Lecture Notes: The Auditory System

Introduction to the Auditory System

• Focus on the auditory system responsible for hearing.
• Sound: Stimulus energy activating auditory receptors.
  • Defined as air vibrations perceived by organisms.
  • Range of audible sounds varies by species.
  • Human hearing is sensitive to minuscule vibrations (Hudspeth, 2014).
  • Humans use hearing to gain information from the environment, e.g., footsteps, voices.

Properties of Sound Waves

• Amplitude: Intensity/loudness measured in decibels.
  • Higher amplitude = louder sound.
  • Exception: Rapid speech vs. slow music.
• Frequency: Number of air compressions per second (measured in hertz).
  • Determines pitch (high frequency = high pitch; low frequency = low pitch).
  • Human hearing range: 15 - 20,000 Hz.
• Timbre: Complexity of sound wave.
  • Allows differentiation between sounds of the same pitch.
  • Harmonics contribute to different sound qualities.
• Prosody: Communication of emotions through variations in pitch, loudness, and timbre.

Structure of the Ear

• Outer Ear: Pinna and external auditory canal.
  • Pinna helps locate sound sources.
• Middle Ear: Evolutionary adaptation for land hearing.
  • Components: Tympanic membrane, malleus, incus, stapes.
  • Function: Amplifies sound vibrations to oval window.
• Inner Ear: Cochlea with auditory hair cells.
  • Hair cells stimulated by fluid movement, activating the auditory nerve.

Theories of Pitch Perception

• Place Theory: Different basilar membrane areas respond to specific frequencies.
  • High frequency = base activation; Low frequency = apex activation.
• Frequency Theory: Entire membrane vibrates in synchrony with sound.
  • Limited by neuronal refractory period.
• Volley Principle: Groups of neurons alternate firing to handle frequencies 100-4000 Hz.

Auditory Pathway

• Auditory input sent to auditory cortex in the brain.
• Primary Auditory Cortex (A1): Essential for auditory imagery.
  • Separate pathways for "what" (anterior temporal cortex) and "where" (posterior temporal and parietal cortex).
  • Motion Deafness: Inability to detect auditory movement due to superior temporal cortex damage.

Sound Localization

• Methods:
  • Time of arrival differences between ears.
  • Intensity differences ("sound shadow").
  • Phase differences for low-frequency sounds.
• Learning: Sound localization requires adjustment as head grows.

Individual Differences in Pitch Perception

• Amusia (Tone Deafness): Difficulty detecting small frequency changes.
• Absolute Pitch: Ability to identify notes accurately without reference.

Hearing Loss

• Conductive Deafness: Middle ear bones fail to transmit sound.
  • Causes: Disease, infection, tumors.
  • Can be corrected with surgery or hearing aids.
• Nerve Deafness: Damage to cochlea or auditory nerve.
  • May include tinnitus (ringing in the ears).
  • Causes: Hereditary factors, disease, loud noise exposure.

Hearing Challenges in the Elderly

• Difficulty following conversations despite hearing aids.
  • Possible reasons: Language cortex deterioration, attention issues.
  • Visual attention to speakers aids auditory focus.

These notes provide an overview of key concepts related to the auditory system, including the properties of sound, the ear's structure, theories of pitch perception, sound localization, individual differences, and hearing loss.