ch 8

Sound and Hearing

Origin of Sound

  • Sound originates from the motion or vibration of objects, e.g., vocal cords, instruments, aircraft.

  • This vibration causes changes in pressure in the surrounding medium (usually air).

  • Human auditory system detects a wide range of air pressure changes (0.00002 to over 100 Pascals).

Auditory Processing

  • Hearing is not just about detection; it's about constructing an internal model of the world.

  • This model is based on sensory input and past experiences.

  • The hearing brain seeks constancy in sensory input, mirroring principles found in visual processing (e.g., color constancy).

Differences Between Auditory and Visual Senses

Sensitivity to Temporal and Spatial Information

  • Auditory system excels in detecting rapid temporal changes (e.g., speech sounds).

  • It struggles with spatial location compared to vision.

  • Example: Morse code is easier to process audibly rather than visually.

Auditory System Basics

Sound Waves

  • Pure tones have a sinusoidal waveform and a specific pitch corresponding to frequency (measured in Hertz).

  • Human hearing ranges from 20 Hz to 20,000 Hz.

  • Loudness is related to sound intensity (amplitude), measured in decibels.

Psychological vs. Physical Properties

  • Distinction between physical (frequency/intensity) and perceived properties (pitch/loudness).

  • Example: Loudness can affect perceived pitch.

  • Sounds are rarely pure tones; most are combinations of multiple frequencies.

Sound Processing Anatomy

Ear Structure

  • The ear has three main parts: outer ear, middle ear, and inner ear.

  • Outer ear gathers sound waves; middle ear converts vibrations; inner ear (cochlea) transforms these into neural impulses.

  • Cochlea has a basilar membrane that responds to different sound frequencies.

From Sound to Neural Signals

  • Auditory nerve transmits signals from the ear to the brain.

  • Signals pass through several nuclei before reaching the primary auditory cortex in the temporal lobe.

  • Non-uniform sensitivity in the basilar membrane: high frequencies stimulate one end, while low frequencies stimulate another.

Cortical Auditory Processing

  • Primary auditory cortex (A1) is critical for processing sound; it relies on adjacent secondary auditory areas for more complex processing.

  • Damage in primary auditory areas may not cause deafness but can impair sound identification and localization.

  • The auditory cortex exhibits tonotopic organization: spatial arrangement mirrors frequency responsiveness.

Sound Localization Strategies

Inter-aural Differences

  • Sounds arriving at ears at different times (inter-aural time difference) and intensities provide spatial cues for localization.

Head-Related Transfer Function (HRTF)

  • Personal anatomical features affect sound perception; the brain uses this information to localize sounds.

Auditory Memory and Stream Segregation

Sensory Memory in Auditory Processing

  • Auditory memory aids in grouping sounds into distinct streams for comprehension.

  • The mismatch negativity (MMN) component highlights responsiveness to auditory deviations.

The Cocktail Party Problem

  • Ability to focus on one auditory stream amidst competing noises is a key challenge for auditory processing.

Music Perception

Biological Basis of Music Perception

  • Music perception has innate components and is universally present across cultures.

  • Humans inherently perceive rhythms and pitches without needing formal training.

Music Neural Processing

  • Evidence shows different neural pathways for music and speech, with emphasis on pitch and rhythm processing.

  • Potential hemispheric specializations exist (right hemisphere for pitch; left for timing).

Voice and Speech Perception

Importance of Voice Information

  • Voices convey social information, including identity and emotional state.

  • The auditory system has specialized pathways for processing these signals.

Speech Processing Mechanisms

  • Speech involves complex processing, distinguishing phonemes despite acoustic variability.

  • Different theories (e.g., motor theory of speech perception) describe how auditory and motor systems interact for speech understanding.

Categorical Perception

  • Continuous auditory signals mapped to discrete categories aids in speech recognition and understanding.

Summary of Key Concepts

  • Sound perception involves extracting features like loudness and pitch.

  • Auditory processing is hierarchical and involves distinguishing "what" (content) vs. "where" (location) information.

  • Music and speech processing share some neural pathways but also exhibit distinct characteristics.