Perception and Cognition: Hearing

• What is hearing? What benefits does it have?

  → the basis to understand the perception of speech and music

  → design systems for telecommunications, entertainment systems, auditory alerts and warnings

  → mimic human behaviour (automatic speech recogniser ASR)

  → help people with impaired hearing; aids, cochlear implants
• What is sound? What does it arise from?

  → 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 determines pitch/time/what you sound like?

  The rate at which vocal cords vibrate determine the pitch, time, and what you sound like; obstacle in larynx.
• What is pitch (frequency)? Measurements?

  → expressed in Hz, which is the number of times a period is repeated every second

  → sounds can be ordered on a musical scale

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

  → loudness is a psychological experience, as is pitch
• What does a spectrum represent sound as, on a graph?

  → Spectrum represents sound as a consequence of amplitude and frequency

  → on a graph the amplitude does not change, bar of the same height as the intensity is the same but the waves become more frequent as there is more cycles due to increasing pitch
• What are the features of repetitive periodic sound, above and below the line?

  Repetitive periodic sound has two features. Above line is expansion and below is compression over time of air molecules.
• How can a pure tone be summarised?

  → a pure tone can be summarised using only two dimensions

  a pure tone is a sound with a sinusoidal waveform; that is, a sine wave of constant frequency, phase-shift, and amplitude.
• What is amplitude? Two definitions.

  Loudness; the attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud

  Decibels; convenient scale for measuring the intensity of one sound compared with another.
• What is the decibel scale examples?

  The decibel scale:

  Hearing threshold - 20 micropascals or 0 dB

  Pain threshold - 200,000,000 micropascals or 140 dB
• What does increasing amplitude do to a sound, and on a diagram?

  Increasing the amplitude makes the sound louder, and on a diagram the waves become higher and more expanded, but frequency does not change only the height of the waves therefore amplitude.
• What is timbre? Is it complex?

  → 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

  → quality or complexity of a sound
• What is needed for a sound to have a timbre?

  Harmonic complex tone.

  → in order to have a timbre, a sound must have more than one frequency; must be a harmonic complex tone (harmonics are always multiples of the fundamental frequency

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

  → harmonic complex tone can be created by adding pure tones together
• What is the cochlea? What is important about what it is filled with?

  → fundamentally a 'frequency analyser'

  → convert energy at different frequencies into neural activity in different fibres of the auditory nerve

  
  

  fluid filled, harder for vibrations to travel
• What is the outer ear made of? What are the functions of the pores of one of these structures?

  pinna, ear canal

  
  

  (sound elevation and where sound is coming from) Pores in the pinna will gather sounds and attenuate frequencies to determine where the sound is coming from
• What is the middle ear made of?

  e, rw, ow, oss

  eardrum, round window, oval window, ossicles (middle ear bones; hammer, anvil, stirrup. amplify vibrations)
• What is the inner ear made of?

  et, v, c, an

  eustachian tube (connects with throat), vestibule, cochlea, auditory nerve
• what are the features and location of the organ of corti?

  organ of corti is full of neurons

  located on the basilar membrane

  one row of inner hair cells, three rows of outer hair cells
• What are the functions of the basilar membrane? Different areas and what they respond to? think


  → vibrates when the stapes moves fluid in the inner ear

  → vibrates maximally at different points on its surface depending on the frequency of sound

  
  

  wide and floppy, vibrates easily, and responds to low frequencies; apex

  thin, taught, respond to high frequencies; base
• What are the functions of the outer hair cells?

  put energy back into the basilar membrane
• What are the functions of the inner hair cells? When do they fire?

  detect the movement of the basilar membrane

  → rub against tectorial membrane triggering an action potential in the neural fibre

  
  

  inner hair cells fire in synchrony with peaks in the waveforms of tones
• What can each auditory nerve fibre respond to? Do they overlap?

  How can we demonstrate that different places on the basilar membrane code varying frequencies?

  Each auditory nerve fibre responses only to a narrow range of frequencies; reflects the tuning of the basilar membrane at the place which excites the fibre

  
  

  There are many overlapping single-fibre tuning curves in the auditory nerve. Their response patterns confirm that the cochlea acts as a frequency analyser.
• What is the stapedial reflex? What happens to the muscles?

