Lecture on Auditory Processing and Tonotypical Mapping

Introduction

• Topic: How the brain distinguishes between different sounds.
• Key Structures: Cochlea, basilar membrane, primary auditory cortex.

Key Concepts

Frequency Differentiation

• Frequency Difference: Base drum (low frequency) vs. bee's wings (high frequency).
• Range of Hearing: 20 Hz to 20,000 Hz.

Role of the Cochlea

• Function: Differentiates sounds of varying frequencies.
• Basilar Tuning: Mechanism within the cochlea for frequency differentiation.
  • Basilar Membrane: Contains hair cells.

Structure of the Cochlea

• Membrane Unrolled:
  • Base: Activated by high-frequency sounds.
  • Apex: Activated by low-frequency sounds.
• Hair Cells: Distributed along the basilar membrane.
  • Base hair cells: Respond to high frequencies (e.g., 1,600 Hz).
  • Apex hair cells: Respond to low frequencies (e.g., 25 Hz).

Sound Processing

Sound Wave Travel

• Path: Ear → Cochlea.
• Example: 100 Hz frequency activates specific hair cells along the basilar membrane.

Action Potentials

• Trigger: Hair cells at specific frequencies generate action potentials.
• Transmission: Signal travels via auditory nerve to the brain.

Primary Auditory Cortex

• Location: Brain region responsible for receiving cochlear information.
• Tonotopic Organization: Cortex areas correspond to specific frequencies (e.g., 0.5 Hz, 16 Hz).

Importance of Basilar Tuning

• Purpose: To enable the brain to distinguish between different sounds.
• Mechanism: Prevents all hair cells from firing simultaneously for any sound.
• Tonotypical Mapping: Organized mapping of frequencies allowing precise sound differentiation.

Summary

• Process:
  1. Sound waves enter the ear.
  2. Sound waves hit the cochlea, activating specific hair cells based on frequency.
  3. Activated hair cells send axons forming the auditory nerve.
  4. Auditory nerve transmits signals to the brain.
  5. Brain processes the signals in the primary auditory cortex, mapped specifically to different frequencies.