are approximately related to how we perceive the sound
Amplitude
measured in ___
Physical quality ____
perceptual quality ____
physical qualities which are approximately related to how we perceive the sound
overtones
measured in ___
Physical quality ____
perceptual quality ____
physical qualities which are approximately related to how we perceive the sound
basic pathway - sensation to perception
Receptors transduce the physical signal into a neural signal
• Before the receptors:
▫ Outer ear (pinna), auditory canal ▫ Middle ear (eardrum, ossicles)
• Outer and middle ears funnel signal into the inner ear
• Receptors are in the inner ear
▫ Hair cells located in the cochlea
Sensation: stimulus affects a receptor
• Receptor transduces the stimulus: turns a
physical stimulus into a neural signal
Perception:
interpretation of sensations to create a model of the world
Outer ear
Pinna captures sound waves and funnels them inward
• amplifies certain frequencies: important for locating sounds
Middle ear
Ossicles (malus incus and stapes)
tympanic membrane = ear drum
Inner ear
COCHLEA —
Auditory nerve
Transduction in the cochlea
Pressure waves in fluid of cochlea cause basilar
membrane to move up and down
• Movement affects hair cells in the organ of Corti
▫ Movement causes the hair cells to fire action potentials
▫ Hair cells are thus the receptors for sound.
(Movement of the basilar membrane shifts the tectorial
membrane, which moves the sterocilia) = causes neurons to fire
Stereocilia
Hair cell transduction
Tip link • Movement of stereocillia
pulls on “tip links”
• Pulling of tip links open
ion channels; K+ enters
cell, depolarizing it and
causing neurotransmitter
release
• Stimulates neurons of
auditory nerve to fire action
potentials
Sound amplitude and the basilar
membrane
Higher amplitude sounds make larger ripples in the
basilar membrane, causing greater opening of ion
channels
Sound frequency and the basilar
membrane
• Basilar membrane is wedge shaped
▫ Goes from being narrow and stiff to wide and floppy
Sound frequency and the basilar
membrane
• Peak movement of the basilar membrane depends on
frequency
• Different frequencies vibrate different parts of the
membrane, activating different hair cells
Auditory Pathway
MGS
MEDIAL GENICULATE NUCLEUS
Auditory cortex in temporal lobes
Auditory nerve
Auditory Pathway
Axons from auditory neurons come together to form auditory nerve
Medial geniculate nucleus (of the
thalamus)
Primary auditory cortex in temporal lobes
Primary auditory cortex (A1)
•AKA “Core” region
• Located in superior temporal lobe
•Receives input from thalamus
• Encodes simpler features of auditory stimulus
▫ E.g. neurons respond to a narrow range of
frequuencies
• Tonotopically organized
Tonotopical organization
Secondary auditory cortex
- of features, e.g. aBelt and parabelt regions
- Respond to more complex features
▫e.g. broader range of frequencies; sums up information from A1
▫May also respond to combinations certain loudness and location
•Notice this is like the hierarchy of processing we saw in the visual system.
**
Notice this is like hierarchy in the visual system
Kind of like how simple cells combine information from LGN center-surround cells
“What” Versus “Where”
•Some evidence for two routes:
▫“What” route along the temporal lobes
▫e.g. anterior A2 responds to monkey calls (irrespective of location)
•“Where” route projecting to the parietal lobes
e.g. posterior A2 responds to location of sound (irrespective of who is making it)
***
Are there different parts of the brain that are specialized for processing what and where?
What route:
Different monkeys have different calls, like how we all have different voices
Cells in anterior A2 will respond to particular identities (Monkey Phil) regardless of location (where Monkey Phil is)
Like the what pathway in the visual system: respond to the same object regardless of where it’s located
Where route:
Responds to a call from a particular location, regardless of who is making the call (Monkey Phil or Monkey Steve)
“Where” Processing:
Locating Sounds
Horizontal plane
•Horizontal plane: Inter-aural differences
▫Time differences
Sound takes longer to arrives at the ear that is furthest from the source
▫Intensity differences
Sound waves decay with distance
Less intense in ear that is further from source
horizontal plane
Horizontal plane: take advantage of having two ears
If the sound reaches my left ear before my right ear, where must it be coming from?
What if the sound reaches both ears at the same time?
If the sound is louder in my right ear than in my left ear, where must it be coming from?
Scent localization works on intensity differences
“Where” Processing: Locating Sounds
vertical plane
- Vertical plane: Distortions by pinnae
- Increases some frequencies, decreases others depending on vertical location
- Brain uses a learned model of how sounds get distorted (“head-related transfer function”) to infer sound location
- What about animals that don’t have pinnae?
***
Vertical: neither interaural delay nor intensity is going to differ as an object moves up and down
Why do we have those wrinkles in our ears? Why aren’t they smooth?
If you fill in the crinkles in the ears, people can’t tell where something is coming from vertically
All of our ears are shaped differently, learn the shape of our own ears
If you listen through someone else’s ears, can’t figure out the vertical location
Owls: ears are at different heights on their heads, so can used horizontal localization techniques
comparing hearing and vision
Overall summary
•Hair cells in the cochlea transduce pressure waves into a neural code
▫Different frequencies stimulate different parts of the basilar membrane
•Signal is passed through the MGN of the thalamus to auditory cortex in the temporal lobe
▫Separate pathways for processing “what” and “where”
•Location can be determined by analyzing differences between the two ears (horizontal) and distortions by the pinnae (vertical)
Outer ear
Pinna captures sound waves and funnels them inward • amplifies certain frequencies: important for locating sounds
Ear drum aka _____
Tympanic membrane
Middle ear Function? Process
Function: transmit sounds from outer ear to inner ear • Pressure changes make ear drum (“tympanic membrane”) vibrate Bones (ossicles) transmit the vibration to the inner ear. • Not a passive transmission, they amplify the signal (pressure changes) captured by the eardrum. • Push back and forth on the oval window
Ossicles
Malus Incus Stapes “Bones”
Inner ear
• Contains the cochlea • Cochlea is filled with fluid • Action of ossicles creates waves in the fluid
……..?.How do we hear? (Sensation)
Vibrations in air are Liquid Neurons