  Stapedial reflex; automatic muscle contraction in the middle ear in response to loud sounds (around 10-20 dB below the pain threshold)

  → muscles stiffen the ossicular chain, pulling the stapes away from the oval window

  → auditory reflex prevents damage to the cochlea; inner ear
• How does the cochlea encode intensity, complexity, and frequency?

  bottom chamber

  scalar vestibuli

  scalar tympani

  vibrations travel through the cochlea to the bottom chamber vibrations travel through the scala vestibuli to the end then to the scala tympani and the oval window
• fibre with low threshold and high spontaneous rate, narrow dynamic range, what type of sounds is it good at detecting?

  This fibre is good for detecting sounds with very low amplitudes, but is not good at indicating that a sound with a high amplitude is more intense than a sound with a medium amplitude.
• fibre with high threshold, low spontaneous rate, wide dynamic range, what types of sounds is it good at detecting?

  This fibre is good for indicating that a sound with a high amplitude is more intense than a sound with a medium amplitude, but is not good at detecting sounds with very low amplitudes.
• What is the role of the cochlear nucleus? What does it send, and where?

  The Cochlear Nucleus acts as a relay station. It sends neural activity to other nuclei in the brain-stem for further analysis.
• What is the role of the SO? What does it analyse, and when?

  The Superior Olive analyses the location of sources of sound. This happens early in the ascending auditory system because it relies on very precise timing - of the order of millionths of a second.
• What are the roles of the IC and Medial Geniculate? When do they complete their role?

  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.
• What is the role of the primary auditory area?

  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.
• Where are the primary, secondary, and associative cortices located?

  Temporal lobe
• The organ of Corti contains hair cells that are responsible for converting sound waves to electrical signals.
• The basilar membrane separates the scala media from the other two chambers.
• What is azimuth?


  Azimuth: Horizontal direction measured as the angle between the observer's heading and the direction of an object.
• What is the importance of locating the sources of sound? (3) attention and sequences

  Localization Importance: Locating sound sources is crucial for various reasons, including providing valuable information, directing visual attention, and distinguishing between different sound sequences.
• What does the auditory system combine for sound localisation?

  Cues Combination: Auditory system combines interaural time differences, interaural level differences, and reverberation cues for sound localization.
• What do low and high frequencies rely on? Think about the duplex theory.

  Frequency Dependency: Low frequencies rely on ITD, while high frequencies rely on ILD for localization.

  
  

  
  

  Lord Rayleigh's "duplex" theory

  → listeners use ITDs to localise low-frequencies (750-) and ILDs to localise high-frequencies (1500+) (when in azimuth)

  → because there is less diffraction and therefore a greater head shadowing effect; can be combined to produce reasonable spatial resolution across a range of pure tone frequencies (Rayleigh 1907)
• What is an ITD, give an example? When is it most important?

  Interaural Timing Differences (ITD): Time lag between when a sound reaches one ear versus the other, crucial for localizing low-frequency sounds.

  
  

  → when the source of sound is on the left, the sound reaches the left ear before the right ear.

  → distance the sound wave has to travel as it bends around your head

  → most important when low frequencies are present
• How does ITD vary with azimuth? What is the smallest ITD that listeners can distinguish from an ITD of 0 microseconds?

  → When a sound comes from one side of an adult head (azimuth +90 degrees), the ITD is 0.65 milliseconds (650 microseconds)

  
  

  → 10 microseconds

  → corresponds to a difference of 1 degree from straight ahead

  → the auditory system has exquisite sensitivity to interaural timing differences
• What is an ILD? What is head shadowing?

  Interaural Level Differences (ILD): Differences in sound intensity between the ears, mainly significant for high-frequency sounds.

  
  

  → when the source of sound is on the left, the head shields the right ear from the sound. This is the 'head shadow'

  - head will prevent some of the energy from a sound soruce on the right from reaching the left ear

  → level of the sound at two ears; a sound from the right will be more intense at the right than the left ear

  → sound intensity decreases as the distance from the source increases

  → it generates an interaural level difference (ILD)
• How does ILD vary with azimuth? What is the smallest ILD that listeners can distinguish from an ILD of 0 dB?

  How does ILD vary with azimuth?

  → when a sound comes from one side of an adult head (azimuth +90 degrees), the ILD is about 20 dB at 6000 Hz, diminishing to 0 dB at 200 Hz.

  What is the smallest ILD that listeners can distinguish from an ILD of 0 dB?

  → 1dB; sensitivity is good across a wide range of frequencies