NEUR305: Auditory

What is sound?

Pressure waves through air evoked by quickly moving objects

Purpose of signaling?

Show presence of objects, prey/predators (avoiding danger), animal communication

Properties of signals?

• Frequency - Pitch

• Amplitude/Intensity - Loudness

• Timing/Phase

• Complexity - Timbre

Structure of the ear

Outer ear (pinna & ear canal), middle ear (ossicles), inner ear (cochlea)

Outer ear function?

Amplifies sound in 3k Hz range (range of human speech)

ex. bats with big ears have ridges that create frequency dependent sound level differences

Middle ear function?

Condenses air waves into signals

Consists of the malleus, incus, stapes, and tympanic membrane

What is impedance matching?

Inner ear is fluid-filled and the rest is air!

Expected loss of signal from the switch of mediums (due to reflection of signal), is accounted for in an amplification of the signal.

How does the middle ear deal with impedance?

Mechanical advantage through tympanic ossicles that act as levers; focuses the force thru the ration between tympanic membrane and oval window

Inner ear function?

Waves enter scala vestibuli through oval window

Waves exit through scala tympani out round window

What is the cochlea + 3 steps?

Transduces mechanical energy into electrical signals!

• hair cells detect motion of wave

• cilia motion transduces to neurotransmitter release

• spiral ganglion cell axons form the cochlear nerve

What converts the stimulus to action potentials in the auditory system?

Spiral ganglion cells!

What’s the tonotopic map of cochlea?

Base = higher frequency

Apex = lower frequency

What’s the auditory pathway?

auditory nerve → cochlear nuclei → superior olivary nucleus → inferior colliculus → thalamus (MGN) → primary auditory cortex

What’s the auditory cortex function?

complex selectivity for sound features; context dependent

What’s the thalamus function?

associating sound w/ reward; attentional modification; shaping to process complex sound

What’s the inferior colliculus function?

full spatial map of sound location; spectrotemporal processing; motor structures & startle response

What’s the superior olivary nucleus function?

LOCALIZE SOUND

Medial superior olive is interaural time difference (ITD) or time differences (time to for sound to reach the two ears).

Lateral superior olive is interaural level difference (ILD) loudness differences (difference in sound pressure level reaching two ears)

What’s ITD (Interaural Time Differences)?

ITD look at the time sound reaches the two ears!

Medial Superior Olive

Same distance = no time difference

Smaller heads have small ITDs

What’s ILD (Interaural Level Differences)?

Looks at the intensity of the sound reaching both ears; greater intensity = louder

First ear to be stimulated activates entry level, which inhibits contralateral; only side with high excitation travels up!

What happens as we move up the auditory “hierarchy”?

Stimulus selectivity increases! It may be multi-dimensional: frequency, bandwidth, intensity, modulation frequency, spatial location

What are secondary auditory areas?

It’s the belt area; prefers complex sounds; what and where streams

How is sound perceived differently based on relevance?

Belt area is more lateralized than the core!

In speech, the left belt is more active.

In music, the right belt area is more active.

What is the auditory processing of barn owl?

Owl’s ears are not symmetrically aligned which leads to differences in sound input.