Psych 351C: Test 1 Review (Auditory System)

What are the two goals of the auditory system

1. Receive sound using auditory receptors

2. Differentiate stimuli by intensity and frequency

What are the 3 parts of the ear

Outer ear

Middle Ear

Cochlea (inner ear)

Outer ear

The part of the ear that exists outside of the body, is shaped specifically to direct/funnel sound into the middle/inner ear

Middle ear

Made up of the ossicles and tympanic membrane, sound hits the membrane and then is transferred through the ossicles

The bones amplify the sound pressure (x22) onto the oval window, part of the cochlea (much smaller thus there is a pressure increase)

What 3 bones make up the ossicles

Malleus, incus and stapes, smallest bones in the body

Organized like a lever system, the maleus hits the incus which hits the stapes

Cochlea

A hollow tube that is spiraled in the ear, turns mechanical vibrations into the neural signals

Part of vestibular system, so plays a role in balance

What are the two membrane covered holes at the base of the cochlea

Oval window - entry point

Round window - pressure release (exit point)

What are the 3 fluid filled membranes of the inner ear and what forms them

Scala tympani - formed by round window

Scala media - formed by vestibular membrane and basal membrane

Scala vestibule - formed by the oval window

What are the 5 steps of how sound flows through the cochlea

1. Stapes impact oval window

2. Movement in oval window creates pressure in fluid of scala vestibule

3. Pressure change causes basilar membrane to ripple

4. Basal membrane induces pressure in scala tympani

5. Scala tympani pressure creates movement in round window

Basilar membrane

A line that exists in the centre of the cochlea, is wider and floppier at the apex and gets thinner as it moves towards the base

Has a tonotopic response, meaning that it resonates at specific areas based on frequency (lower frequency closer to apex)

Why is it said that the basilar membrane is organized tonotopically

Similar frequencies are processed in similar areas on the membrane

How is a lower/higher frequency transmitted to the cochlea

By how quickly (frequently) the stapes knocks against the oval window

slower = lower frequency and vice versa

Hair cells

Cells that are connected to the basilar membrane that move when it flexes

Turn the movement (mechanical vibrations) into a neural signal

What do hair cells synapse onto

Spiral ganglion cells which form the auditory nerve (similar to what is seen in optic nerve in the retina)

Tone Mapping

Spiral ganglion cells receive input from one hair cell at a specific location on the basilar membrane leading to them responding to a specific frequency

Characteristic frequency

The specific frequency that a ganglion cell responds to, also modulated by intensity where louder sounds lead to increased AP firing

When the output of a ganglion cell is recorded and graphed what can we expect regarding characteristic frequency and shape of the graph

The graph is not flat, it is bell shaped where the peak of the bell is the characteristic frequency of that specific cell

What is important to note regarding the shape of a ganglion cell firing when graphed

The bell shape implies that it fires when presented with relatively similar frequencies, not only a specific frequency with less similar frequencies leading to less firing

What is the human auditory spectrum

About 20 to 20 000 Hz, decreases with age

What are the 4 stops as part of the auditory pathway

1. Ganglion cells form auditory nerve travel directly to the medulla (part of brain stem)

2. Travels from medulla to inferior colliculus (part of brain stem)

3. Travels to medial geniculate nucleus (MGN)

4. Travels to auditory cortex located in temporal areas of the brain

What is the role of the medulla

Involved in sound localization, houses the olivary nucleus the area where input from both ears is first compared

Coincidence detectors

Cells located in the olivary nucleus that respond best to specific time differences between the ears

Interaural time delays

Delay between the sound reaching the separate ears

What is the delay if sound comes from directly ahead, perpendicular to the head or 45 degrees from the head

Ahead - - 0.0 ms delay

Perpendicular - 0.6 ms delay

45 Degrees - 0.3 ms delay

Intensity differences

Closer sounds are more intense, so differences between ears can be used to localize sounds

What is the theory regarding how sounds are localized with regard to their frequency and intensity

Suggests that low frequency sounds use time delay while high frequency sounds use intensity differences

Likely because low intensity sounds are not as effective at traveling through the head

Auditory cortex

Collection of cortical neurons in the primary auditory cortex (A1) that are frequency tuned and tonotopically mapped

What is seen with frequency curves in A1

They are sharp/narrow, suggests that each cell maximally responds to specific frequency and intensity

What is seen with frequency curves in A2

Broader curve due to broader receptive frequencies, suggests that cells focus only on one category (frequency/intensity) and ignore the other

What are the left and right hemispheres of the auditory cortex specialized for respectively

Left - language

Right - music and environmental sounds

Why does it mean for the auditory cortex to be internally hierarchically organized

Simpler areas are more medial and complex areas are more lateral

What are the 3 parts of the auditory cortex and what to they respond to

1. Core (A1) - responds to very simple sounds at specific frequencies and intensities

2. Belt (A2) - responds to complex patterns of sounds

3. Parabelt (A3) - responds to complex patterns of sounds

T or F: The auditory cortex is also organized anteriorly and posteriorly

T, more posterior parts deal with “where” and anterior parts deal with “what”

What/Where Auditory Experiment

Had participants complete two tasks and then applied TMS to anterior/posterior regions and measured participants RT on tasks

What where the two tasks of the what/where auditory experiment

Spatial - participant hears a tone and then a second tone that is either to the left/right of the original, asked to determine if second tone is to the left or right of original

Identification - participant hears a tone then a second higher/lower tone and asked to determine if second tone is higher/lower than original

What were the findings of the what/where auditory experiment

TMS applied anteriorly - impaired on identification but not spatial (impaired on what)

TMS applied posteriorly - impaired on spatial but not identification (impaired on where)

What are the 3 types of neurons in the auditory cortex and what do they respond to

Simple - respond to simple tones

Complex - respond to vocalizations

More complex - respond to spoken language functions

What happens if a more complex neuron is damaged

One will be impaired on their linguistic functions

What are the two ways deafness can occur with regard to the auditory system

Damage to ears/cochlea - deafness occurs in same ear as damage

Damage to auditory cortex - unilateral lesions typically lead to problems with localization, not deafness