4.2 Auditory Perception

Our ears are incredible sound detectors, transforming vibrations into electrical signals our brains can understand. From the outer ear to the auditory cortex, each part plays a crucial role in processing sound waves and creating our auditory experience.

Auditory perception isn't just about hearing sounds; it's about making sense of them. Our brains use complex processes to determine pitch, loudness, and location. Attention also plays a key role, helping us focus on specific sounds in noisy environments.

Auditory System and Sound Processing

Structures of auditory system

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• Outer ear
  • Pinna collects and funnels sound waves (concha)
  • Ear canal channels sound waves to eardrum (2-3 cm long)
• Middle ear
  • Eardrum vibrates in response to sound waves (0.1 mm thick)
  • Ossicles amplify and transmit vibrations (malleus, incus, stapes)
  • Eustachian tube equalizes air pressure (opens when swallowing)
• Inner ear
  • Cochlea contains hair cells for sound transduction (snail-shaped, 35 mm long)
  • Auditory nerve transmits electrical signals to brain (30,000 nerve fibers)
• Auditory cortex processes and interprets auditory information (temporal lobe)

Process of auditory transduction

1. Sound waves enter ear canal
2. Eardrum vibrates
3. Ossicles amplify vibrations
4. Cochlear fluid moves
5. Hair cells stimulated
   • Stereocilia bend opening ion channels
6. Neurotransmitters released
7. Action potentials generated in auditory nerve fibers
8. Signals transmitted to auditory cortex

Auditory Perception and Attention

Principles of sound perception

• Pitch perception
  • Place theory activates specific basilar membrane regions (high frequencies at base, low at apex)
  • Temporal theory neural firing patterns match sound wave frequency (phase locking)
• Loudness perception
  • Amplitude of sound wave correlates with perceived loudness (measured in dB)
  • Weber-Fechner law perceived loudness increases logarithmically with intensity ($\Delta I / I = k$)
• Sound localization
  • Interaural time difference sound reaches one ear before other (up to 0.7 ms)
  • Interaural level difference sound louder in one ear (high frequencies)
  • Head-related transfer function spectral cues from ear shape and head position (pinna notches)

Role of auditory attention

• Selective attention focuses on specific stimuli while ignoring others (dichotic listening tasks)
• Cocktail party effect focuses on single conversation in noisy environment (uses visual cues)
• Bottom-up attention automatically orients to salient auditory stimuli (sudden loud noises)
• Top-down attention voluntarily focuses on specific auditory information (searching for a voice)
• Auditory scene analysis groups and segregates auditory streams (based on pitch, timbre, location)
• Attentional capacity limitations in auditory processing (7 ± 2 items)
• Impact of attention on auditory perception
  • Enhanced processing of attended stimuli (increased neural response)
  • Reduced processing of unattended stimuli (attentional filter)
• Inattentional deafness fails to perceive auditory stimuli due to attentional focus elsewhere (gorilla experiment)