Hearing, Olfaction

PSYCH 230 Exam 2

Sound

Vibrations that travel through air or other medium; traveling waves that are more/less condensed in fixed cycle/frequency

• Speed in air/room temp = 340 m/s

Sound frequency

Determines our sense of pitch —> measured in cycles per second, Hz

• Higher frequency = higher pitch we perceive

Sound amplitude

Determines our sense of loudness —> measured in decibel, dB

• Higher amplitude = louder

Pure tone

A sound with a perfect sinusoidal waveform

• Most sounds are not —> complex sounds

  • Ex: speaking — different syllables; composition of multiple frequencies

Fourier transform

Decomposition of a sound (or other signal) to the frequencies that make it up

• Any complex sound, no matter how complicated, we can decompose (break) into sum of pure sinusoids

Power spectrum

Receipt of how to get complex waveform — how much of each frequency to get original complex sound

• Power spectrum plot shows frequency of composition of sounds

  • What is the frequency content of a complex sound

  • X axis = frequency, y axis = power

Logarithmic scale

Each increase in 10 dB represents a 10-fold increase in sound intensity and is perceived by humans as twice as loud

The ear auditory system

• Pinna (outer ear) collects sound vibrations and channel to ear canal

• Sound air pressure waves strike tympanic membrane — causes vibrations

• Tympanic membrane forwards to inner ear

  • Middle ear bone (malleus, incus, stapes) passes vibrations to the cochlea

Cochlea

Coiled tube where translation of a vibration to neural signal happens

• Like retina in vision

• Contains basilar membrane — has thicker basal end and thinner apical end

  • Differences in thickness and rigidity across basilar membrane 

  • Organized tonotopically —> where one side is for higher and other lower

• Sound waves cause basilar membrane to vibrate but basal end is thicker so it will be vibrated by higher frequencies vs apical end with lower frequencies

  • Basilar membrane decomposes complex sounds into component frequencies

Hair cells

Converts sounds to electrical signals

• Vibration of the basilar membrane causes movement of hair cell stereocilia

  • Similar to photoreceptors in retina

  • Tectorial membrane = where inner hair cells are anchored

• Movement opens K+ channels, depolarizing cell

  • Depolarization causes neurotransmitter release — no action potentials

• Auditory nerve sends singals from hair cells to cochelea nucleus in the brainstem

Hearing loss

1 in 3 people in US lose hearing at 65-74, ½ of 75+ = difficulty hearing

• Enhanced by excessive exposure to loud sounds across lifetime

• Also a component of just aging

• Often due to death of hair cells 

Hearing aid 

Small electronic device that amplifies sound

• 3 basic parts: microphone, amplifier, speaker

• Send to middle ear, then cochlea

Cochlear implant 

Used when have complete or near-complete deafness

• Ex: no hair cells at all — hearing aid won’t do the job, therefore need implant

• Bypasses/replaces hair cell to directly electrically stimulate the auditory nerve (basilar membrane)

  • Similar to bionic retina

Superior olivary nucleus

Brainstem nuclei critical for sound localization —> know where sound came from

• Cochlear nuclei sends info here

• Computes difference between ears — can hear difference in loudness and earlier/later

  • Having two ears provides cues to localize sounds

    • Sound location has to be computed; it is not encoded in peripheral receptors

Interaural time difference (ITD)

How much earlier it comes to one ear relative to the other

Interaural level difference (ILD)

How much louder in one ear relative to the other 

Medial superior olive (MSO)

Detects ITD

Lateral superior olive (LSO)

Detects ILD

Auditory cortex (AC)

Higher level auditory processing

• Lesions typically do not cause deafness — impacts high level auditory perception

  • Animals can still detect and respond to presentation of a sound; lower parts are sufficient

  • Rather, impair recognition of complex sounds — speech, sound localization, and hearing in noisy environments

• Neurons respond to both simple and complex sounds — “promiscuous”, not very selective

Frequency response area (FRA)

Receptive field for auditory cortex

Best/Characteristic Frequency

The sound frequency that makes a neuron respond strongest

• Around 11Hz

Tonotopic organization

How the AC is organized

• Neurons responding to high frequencies are located in the posterior end of AC and neurons responding to low frequencies reside in anterior end

  • Gradient of preferred frequencies

Experience-dependent plasticity

Ongoing exposure to sound can cause expansion in the tonotopic map

• Repeatedly exposing rat pups to sound (7Hz) increased representation of that sound in AC

• Single cells in AC can shift their “best frequency” following experience 

  • Spiking responses of neuron before and after pairing the tone with a footshock

T-butyl mercaptan

Harmless, non toxic chemical that is added to odorless natural gas (heating, cooking) to make it easier to detect in case of a leak

• Rotten egg smell

Odorant

Molecule that has a smell

• Aroma compound; like pure tone

Odor

The sensation that a mix of odorants gives — smell, scent

Olfactory epithelium

Found at top of nasal cavity

• Odorants either enter through nose or mouth via back of throat

• Odorants dissolve in the mucus covering it

Olfactory transduction

• Odorants bind to cilia receptors — first cells that transduce into neural signals

  • aka olfactory sensory neurons / olfactory receptor cells

    • Undergo constant turnover ever 4-6 wks

  • Binding of odorant causes olfactory receptor to interact with G-protein —> causes chain reaction inside neuron: influx of Ca++ and Na+, depolarization, action potentials

Combinatorial coding

Each olfactory sensory cell in epithelium expresses only 1 type of receptor BUT each receptor can bind to a number of odorants

• Each cell only has one type of “lock” (receptor) but multiple “keys” (odorants) can open each lock

• Brain decodes — knowing the pattern of which neurons are activated, can tell what is being sniffed

Olfactory receptor cells to olfactory bulb to brain

• Olfactory receptor cels project to glumeruli in olfactory bulb

  • Each glomerulus receives input from just one type of odorant receptor

• Output from olfactory bulb send info directly to olfactory cortex via olfactory tract

Hyposmia

Reduced sense of smell

Anosmia

Loss of sense of smell

• Permanently or temporarily