23- The Auditory System (10.17.25)

PBSI 320

The auditory system

10.17.25

Fundamentals of the Human Auditory System, Part II (moving onto the brain)

Recap of the Parts

Place Code for Frequency

The intensity of sound required to produce an above-baseline response of a hair cell

Note the correspondence of the sensitivity of a neuron to sound and the displacement of the basilar membrane

Note the increased sensitivity for higher frequencies, aiding discrimination ability at these frequencies

Place Code for Frequency

Like a tuning function for orientation in V1

Place Code for Frequency

Psychophysical evidence from the amplitude of noise needed to mask perception of a tone

Temporal Code for Frequency

Volley principle: action potentials occur in sync with the peak of the eliciting sound wave (although they do not fire at every peak) Temporal code found in the firing rate across the population of cells Not informative above ~5,000 Hz

Amplitude Representation

Louder tones tend to activate a broader range of hair cells (more total) Can combine with which cells are active to provide amplitude information

Amplitude Representation

Louder tones tend to activate a broader range of hair cells (more total) Can combine with which cells are active to provide amplitude information Different cells with the same characteristic frequency can have different sensitivities

Hearing Tests

Tested over a range of frequencies and intensities Measure the minimal intensity needed to elicit perception at each of a range of frequencies

Causes of Hearing Loss

Conductive hearing impairments transmission of sound to the cochlea is impaired possible causes: damage to the ossicles damage to the tympanic membrane blockage of the auditory canal inflammation of the middle ear (“earache”) Some possible treatments: amplify sounds (hearing aid) prosthetic ossicles

Causes of Hearing Loss

Sensorineural hearing impairments damage to the cochlea, auditory nerve, or auditory pathways of the brain some causes are congenital often a recessive gene (needs to be inherited from both parents) seen in ~1 in 1,000 births (commonly tested very early in development) Commonly occurs with aging and can be caused by exposure to loud noises

Hearing Loss with Age: Presbycusis

Buildup of exposure to factors such as noise, environmental toxins, head trauma, and poor nutrition more evident at higher frequencies more pronounced in men

The Consequence of Noise on Hearing Loss

Hearing loss can result from: prolonged exposure to 85+ dB short-term exposure to 120+ dB (will also be painful) single brief but high-intensity impulse often maximal ~4,000 Hz

The Consequence of Noise on Hearing Loss

Some causes of noise-related hearing loss: mechanical damage due to high-amplitude pressure waves tearing the basilar membrane from the walls of the cochlea tearing out stereocilia breaking tip links between stereocilia hair cell death overstimulation (excitotoxicity) reduced blood flow to the cochlea due to mechanical damage production of oxygen-based free radicals, which damage tissues can develop days and weeks after noise exposure

Tinnitus

Perception of sound when there is none (“ringing in the ears”) common (50+ mil in US, interferes with sleep in ~5% of adults over 50) can be continuous or intermittent not well understood damage to cochlea irritation or pressure on auditory nerve (blood vessels or tumor) changes in neural circuits within auditory cortex associated with noise-induced hearing loss, but no direct correlation varying treatments drugs that reduce auditory neural activity hearing aids to increase “signal to noise” stimulation of auditory nerve or cortex to reduce excitability

Cochlear Implants

Sound system generates electrical impulses through a Fourier analysis that are transmitted to the cochlea to stimulate nerve fibers

Cochlear Implants

After the ear, where to next?

Ascending Pathways:From the Ear to the Brain

• Axons from the inner and outer hair cells = the auditory nerve.
• The first stop is the cochlear nucleus of the medulla.
• Some go to the dorsal nucleus while others go to the ventral nucleus.
• Next is the superior olivary nucleus then the inferior colliculus
• Alt route: bypass the superior olivary nucleus & go directly to the inferior colliculus.
• From the inferior colliculus, the neurons travel to the medial geniculate nucleus of the thalamus
• Finally, from there to the auditory cortex (areas 41 and 42).

From the Ear to the Brain

Auditory processing begins in the brainstem

• whereas visual processing in the thalamus

There is a single overarching pathway (midbrain and thalamus connected) Less strongly lateralized than vision

Tonotopic Representation in Auditory Cortex

Similar principle to the organization of visual cortex

Tuning Curves in Primary Auditory Cortex (A1)

• Similar principle to the representation of orientation in visual cortex
• Considerable variety in the shape of the tuning function
• As with orientation, frequency is represented in the population code (cannot be resolved from a single neuron)

Dorsal and Ventral Pathways in Cortical Processing of Auditory Information

• Similar organization to visual information processing
• Strongly supported by selective deficits seen in patients with brain damage
• Note the neuronal populations that respond to both modalities

Localizing Sound

Thinking about sound in head-centered coordinates

Azimuth: side-to-side dimension (left or right of median plane) Elevation: up-down dimension (above or below horizontal plane) Distance: from center of head (total, any direction)

Localizing Sound

Measuring the accuracy of sound localization

Same/different judgment Minimum audible angle: 75% correct threshold < 10 degrees, can be as low as 1 degree How? Not directly represented in the cochlea (tonotopically organized)

How Sound is Influenced by the Head

• Acoustic shadow cast by the head
• Frequency-modulated
• less severe for lower-frequency sound
• Causes differences in the intensity (dB) of sound between the two ears for sounds off of the median plane

Intraural Level Differences

Interaural level difference caused by the acoustic shadow peaks at 90 degrees azimuth

Intraural Time Differences

• Very small difference in arrival time to each ear due to the speed of sound (on the order of microseconds)
• Normal human sensitivity, as measured in laboratory experiments using artificial stimuli, is very high (< 100 μs)

Ambiguity in Sound Localization

• Sounds from corresponding points in front of and behind a person will produce essentially identical intraural differences
• Insufficient information to resolve the source of the difference in neural activity

Resolving Ambiguity with Head Motion

• By turning the head, intraural differences emerge immediately
• Often done without thinking (unconscious)

Perceiving Elevation

• The pinna causes incoming sounds to reverberate (echo)
• Whether these reverberations amplify or attenuate a sound depends on frequency and elevation (in addition to azimuth)
• You can learn to interpret these differences in terms of elevation

Perceiving Elevation

• Spectral shape cue: the pinna-induced modification of the sound spectrum
• This ability to learn how to interpret the spectral shape cue is important because each person’s pinnae (plural) are unique, like a fingerprint

Perceiving Elevation

• Spectral shape cue: the pinna-induced modification of the sound spectrum
• This ability to learn how to interpret the spectral shape cue is important because each person’s pinnae (plural) are unique, like a fingerprint

Distance Cues for Sound

• Comparing loudness to familiar sounds
• very assumption-heavy
• Sound quality and distance
• reduction in sound level is greater for higher frequencies
• creates a progressive “blurring” effect
• Sound and movement
• moving towards or away
• Doppler effect: the frequency of a moving sound source is higher in front of the source

Human Echolocation

Daniel Kish- a blind man who taught himself to ”see” the world using echolocation

https://www.youtube.com/watch?v=uH0aihGWB8U

https://www.thisamericanlife.org/544/batman

Separating Echoes from the Source

Precedence effect: Sound will arrive first and more intensely from the source, distinguishing it from echoes

Exit Ticket + Resources

Check out this video of a girl hearing for the first time after getting a cochlear implant.

Don't forget your exit tickets!