lecture 9

Question 1

How is vibration in air detected?

Answer 1

Receptor: inner hair cell

Range: 20 - 20,000 Hz

Sensitivity: picometers (re: movement of air) to 100dB

Receptive field: peak of travelling wave?

Question 2

What are key concepts in sensory system description?

Answer 2

• transduction (how is energy captured and transformed)
• receptive field (what stimuli can change neuronal activity)
• sensitivity and range
• resolution (spatial and temporal)

Question 3

What is being detected and what are the experiential dimensions it is mapped to?

Answer 3

Percepts related to sound energy parameters

• wavelength (pitch)
• amplitude (loudness)
• waveform (tone/timbre)

Question 4

Why music?

Answer 4

• peaks in human speech tend to occur where we have the octave, the fifth, the third and at each note on the western 12 note scale
• part of our evolutionary, very nuanced capacity to resolve sounds in peoples’ speech

Question 5

What is the anatomy of the human ear?

Answer 5

Outer ear

• Pinna: cartilaginous flap of human skin, functional significance, helps to lessen the ambiguity of the location of a sound source
• concha (ear hole) is a particular size and shape so that it maximises certain frequencies of sound, sort of tuned to around 3000Hz
• auditory meatus - ear canal

Middle ear:
tympanic membrane, taught membrane that vibrates with the air coming in. Compressions of air that hit water sort of bounce off because of the difference in the way they attenuate sound. So we use a mechanical lever system to transmit sound energy into fluid. Bones.

Inner ear:

• cochlea
• chamber divided into three compartments
• fluid filled tube divided by a chamber creating three chambers (scala vestibuli, scala media, scala tympani)
• organ of corti located in scala media
• when the fluid is vibrated the vibrations travel through the chambers all the way through the scala vestibuli and then back through the scala tympani (via the helicotrema/cochlear apex i.e. where they connect at the end)

Question 6

What is the organ of corti?

Answer 6

big flat board (like a spring board or diving board) and sitting on top of that board (basilar membrane) is an incredibly intricate, tiny clustering of cells (most supporting cells), and inner hair cells and three rows of outer hair cells)
Nerves connect to inner and outer hair cells
- it is these hair cells that start to convert motion into nervous activity
- called hair cells because they have stereocilia
- it is the inner hair cells that do the detecting
- outer hair cells are efferent - have a motor function. Have to do with stiffening and mechanical properties of the basilar membrane and the organ of corti

Question 7

What is spectral decomposition?

Answer 7

• different frequencies will have actions and different parts along the basilar membrane structure
• different frequencies maximally excite vibration at a particular part of the basilar membrane
• the basilar membrane is short and stiff and fat towards the base but becomes flatter and floppier
• high frequencies resonate down at the base, low frequencies you’ll resonate the floppy end of the basilar membrane

Question 8

How do travelling waves initiate auditory transduction?

Answer 8

the cilia of the hair cells are in contact with/embedded in another membrane lying over the top, called the tectorial membrane.

when the basilar membrane vibrates it goes up at down, the tectorial membrane also goes up and down.

However the two membranes have different pivot points causes a shearing force of the two membranes across each other - this will wiggle the cilia embedded in the tectorial membrane

Question 9

What happens when the cilia are moved back and forth?

Answer 9

mechanically gates ion channels in the tips of those stereocilia.
have tiplinks that are important in the transduction process - perhaps connected to the channels pulling them open.
Potassium moves in causing depolarisation (v. different)
Depolarisation in the basal part of the cell causes the opening of Ca2+ channels which cause the release of vesicles containing a transmitter that stimulate the neuron

Question 10

Why is potassium going into the cell?

Answer 10

• potassium concentration is much higher in the cell than outside of it so when the potassium channels open due to shearing forces, the ions move into the cell resulting in depolarisation
• stria vascularis pumps a crazy amount of potassium into the scala media
• the other two fluid chambers are fairly normal in terms of extracellular fluid
• the ciliated ends of the hair cell are exposed to the high potassium environment (-125 mV)

Question 11

What happens if you stimulate a hair cell in a lab?

Answer 11

• depolarisation faithfully follows the frequency of the probe but once you get to around 2000 Hz you get what is called a DC component in which there is a lot of background noise.
  At levels like 5000Hz you are definitely not faithfully following every thing you just have the DC component

Question 12

What is the receptive field of the auditory nerve fibres?

Answer 12

• a frequency range as opposed to a spatial range
• respond over a range of frequencies and they have a slight preference at a peak
• place code: when a particular place in the cochlear is stimulated that tells the brain that a particular frequency has been heard
• they aren’t sharp tuning curves
• to get fine discrimination we have neural mechanisms that can extract the info and compare between individual hair cells
• auditory nerve conveys phase information

Question 13

What are the major auditory pathways into the brain?

Answer 13

• very complex, lots of crossing over etc
• two things:
• • thalamic relay - medial geniculate
• cortical representation in the temporal cortex

Question 14

How can the MSO compute the location of a sound?

Answer 14

e. g. in birds: interaural time differences
- so if you hear a sound on your left side you will hear it a very very short time from your left ear than your right ear

in mammals: usually phase information
acoustic shadow of head - inhibition of the second side that sound comes from

Question 15

How is information mapped in the primary auditory cortex?

Answer 15

• tonotopic / map of basilar membrane

Question 16

What occurs in the secondary auditory cortex (belt area)?

Answer 16

• much more sophisticated analysis of sound

- construct different elements of sound

Question 17

What is Wernicke’s area?

Answer 17

• where sound is decomposed into meaningful language