PSY 324 Audition

Sound Wave Frequency

Frequency = number of cycles per second, measured in hertz (Hz).

1 Hz = 1 cycle per second.

Physical Dimensions of Sound Waves

Frequency → pitch (low = low pitch, high = high pitch)
Amplitude → loudness (high = loud, low = soft)
Complexity → timbre (simple = pure tone, complex = sound quality)

Tinnitus

Perception of sound (ringing, buzzing, etc.) without external source; can be soft/loud, high/low, continuous or intermittent.

Types: objective (physical cause, blood flow cause), subjective (hearing system damage).

No effective cure → prevention (protect hearing).

Sound Wave Complexity

Pure tones = single frequency
Complex tones = multiple frequencies

Determines timbre (sound quality/uniqueness).

Perception of Sound

Sound waves (air pressure) have frequency → pitch and amplitude → loudness.

The auditory system converts these waves into neural signals sent to the brain.

Functional Anatomy of the Auditory System

Ear collects sound waves → converts to neural signals → sent to auditory cortex.

System processes frequency, amplitude, and complexity and helps locate sound.

Sound production and perception systems are closely connected.

Processing Sound Waves

Pinna collects and funnels sound into the ear.

Ear canal amplifies sound and sends it to the eardrum, which vibrates.

Processing Sound Waves (Middle Ear)

Middle ear = air-filled chamber with ossicles.

Ossicles (hammer, anvil, stirrup) transmit vibrations from eardrum to cochlea (oval window).

Processing Sound Waves (Inner Ear)

Cochlea (fluid-filled) converts sound to neural signals.

Basilar membrane → transduction of sound.
Hair cells → detect movement.
Tectorial membrane → interacts with hair cells.

Transducing Sound Waves

Sound creates a traveling wave along the basilar membrane.

High frequency → base; low frequency → apex.

Auditory Receptors

Hair cells convert sound into neural signals.

3500 Inner hair cells = receptors; 12 000 outer hair cells = adjust sensitivity.

Bending of hair cells changes membrane potential → neurotransmitter release.

Auditory Receptors (Outer Hair Cells)

Without inner hair cells → deafness.

Outer hair cells enhance hearing by sharpening sound and adjusting tectorial membrane stiffness.

Auditory Receptors (Cilia Movement)

Toward tallest cilia → depolarization → ↑ neurotransmitters → ↑ nerve signals.

Toward shortest cilia → hyperpolarization → ↓ neurotransmitters → ↓ nerve signals.

Otoacoustic Emissions

The cochlea (outer hair cells) produces sound waves.

These amplify hearing; some sound is emitted back out of the ear.

Pathways to the Auditory Cortex

Inner hair cells → auditory nerve (CN VIII) → brainstem (cochlear nucleus).

Signals go to superior olive and both sides of the brain.

Cochlear nucleus & superior olive → inferior colliculus → medial geniculate nucleus (thalamus).

Then to primary auditory cortex (A1) and nearby cortical areas.

Auditory Cortex

A1 (primary) in Heschl’s gyrus; surrounded by A2 (secondary areas).

Wernicke’s area (left) → speech processing.
Right hemisphere → music processing.

Neural Activity and Hearing

Pitch is coded by tonotopic organization (frequency mapping).

This mapping starts in the cochlea and is preserved through pathways to the auditory cortex.

Hearing Pitch

Cochlear implant converts sound to neural signals to restore hearing.

Low frequencies (<200 Hz) → coded by firing rate, not location (all apex cells respond).

Detecting Loudness

Greater amplitude → higher firing rate.

Stronger waves → more hair cell movement → more neurotransmitter release.

Detecting Location

Sound location is determined by comparing input from both ears.

ITD → difference in arrival time.
IID → difference in loudness between ears.

Locating a Sound

Sound reaches the nearer ear first → brain detects timing difference (ITD).

Brain combines signals → one clear sound direction.

Horizontal = azimuth; vertical = elevation

Localizing Language in the Brain

Language mainly in left hemisphere.

Broca’s area → speech production.
Wernicke’s area → language comprehension.

Aphasia

Damage to language areas → impaired speech or comprehension.

Broca’s aphasia → non-fluent speech, comprehension intact.
Wernicke’s aphasia → fluent but meaningless speech, poor comprehension.

Disrupting and Eliciting Speech

Supplementary speech area → stopping speech (speech arrest).

Motor/somatosensory cortex stimulation → produces vocalization.

Parallels Between Birdsong and Language

Both are partly innate but shaped by experience and learning.

Develop during a critical period; influenced by genes + environment.

Brain has a template; experience refines it (preference for own species/language)