sound wave vs acoustic energy vs sound
sound wave = physical disturbance caused by the movement of energy travelling through a medium
acoustic energy = produced when a sounds waves makes something vibrate, like a drum
sound = definition depends on your perspective, e.g. monist, naive realist
ossicles
malleus → incus → stapes
MIS
acoustic reflex
- involuntary contraction of muscles attached to stapes and malleus when intense sound stimuli (70-100dB)
- reduces movement = reduces amplification
bending of hair cells
vibration along basilar membrane, causes bending and hair cells against tectorial membrane bend too
- deformation = depolarisation and transmitter release
- onto spiral ganglion cells → axons form auditory part of cochlear nerve
innervation of hair cells
- Inner hair cells synapse with afferent fibres of CNVIII (to brain)
- Outer hair cells receive input from efferent fibres of CNVIII (from brain)a
amplification of quiet sounds
- for quiet stimuli (<50dB), brain sends signals via CNVIII to outer hair cells, makes them contract
- pulls basilar and tectorial membranes closer together = amplifies vibration
- also bends inner cells → depolarise → more likely to fire
coding schemes for auditory info
Tonotopic map maintained from basilar membrane = a place code
And fibres of the auditory nerve will fire increasingly in response to increasing SPL (sound pressure level (dB)) = a rate code
projections
- superior olive [medulla]
- inf colliculus ⇒ startle reflex, projections descend to spinal cord
- medial gen nucleus ⇒ final relay station, for processing WHAT and WHERE info
- auditory cortex
sound localisation
Interaural time difference
- MSO cells require coincident input to depolarise
- sound reaches L ear before R ear, so coincident firing on MSO cell furthest from L ear and closest to R ear
- not on other cells, bc takes longer for sound to get to R ear and along pathway
Interaural level difference
– head masks sound coming from one side (acoustic shadow)
- if sound primarily coming into one ear, LSO receives excitatory ipsi input and inhibitory contra. input
- = sound localisation
- used to convey WHERE info upstream
auditory cortex - progression
Info goes CORE → BELT → PARABELT
- 1º = A1 ⇒ specific frequencies & simple tones (lower order)
- ordered low → high frequency (ant → post)
- 2º = The belt ⇒ respond to complex sounds, sort features of sound (higher order)
- 3º = Parabelt ⇒ a/a
dorsal vs ventral stream info
Dorsal stream = WHERE (binaural integration, audiovisual integration - localisation)
Ventral stream = WHAT (sorting sound based on patterns, content, duration and location - identification)
Signal-to-noise optimisation
- ventral belts
- masking when amplitude of noise > signal, or when noise and signal similar frequencies (e.g. speech in crowded room)
- to optimise signal:noise ratio
- head shadow effect (turn head, one ear facing signal, tune in to this one)
- attention
- lip reading
cocktail party phenomenon
- multi-sensory modality: visual input - read lips
- attention: the concentration of awareness on some phenomenon to the exclusion of other stimuli
- PFC (using ACh) → other brain regions, defining where focus is, top-down processing
- Also to respond to sudden external stimuli → bottom up
