Lecture 1 Notes: A Journey Through the Auditory System

Perception & Cognition

  • Hearing? - Sven Mattys
  • Check in code: TBD

Background Readings

  • Book chapters relevant to each lecture are indicated on the VLE and on the first slide of each weekly lecture as background readings.
  • Background readings are meant to help you understand the content of the slides.
  • Materials in these readings that are not covered in the slides will not be tested at the exam.

Plack, C. J. (2013 or 2018) “The Sense of Hearing”

  • Taylor and Francis, Hove, UK.
  • ISBN 978-0-8058-4884-7 or 978-1-1386-3259-2
  • The University Library has copies. Also available as an e -book.
  • Both editions (2 and 3) are acceptable.

Tutorial

  • "Some of the reasons why pitch perception is important"
  • Links back to lectures 1 and 2.

Qs?

  • Discussion board on the VLE (Hearing discussion board).

Four Lectures on Hearing

  • Lecture 1: A journey through the auditory system.
    • Today 19/02 10AM
  • Lecture 2: Frequency selectivity and speech perception
    • 27/02 2PM
  • Lecture 3: Spatial hearing: Where are sounds coming from?
    • 06/03 2PM
  • Lecture 4: Impaired hearing: What goes wrong when hearing is impaired?

Hearing: Lecture 1

  • A journey through the auditory system
  • Sven Mattys
  • Background readings: Plack - Chapters 1, 2, 4.

Objectives of this Week’s Lecture

  • What is hearing used for?
  • What is sound?
  • How does the auditory nervous system work?

Why Study Hearing?

  • Hearing is an amazing sense
  • Basis to understand the perception of speech and music
  • Design systems for
    • Telecommunications
    • Entertainment systems
    • Auditory alerts and warnings
  • Mimic human behaviour
    • Automatic speech recognition (ASR)
  • Help people with impaired hearing
    • Hearing aids
    • Cochlear implants

Listen to This…

  • How many sources of sound can you hear?
    • (1) < 5
    • (2) 5-10
    • (3) 11-15
    • (4) > 15
  • Menti Code: TBD

What Did You Hear?

  • Man starts talking
  • Dog barks
  • Birds twitter
  • Woman replies to man
  • Birds continue to twitter
  • Cockerel crows
  • Baby cries
  • Siren starts
  • Man starts commentating in Italian (?)
  • Woman starts speaking in US English
  • Woman sings

What is Hearing Used For?

  • Who or what is the source of sound?
  • Where is the source of sound located?
  • In which direction is the source of sound moving?
  • What information is the source sending?

Sound Arises

  • Sound arises from the movement or vibration of an object.
  • The movement alternately squeezes air molecules together and pulls them apart.
  • This ‘pressure wave’ spreads outward from the source of sound to the listener.

What is Sound?

  • Waveform - time pressure
  • Period (sec)
  • Amplitude - Intensity “Loudness”
  • Frequency - “Pitch”
  • Pitch is expressed in Hertz (Hz), which is the number of times a period is repeated every second. A sound made of a repeating period that lasts 5 ms has a frequency of 200 Hz.

Attributes of Sound

  • Physical Attribute
    • Amplitude/intensity
      • Intensity: Loudness
    • Frequency
      • Pitch
    • Complexity
      • Timbre
  • Psychological Attribute

Decibel (dB) Scale

  • Sound pressure (µPa)
  • Sound pressure level (dB)
  • Examples:
    • Hearing threshold: 20 µPa, 0 dB
    • Woods: 100 µPa, 20 dB
    • Bedroom: 200 µPa, 30 dB
    • Library: 1,000 µPa, 40 dB
    • Conversation: 10,000 µPa, 60 dB
    • Normal traffic: 100,000 µPa, 80 dB
    • Rock group: 10,000,000 µPa, 110 dB
    • Take-off: 100,000,000 µPa, 130 dB
    • Pain threshold: 200,000,000 µPa, 140 dB

Definitions of Loudness and Decibels

  • Loudness: Attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud.
  • Decibels (dB): A convenient scale for measuring the intensity of one sound compared with another.

Waveform and Spectrum

  • Frequency = 1000 Hz
  • Intensity = 0 dB, +20 dB, +40 dB
  • Increase in Amplitude, Intensity, Loudness

Attributes of Sound

  • Physical Attribute
    • Amplitude/intensity
      • Intensity: Loudness
    • Frequency
      • Pitch

Definition of Pitch

  • Pitch: Attribute of auditory sensation in terms of which sounds can be ordered on a musical scale.

