hearing

sound

• waves traveling, sinusoidal

• lower compression of molecules farther away

• bigger amplitude = louder; higher frequency = higher pitch

amplitude: dB

frequency: Hz (cycles per s)

humans: 20-20,000 Hz

loudness by logarithmic scale: increase in 10 dB represents a 2-fold increase

pure tone

perfect sinusoidal waveform sound

complex sounds

most are NOT pure tones → consisting of multiple frequencies

fourier transform

decomposition of a sound (or signal) to the frequencies that make it up → pure tones within (sum of) complex sounds

power spectrums

frequency domain of frequency (x) vs power (y)

• frequency composition of a sound

pure tone waveform

amplitude (y) vs time (x)

• 10 spikes => peak at 10 Hz in power spectrum

overview of the auditory system

the ear

• pinna: collects sound and directs in down the ear canal

• sound air pressure waves strike tympanic membrane

• vibration to middle ear bones reach the cochlea

the cochlea

coiled tube with basilar membrane which vibrates with sound waves

• thicker basal end, thinner apical end

• basal end: high frequency vibrations

• apical end: low frequency vibrations

tonotopical organization

within the cochlea, the basilar membrane performs fourier transform at complex sounds

hair cells

sound → electrical signal

vibrations of basilar membrane causes movement of hair cell stereocilia → tectorial membrane

movement of hair cells opens K+ channels, depolarizing the cells → neurotransmitter release WITHOUT action potentials!

auditory nerve

send signals from hair cells to the cochlear nucleus in the brainstem

noise and loss

1/3 in the US 65-74 have hearing loss due to aging AND noise loss exposure

• DEATH OF HAIR CELLS!

hearing aid

small electrical device that amplifies sound

• parts: microphone, amplifier, speaker

cochlear implant

used for complete or near complete deafness

• bypass and replaces hair cells to stimulate the auditory nerve

superior olivary nucleus

cochlear nucleus sends information to the SON

• brainstem, critical for sound localization

  • has to computed, not encoded in peripheral receptors

  • 2 ears (differences in speed and loudness)

interaural time difference

delay and time difference

• detected by medial superior olive

coincidence detectors

• ITD = 0; straight ahead

• ITD > 0; RIGHT ear first

• ITD < 0; LEFT ear first

interaural level difference

sound loudness difference between 2 ears

• detected by lateral superior olive