Sound Perception and Properties

Sound Perception

• Sound is the brain's interpretation of sound waves.
• Sound occurs when vibrations cause pressure changes.

Physical Properties of Sound

• Frequency: The number of cycles of a sound wave per unit of time (Hertz, Hz).
  • Higher frequency = higher pitch.
  • Lower frequency = lower pitch.
• Amplitude: The measure of sound pressure changes, related to loudness.
  • Higher amplitude = louder sound.
  • Lower amplitude = softer sound.
• Threshold: Minimum amplitude needed for sound to be perceived.
  • Threshold varies with frequency.
  • Humans typically perceive sounds within a range of 500-5,000Hz.
• Decibel Scale: A logarithmic scale measuring sound intensity.
  • Each 10-fold increase in amplitude corresponds to a 20 decibel increase.
• Timbre: The quality or character of a sound, distinguishing it from other sounds of the same pitch and loudness.

Sensory Processing of Sound

• Receptors and Transduction: Hair cells in the inner ear convert sound waves into electrical signals.
• Neural Pathways: Auditory nerve transmits signals to the brain.
• Relay Stations: Structures like the inferior colliculus, medial geniculate nucleus, and olivary complex process sound information.
• Primary Auditory Cortex: Processes basic sound features.
• Association Auditory Cortex: Interprets complex sounds and meaning.

Ear Structure and Function

• Outer Ear: Pinna funnels sound waves into the ear canal.
• Middle Ear:
  • Tympanic membrane (eardrum): Vibrates in response to sound waves.
  • Ossicles (malleus, incus, stapes): Amplify vibrations.
  • Air-filled cavity.
• Inner Ear:
  • Cochlea: Fluid-filled structure where sound vibrations are converted to electrical signals.
  • Basilar membrane: Vibrates in response to sound frequency, with different parts responding to different frequencies.
    • Base (high frequencies), Apex (low frequencies).
  • Organ of Corti: Contains hair cells – sound receptors.
• Tonotopy: Organization of the auditory system where different frequencies are processed in different locations in the brain. Inner hair cells convert pressure to electrochemical signals. This triggers an electrical signal to the brain.

Sound Localization

• Medial superior olive: Relays information from both ears to localise sound sources.
• Inferior colliculus: Processes location information about sounds.
• Medial geniculate nucleus: Relays auditory information from the thalamus to the primary auditory cortex, maintaining frequency information.

Primary Auditory Cortex (A1)

• Located in the temporal lobe.
• Columns organized by frequency, with high frequencies processed caudally (toward the back).

Secondary Auditory Cortex (A2)

• Analyzes sound complexities.
• Supports auditory memory.
• Wernicke's area (part of A2): Associated with language comprehension; receives input from the primary auditory cortex.
• Wernicke's aphasia: Fluent speech without meaning.

Vestibular System

• Semicircular canals: Detect head movement and aid in balance.
• Utricle and saccule: Sense head position in space.

Hearing Disorders

• Hearing Loss: Sensorineural (cochlea damage), Mechanical (damage before cochlea), mixed.
• Tinnitus: Perception of sound (ringing, buzzing) with no external source.
• Otosclerosis: Abnormal bone growth affecting ossicles.

Auditory Illusions

• Pitch Circularity: How sound frequency is perceived as pitch.
• McGurk Effect: Visual cues influencing auditory perception.

Connection between Hearing and Schizophrenia

• Middle ear disease correlation.
• Difficulty separating inner thoughts from external stimuli.