Frequency

  • Frequency examples: 400 Hz, 800 Hz, 1600 Hz, 3200 Hz

Attributes of Sound

  • Physical Attribute
    • Amplitude/intensity
      • Intensity: Loudness
    • Frequency
      • Pitch
    • Complexity
      • Timbre

Definition of Timbre

  • Timbre: Attribute of auditory sensation in terms of which a listener can judge that two sounds similarly presented and having the same loudness and pitch are dissimilar.
  • Timbre relates to the quality or complexity of a sound.

Spectrum of a Complex Tone

  • Harmonics are always multiples of the fundamental frequency.
  • In order to have a timbre, a sound must have more than one frequency. It must be a harmonic complex tone.
  • Example: 200 Hz (1st), 400 Hz (2nd), 600 Hz (3rd) - Harmonics. Fundamental frequency

Composition of a Harmonic Complex Tone

  • A harmonic complex tone can be created by adding pure tones together.
  • 1st, 2nd, 3rd, 4th Harmonics
  • Fundamental Frequency
  • These are the pure tones
  • This is the harmonic complex tone

Quiz Time…

  • A harmonic complex tone has a fundamental frequency of 150 Hz. What is the frequency of its 2nd harmonic?
    • 1: 2 Hz, 2: 200 Hz, 3: 300 Hz, 4: 450 Hz
  • Menti Code: TBD

Examples of Sounds

  • Examples of sounds that have roughly the same loudness and pitch but have different timbres

Equaliser as a Timbre Modulator

  • 32, 64, 125, 250, 500, 1K, 2K, 4K, 8K, 16K
  • -12 dB, 0 dB, +12 dB

Sounds with Different Timbres

  • The following three sounds have roughly the same loudness and pitch, but they have different timbres:
  • Speech sounds differing in timbre.
  • Vocal cords.
  • Spectrum of sound from larynx.
  • Profile of vocal tract.
  • Spectrum of resonances radiated from lips.

Quiz Time

  • What is this?
    • 1 A spectrum
    • 2 A spectrogram
    • 3 A waveform
    • 4 None of those
  • Menti Code: TBD

Quiz Time…

  • These two sounds are likely to differ in [you can choose more than one answer]:
    • 1 Pitch
    • 2 Loudness
    • 3 Timbre
  • Code: TBD

How Does the Auditory Nervous System Work?

The Human Auditory System

  • Outer ear
    • Pinna
    • Ear canal
  • Middle ear
    • Ear drum
    • Hammer
    • Anvil
    • Stirrup
    • Oval window
    • Eustachian tube (connects with throat)
  • Inner ear
    • Cochlea
    • Auditory nerve
    • Vestibule
    • Round window

Parts of the Middle Ear

  • Three Ossicles = Hammer (Maleus) + Anvil (Incus) + Stirrup (Stapes)

Middle Ear

  • Ossicles in the middle ear viewed from the inside. This image was shot with a fibre optic camera threaded up through the Eustachian tube.

Ossicula Auditus

  • Malleus
  • Incus
  • Stapes

Contact Point

  • This movie depicts the stirrup (stapes) pushing against the oval window
  • Contact point between middle ear and inner ear: Stirrup

Stapedial Reflex

  • Automatic (involuntary) muscle contraction in the middle ear in response to loud sounds (~ 10-20 dB below the pain threshold).
  • Muscles stiffen the ossicular chain, pulling the stapes away from the oval window.
  • This “auditory reflex” prevents damages to the cochlea (inner ear).

The Cochlea

What Does the Cochlea Do?

  • The principal job of the cochlea is to act as a frequency analyzer, that is, to convert energy at different frequencies into neural activity in different fibers of the auditory nerve.

Let's Uncoil the Cochlea

  • Cochlea, uncoiled
  • Round window
  • Auditory nerve
  • Basilar membrane
  • Hair cells of the sensory cells
  • Spiral canal of the cochlea
  • Tectorial membrane
  • Organ of Corti
  • Oval window
  • Scala tympani

The Basilar Membrane

  • The basilar membrane vibrates when the stapes moves the fluid in the inner ear.
  • The basilar membrane vibrates maximally at different points on its surface depending on the frequency of sound.

