Journey through auditory system Flashcards
Why study hearing
- BasIS to understand perception of speech + music
- Hearing is first sense dev
- Mimic human beh : Automatic Speech Recognition (ASR)
- Help people with impaired nearing: hearing aids, cochlear implants
What is hearing used for
- identify who or What source of sound is
- locating source of sound
- Direction of sound moving
- info source is sending us
what is sound
- Arises from movement/ vibration of an object
- movement squeezes air molecules together + pulls them apart
- Air pressure change travels from source to detector
-pressure wave spreads out from source to listener - Amplitude comes from size of pressure = intensity = loudness
- period between air Molecules = frequency = pitch
Amplitude + frequency ( pitch)
- plot amplitude along frequency
- when location of bar changes = different pitch
- when height changes = different volume
3 attributes of sounds
- Amplitude = loudness
- Frequency = pitch
- complexity = timbre
Loudness - decibel scale
-Very large scale
- compress it logically
- speech is 55-65 d B
Definition of loudness + decibels
- loudness = attribute of auditory sensation in Terms of which sound can be ordered on a scale extending from quiet to loud
- decibels measure intensity of one sound compared to another
-loudness represented via amplitude with frequency being constant
-size Of peakes + valleys
Pitch
- attribute of auditory sensation in terms of which sounds can be ordered on a musical scale
- loudness is same (amplitude)
- Amount/frequency of peaks + troughs increases as pitch increases
Timbre Definition
- auditory sensation in terms of which a listener can judge that 2 sounds similarly presented + having the same loudness + pitch are dissimilar
- Quality or complexity of sound
- Timbre exists when themes more than 1 frequency
- Must be harmonic complex tone
-Harmonics are multiples of the fundamental frequency (first harmonic) - 1st = 200, 2nd = 400, 3rd: 600 (400+200)
composition of a harmonic complex tone
-created by adding pure tones together
- waveform = peaks + valleys of of pure tones added -together- harmonic complex tone
speech sound differing in timbre
-vocal cords tighter = higher frequency/pitch sound
-vocal cords looser = flap a bit = low frequency
- Larynx = vocal cords
- spectrum of sound from larynx
- Base spectrum
- from base spectrum some are amplified, some reduced
- The way you move your mouth amplifies different frequencies to make different vowels
Human auditory system - outer ear
- Pinna = flap of flesh - perception of elevation (where is sound?)
- Ear cannal = conveys air pressure change to ear drum
Human auditory system -middle ear
- Eardrum -> seals off middle ear from outer world + has 3 ossicle bones, converts air pressure change to fluid pressure change
- ossicle bones-> amplify vibrations through lever effect, transfer vibrations to oval window (this is much smaller so acts as amplification due to increased pressure)
Human auditory system - inner ear
- cochlea - filled with fluid which reduces hearing because pressure changes travel difficulty through fluid
- Stirrup = contact point between middle + inner ear + it pushes against oval window
stapedial reflex
- Automatic (involuntary) muscle contraction in middle ear in response to loud sounds
- muscles stiffin ossicular chain pulling stapes away from oval window
- contract if detect a sound causing too much pressure change -stops vibrations
-prevents damage to cochlea
the cochlea
- Acts As frequency analyser > breaks down complex harmonic sound into pure harmonic tones t converts energy at different frequencies into act nerval activity
-End of middle ear, stabe vibrate = oval window vibrates = cochlea vibrates at same rate
-Basilar membrane from base of cochlear to apex
Basilar membrane
- Becomes wider from base to apex + thinner , stiff at base, floppy at apex
-vibrates when stabes moves fluid in inner ear - High frequency makes base vibrate most
-vibrates maximally at diff points on its surface depending on sound frequency - Allows breakdown of sound into component frequencies
Organ of corti
- sits on basilar memb
- contains auditory fibres/ receptive cells
-Hair cells
organ of corti inner hair cell
- detects movement of basilar memb
- when hair is bent it triggers an AP
- As memb vibrates, pushes hair against tectorial memb
-Don’t replicate it exactly for every vibration bVt most of the time when its bent right it triggers an AP - chain of vibration carried from outer ear + converted to impulses
Organ of corti outer hair cell
- Amplify movement of basilar Memb
-Puts energy back into basilar memb
-Press tectorial memb up + down as it detects vibration - makes it easier for inner hair cell to detect movement
Demonstrating different places on basilar memb coding for different frequencies - Turning curves
-put microelectrode into 1 fibre along memb
-see now it responds to range of frequencies with different intensities
- fibres respond more specifically to certain frequencies = specialised
-High frequency specific when play low frequency sound (+ visa versa)
-Many overlapping single- fibre turning curves
Demonstrating different places on basilar memb coding for different frequencies
- frequency coded by location on basilar memb + frequency of fibres firing
- frequency of tone coded by when + how fast fibres fire
- inner hair cells fire in synchrony with peaks in waveform of tones
- single fibre creates neural discharge on same location of waves of pressure
- time intervals between spikes indicate period of stimulus
frequency-pitch
-coded by which fibres are active in auditory nerve + when (how fast) fire
complexity - Timbre
- coded by which combination of fibres are active at same time
Intensity - loudness
- fibres specialise in type of intensity they code for
- loudness is now quickly fibres discharge in auditory nerve
-related to total neural activity evoked by the sound - low threshold= good detecting low intensity but can’t distinguish small intensity between sound
- High threshold = good detecting small differences in medium - high intensities not low
Auditory nervous system - cochlear nucleus
- acts as relay station
-monaural - sends neural activity to other nuclei in brain stem for further analysis
Auditory nervous system - superior olive
- analysis the location of sound
-binaural - early process relies on very precise timing between ears
Auditory nervous system - Inferior colliculus + Medial geniculate
- analyse pitch of sounds
- carry in ascending auditory system-relies on precise timing
Auditory nervous system - Primary auditory area
- cerebral cortex
- analyses higher-order features of sounds incl. spectral shape
- later in path -> less reliant on precise timing
consequences of disabling outer hair cells on sound perception
-reduced loudness
- detection failure- unlikely to make all sound indetectable high intensity should be detectable
Location of auditory
- primary, secondary + associative cortex located in temporal lobe
