JL

Psych 5.4 Hearing

The Auditory System: Anatomy, Function, & Perception

Learning Objectives

By the end of this section, you will be able to:

  • Describe the fundamental anatomy and function of the auditory system.

  • Explain the mechanisms involved in encoding and perceiving pitch.

  • Discuss how the auditory system localizes sound in the environment.

Introduction to the Auditory System

The auditory system is a sophisticated biological mechanism responsible for converting mechanical pressure waves into meaningful electrical signals that the brain interprets as sound. This intricate process allows us to engage with the world through sound, facilitating appreciation for natural sounds, music, and the complexities of human communication via spoken language.

This overview will delve into:

  • The basic anatomy of the ear and its components.

  • How sensory stimuli (sound waves) are transduced into neural impulses.

  • The brain regions involved in processing auditory information.

  • The theories explaining how we perceive different pitches.

  • The mechanisms by which we determine the origin of a sound.

Anatomy of the Auditory System

The ear, the primary organ of hearing, is structurally divided into three main sections:

1. The Outer Ear

  • Pinna: The visible, external part of the ear that protrudes from the head. It collects sound waves.

  • Auditory Canal: A tube leading from the pinna to the eardrum, through which sound waves travel.

  • Tympanic Membrane (Eardrum): A thin, taut membrane that vibrates in response to sound waves.

2. The Middle Ear

This air-filled cavity contains three tiny bones, collectively known as the ossicles, which amplify and transmit vibrations:

  • Malleus (Hammer): Attached to the tympanic membrane.

  • Incus (Anvil): Connects the malleus to the stapes.

  • Stapes (Stirrup): Presses against the oval window of the cochlea.

3. The Inner Ear

This fluid-filled labyrinth contains structures crucial for both hearing and balance:

  • Semicircular Canals: These structures are primarily involved in maintaining balance and detecting head movements (part of the vestibular sense), not directly involved in hearing itself.

  • Cochlea: A fluid-filled, snail-shaped structure that is the primary organ of hearing. It houses the sensory receptor cells (hair cells) of the auditory system. (Refer to Figure 5.18, which illustrates these divisions).

Function of the Auditory System: The Pathway of Sound

From Sound Wave to Neural Impulse

The process of converting sound waves into neural signals involves a precise sequence of mechanical and biological events (as depicted in Figure 5.18):

  1. Sound Wave Entry: Sound waves enter the auditory canal.

  2. Tympanic Membrane Vibration: The sound waves strike the tympanic membrane, causing it to vibrate.

  3. Ossicle Movement: These vibrations are transferred to the malleus, then the incus, and finally the stapes, causing the three ossicles to move in a coordinated manner.

  4. Oval Window Pressure: The stapes presses into a thin membrane of the cochlea called the oval window.

  5. Cochlear Fluid Movement: The pressure from the stapes causes the fluid inside the cochlea to move.

  6. Hair Cell Stimulation: The fluid movement stimulates specialized hair cells, which are the auditory receptor cells. These hair cells are embedded in the basilar membrane, a thin strip of tissue running along the inside of the cochlea.

  7. Mechanical Activation: The stimulation of hair cells is a mechanical process that ultimately leads to their activation.

  8. Neural Impulse Generation: Once activated, hair cells generate neural impulses.

  9. Auditory Nerve Transmission: These neural impulses travel along the auditory nerve towards the brain.

Brain Processing of Auditory Information

Upon reaching the brain, auditory information follows a specific pathway for processing:

  1. Inferior Colliculus: The initial processing center.

  2. Medial Geniculate Nucleus of the Thalamus: A relay station in the thalamus.

  3. Auditory Cortex: The final destination for processing, located in the temporal lobe of the brain.

Similar to the visual system, research suggests that auditory information related to both sound recognition (what the sound is) and sound localization (where the sound is coming from) is processed through parallel streams within the brain (Rauschecker & Tian, 2000; Renier et al., 2009).

Pitch Perception

Our perception of pitch is directly related to the frequency of sound waves. Low-frequency sound waves are perceived as lower-pitched sounds, while high-frequency sound waves are perceived as higher-pitched. The auditory system employs sophisticated mechanisms to differentiate between these various pitches, explained by two key theories:

Theories of Pitch Perception

  1. Temporal Theory (Frequency Theory):

    • Assertion: This theory proposes that the frequency of a sound wave is encoded by the activity level (firing rate) of a sensory neuron. In this model, a hair cell would fire action potentials at a rate directly corresponding to the frequency of the incoming sound wave.

    • Limitations: While initially intuitive, the temporal theory alone cannot account for the entire range of human hearing, which spans approximately 20 ext{ Hz} to 20,000 ext{ Hz}. Neurons have a maximum firing rate due to the properties of sodium channels involved in action potentials (Shamma, 2001). This inherent physiological limitation means that a single neuron cannot fire fast enough to encode the highest frequencies we perceive.

  2. Place Theory:

    • Assertion: This theory suggests that different portions along the basilar membrane within the cochlea are selectively sensitive to sounds of different frequencies (Shamma, 2001).

      • The base of the basilar membrane (closer to the oval window) responds most effectively to high frequencies.

      • The tip (apex) of the basilar membrane responds most effectively to low frequencies.

    • Mechanism: Therefore, hair cells located in the base region are effectively