Frequency Analysis by the Basilar Membrane

  • Animation of a basilar membrane responding to music

The Organ of Corti

  • Tectorial membrane
  • Outer hair cells
  • Inner hair cell
  • Auditory nerve
  • Basilar membrane
  • Scala vestibuli
  • Scala media
  • Scala
    tympani
  • Sulcus

Hair Cells

  • Inner hair cells
  • Outer hair cells

Function of Hair Cells

  • Inner hair cells detect the movement of the basilar membrane.
  • Outer hair cells amplify the movement of the basilar membrane.

Place Coding

  • The inner hair cells detect the movement of the basilar membrane
  • Inner hair cells rubbing against the tectorial membrane, triggering an action potential in the neural fibre.

Frequency and Basilar Membrane

  • Decoding high frequency with maximum wave amplitude at the base
  • Decoding low frequency with maximum wave amplitude at the apex

Tuning Curves

  • Each auditory-nerve fibre responds only to a narrow range of frequencies. This reflects the tuning of the basilar membrane at the place which excites the fibre.

Cochlea as Frequency Analyzer

  • There are many overlapping single-fibre tuning curves in the auditory nerve.
  • Their response patterns confirm that the cochlea acts as a frequency analyzer.

Frequency Coding

  • The frequency of a tone is also coded by when and how fast fibres fire.
  • Inner hair cells fire in synchrony with peaks in the waveforms of tones.
  • Aggregated over many fibers, the time intervals between spikes indicate the period of the stimulus.

Frequency – Pitch

  • The frequency (pitch) of a tone is coded by which fibres are active in the auditory nerve, and by when (how fast) those fibres fire.

Complexity – Timbre

  • The complexity (timbre) of a tone is coded by which combination of fibres are active at the same time.

Intensity – Loudness

  • Fibres specialize in the type of intensity they code for:
    • Low threshold: good for detecting sounds with very low intensities but is not good at detecting small intensity differences between sounds.
    • High threshold: good for detecting small differences between sounds ranging from medium to high intensity but is not good at detecting sounds with very low intensities.

Intensity – Loudness

  • The intensity (loudness) of a tone is coded by how quickly fibres are discharging in the auditory nerve.
  • To a first approximation, the loudness of a sound is related to the total neural activity evoked by the sound.

Auditory Nervous System: Cochlea

  • The Cochlea acts as a frequency analyser. It converts sound energy at different frequencies into neural activity in different fibres of the auditory nerve.

Auditory Nervous System: Cochlear Nucleus

  • The Cochlear Nucleus acts as a relay station. Monaural.
  • It sends neural activity to other nuclei in the brain-stem for further analysis.

Auditory Nervous System: Superior Olive

  • The Superior Olive analyses the location of sources of sound. Binaural!
  • This happens early in the ascending auditory system because it relies on very precise timing between the ears – of the order of millionths of a second.

Auditory Nervous System: Inferior Colliculus and Medial Geniculate

  • The Inferior Colliculus and Medial Geniculate analyse the pitch of sounds.
  • This happens quite early in the ascending auditory system because it relies on relatively precise timing – of the order of thousandths of a second.

Auditory Nervous System: Cerebral Cortex

  • The primary auditory area in the cerebral cortex analyses higher-order features of sounds, including their spectral shape.
  • This happens later in auditory pathway than analyses of location and pitch because it is less reliant on precise timing.

Auditory Cortex Areas

  • The primary, secondary, and associative cortex are located in the temporal lobe.
  • Primary Auditory Cortex (BA 41)
  • Secondary Auditory Cortex (BA42)
  • Auditory Association Cortex (BA 22) (Wernicke’s Area)

Quiz Time…

  • What are the likely consequences of disabling outer hair cells on sound perception?
    • 1 Alteration of pitch perception
    • 2 Reduced loudness
    • 3 Detection failure (won’t hear the sound)
    • 4 Alteration of timbre perception
  • Inner hair cells
  • Outer hair cells
  • Code: TBD

Summary

  • What is hearing used for? What, who, where
  • What is sound? Three dimensions: frequency (pitch), intensity (loudness), complexity (timbre)
  • How does the auditory nervous system work? Cochlea = frequency analyzer, more complex as impulses are relayed to cortex

Next Week

  • How do we use frequency selectivity to identify speech sounds and tell speech sounds from one another?
  • Check in code: TBD