Week 3 - Hearing

Nature of sound

  • Changed by air-pressure

  • Pressure waves characterised by amplitude, frequency, phase

Human hearing range is 20-20000 Hz, varying based on age

Most auditory experiences lie in fraction of this range, 80-1100 Hz

Complex sounds can be built up of various sine waves of varying amplitude, frequency and phase

Sounds can decompose into their sine wave components with a Fourier analysis

Our auditory system does essentially the same thing

Lowest frequency component of a sound is called the Fundamental

Many complex sounds are made up of harmonics - integer multiples of the fundamental

If fundamental is 440Hz, next harmonic will be 880hz etc

Auditory system made out of various components:

Outer ear:

  • Pinna: increases sound amplitude

  • External auditory canal: provides protection, increases amplitude

  • Eardrum (tympanic membrane): vibrates in response to sound waves, moves bones in the middle ear

Middle ear:

  • Ossicles: Malleus, Incus, Stapes (Smallest bones in human body)

  • Ossicles transmit vibration of the eardrum (with some more amplification) to cochlea through lever actions

  • Also provide protection against high amplitude sounds, as muscles attached to ossicles restrict the bones movements

Inner ear:

  • Semicircular canals (important for vestibular sense i.e. sense of orientation

  • Cochlea: Containing auditory sensory receptors, Oval window is a membrane covering an opening in the cochlea, stapes is directly attached to this oval window, and oval window is much smaller than the ear-drum, this size difference helps amplify sound waves. Cochlea is also filled with a watery liquid which moves in response to vibrations coming from middle ear

  • Vestibular Canal

  • Tympanic Canal

  • Cochlear Canal

  • Reissner’s membrane and Basilar membrane separate these. Basilar membrane has auditory receptor cells (hair cells). These membranes vibrate in response to vibrations of the oval window

Mechanism:

When the Basilar membrane vibrates, hair cells also set in motion, which convert this vibration into neural signals

Central Auditory Pathways:

Nerve fibres from each cochlea synapse in a number of sites on the way to the primary auditory cortex:

  • The cochlear nucleus

  • The superior olivary nucleus

  • The inferior colliculus

  • The medial geniculate nucleus

The signal arriving at the cochlear nucleus splits and goes to each of the superior olivary nuclei, beyond this point, input from both ears is present in both hemispheres

A number of tasks can be done prior to signal reaching auditory cortex, and animal studies show these can include responding to:

  • The onset of sound

  • Changes in sound intensity

  • Changes in sound frequency

Similar studies reveal tasks which cannot be performed without the cortex. These include:

  • Discriminating the patterns of several tones

  • Discriminating the duration of sounds

  • Localising sounds in spaces

Thus it seems the cortex deals with more complex auditory tasks while the lower structures deal with simpler aspects of sound

Speech perception requires structures beyond the primacy auditory cortex

Frequency coding:

Basilar membrane is about 30mm long and varies in stiffness and width, with travelling waves moving along the membrane peaking at different points depending on frequency

Thus, the location of the peak identifies the frequency of a sound

When people have damage to a specific part of the cochlea, they tend to suffer from frequency-specific hearing loss

Stimulating auditory nerves at different cochlear locations leads to perception of sounds in different pitch

Hair cells are tuned to different ranges of frequency according to the location along the Basilar membrane

Auditory neurons are arranged in an orderly manner, with this organisation seen repeatedly in auditory pathways. These maps of arrangement are called tonotoptic maps

Pitch perception:

Structures beyond the cochlear nucleus should be contributing to pitch perception

Loudness perception:

two basic mechanisms

  • Overall firing rates

  • Range of firing

Factors that affect loudness perception are

  • Sound duration

  • Frequency of sound waves

Higher frequency sounds tend to be perceived to be louder (up to about 5000hz) sounds in 3000-5000Hz range are perceived to be loudest

Auditory space perception:

Determining a sounds:

  • Horizontal direction

  • Vertical direction

  • Distance

Vision is more accurate for providing information about an objects location

Nothing on Basilar membrane directly indicates sound locations

Auditory space perception is a binaural process, with inter-aural time differences relating to onset and phase influencing this ability

Inter-aural time difference:

Unless a sound is directly in front of you, it reaches both ears at different times, and this onset difference can be calculated by our brains to determine angle of sound coming from.

Simple “delay line” mechanism in the brain is used to detect Horizontal direction.

Inter-aural intensity difference:

Same sound should be a bit more intense in ear its closer to, as the energy of a sound wave decreases the further it travels, and the head works as a barrier for this effect as well (effect of this is more pronounced in higher frequency sounds

Neither of these cues work particularly well for pure tones around 1000-3000Hz

Head movements are generally helpful for auditor localisation, and by changing the positions of the ears, you can experience changes in inter-aural time and intensity differences

Humans perceive horizontal directions better than vertical directions through auditory cues

  • Pinnae are more effective at differentiating front/back than above/below

  • Ear positions can be varied freely along the horizontal dimension, and greater range of head movement is also available across this plane than vertical

Limits of auditory localisation:

  • Most of the auditory localisation cues are dependent on the distance between a sound source and ears

  • As a result, it is difficult to distinguish locations of sounds that as equidistant from an ear

Cues for auditory distance perception:

  • Loudness

  • Energy ratio of direct and reverberant sound (not available in open spaces)

Utility of these cues is limited, loudness can tell us only about relative distance and reverberation cues cary depending on various properties of reflection surface for sound wave

The McGurk Effect:

When we visually capture where a sound “should” be coming from, this can override our auditory localisation through hearing alone

Vision does not always dominate however, we give more weight to information that is more informative.

Vision is usually a good source of spatial information, and Audition is usually a good source of temporal information

Auditory information can influence our visual perception by providing conflicting temporal information, exemplified by the Sound-induced Flash illusion