Audition

• Functions of hearing


  Sounds carry important information about others and our surroundings. Through hearing, we can detect certain attributes of sound - complexity, intensity and frequency
• What is sound?


  Sounds are produced by vibrating objects - these vibrations displace the surrounding medium (liquid air) which creates pressure changes

  
• Psychophysical properties of sound - frequency

  Cycles per time unit. Measured in Hertz; 1Hz = 1 cycle per second. This is perceived as pitch - low frequency = low pitch, high frequency = high pitch

  
• Psychophysical properties of sound - amplitude

  Changed in the magnitude of sound, still same frequency. Measured in decibels (dB). This is perceived as loudness - high amplitude = loud sound, low amplitude = quiet/soft sound

  
• Psychophysical properties of sound - complexity


  This is the frequency composition. It can vary from a pure tone (simple, single frequency) to a complex, mis of frequencies - e.g. fire alarm you hear the exact frequency beep, piano you can hear some vibrations after the initial sound. This is perceived as sound quality

  
• Frequency cont


  Different animals are sensitive to or can detect sounds within different frequency ranges e.g. we can't hear dog whistles but for dogs it is very clear

  
• Amplitude cont


  Everyday sounds have different intensities e.g. leaves rustling is only 20dB but a jackhammer is just over 100dB

  
• Complexity cont


  Complex sounds are most common as it is actually quite rare to hear pure tones in the environment. See example in image

  
• How do we perceive sound?

  - The auditory system can detect changes in air pressure across time in a frequency specific manner

  - Human ears can perceive each individual frequency and its amplitude variation independently

  - The brain receives the info of sound detection and assigns meaning to it

  - Sound perception is just the beginning of auditory experience!
• Human ear - structure and function

  Divided into the outer, middle and inner ear.

  Outer ear - captures and amplifies sound waves

  Middle ear - amplifies and transmits vibrations

  Inner ear - translates vibrations into neural activity

  
• Middle ear - sound transmission

  Air filled occupied by ossicles, the 3 smallest bones in the human body - malleus, incus and stapes

  Ossicles vibrate in response to tympanic vibration which amplifies and transmits sounds to the inner ear (oval window)

  
• Inner ear - structure and function

  Has very complex structure. Oval window - sound communicated from middle ear, cochlea - where sound is detectedTwo functions;equilibrium- vestibular organs filled w/ solution, cells detect movement of this solution - what indicates if we are straight up, upside down or moving around etc.,sound information

  
• Basilar membrane


  Apex 5x wider and 100x less stiff than at base of cochlea, sensitive to dif sound frequencies. When sound enters, creates actual wave in basilar membrane which reaches it's peak at the part of the membrane for that frequency.

  
• Organ of Corti


  Organ within cochlea, near to basilar membrane. Has hair cells which when moved due to vibration in basilar membrane, opens ion channels that release neurotransmitters to propagate signal to nerves and through to the brain.

  
• Pressure transmission along the canals

  Vibrations of stapes push/pull flexible oval window in/out of vestibular canal at base of cochlea. Pressure waves deflect the basilar membrane in frequency specific manner. All pressure ends up moving round window and dissipates
• Cochlea cross section - 3 canals

  Vestibular canal, tympanic canal, and middle canal

  
• Inner and outer hair cells

  Tectorial membrane is like a flap that moves depending on vibrations. We have about 3.5k inner hair cells per ear, and 12k outer hair cells per ear. Inner hair cells are key for detecting sound and once we lose them, they are gone forever - very sensitive so if too loud, they will die.

  
• Inner and outer hair cells - tectorial membrane

  It is attached at one end and projects into middle canal. The membrane floats above the inner hair cells and touches the outer hair cells. Vibrations of the basilar membrane change how the tectorial membrane 'sits' which bends the stereocilia

  
• Vibration to neural activity translation in inner hair cells

  Stereocilia are 6the 'hair' bit of hair cells, molecular filaments (aka tip links) connect the rips of cilia to neighbouring K+ channels.

  Resting state = basal K+ influx and NT release

  Basilar membrane vibration = bend stereocilia, increase K+ influx,increase NT release at cell base

  
• Coding of frequency


  Place code - frequency info coded by place along cochlea w/ greatest mechanical displacement

  
• Coding of amplitude


  Louder sounds produce larger vibration of basilar membrane, makes inner hair cells release more NT

  
• Auditory pathways pt1


  Hair cell NT release activates bipolar cells that form auditory nerve (cranial nerve VIII)

  Auditory nerve enters medulla, making synapsis in a tonotopic manner (frequency spatial representation of basilar membrane maintained)

  Axons from cochlear nuclei ascend to superior olivary complex in pons

  
• Auditory pathways pt2


  Inputs from each ear are processed by both olivary nuclei (sound source spatial location - if no lag = in front of u, if lag between each ear's info reaching it = coming from a certain side).

  Series of ascending projection along midbrain ends up in primary auditory cortex (A1) - the tonotopic representation is preserved up to A1

  
• Hearing loss


  Hearing declines w/ age or damage (permanent or transitory) to any components of auditory pathway
• Transitory hearing loss


  Obstruction of ear canal (e.g. excessive wax), damage to tympanic membrane
• Transitory hearing loss; conductive hearing loss


  Caused by problems in ossicles (i.e. otitis media during ear infections)
• Permanent hearing loss; Otoscelrosis


  Excessive growth of ossicles, requires surgery
• Permanent hearing loss; Sensorineural hearing loss

  Most common, due to defects in cochlea or auditory nerve

  Damage to hair cells caused by toxicity or excessive exposure to noise
• Age related hearing loss


  Frequency sensitivity depends on age, more pronounced in men

  
• Hearing aids - cochlear implants

  These bypass the degenerated inner hair cells. A miniature flexible electrode array is surgically implanted to cochlea via oval window.

  A receiver detects and processes sound in radio signals which are sent to stimulator (implanted in skull during surgery)

  Miniature electrodes positioned in frequency specific regions emit electrical signals which activates nearby bipolar cells and auditory nerve