Perceptual processes ch 10

• Chapter 10 discusses hearing in the environment

Psychoacoustics

• Psychoacoustics correlates psychological dimensions of acoustics
• Psychoacoustics studies how the auditory system works by examining sound perception
• Psychoacoustics studies physical characteristics of sound alongside psychological sensations
• Loudness is the psychological sensation of physical amplitude or intensity dimensions
• Pitch is the psychological sensation of the physical frequency dimension

Measuring Sound Perception

• Audibility threshold refers to the lowest reliably detectable sound pressure level at a given frequency
• Equal-loudness curve plots sound pressure level (dB SPL) against frequency for constant perceived loudness

Equal Loudness Data

• Lower frequency sounds often need more amplitude to be perceived as equally loud
• Higher frequency sounds usually need less amplitude to be perceived as equally loud
• Frequency composition determines sound perception, aligning with auditory system's tonotopic organization

Temporal Integration

• Temporal integration is a process where constant sound is perceived louder with greater duration
• The term temporal integration also applies to perceived brightness depending on light duration
• Tones presented for 100ms are perceived as quieter than those presented for 300ms
• The effect of temporal integration occurs up to the 300ms threshold
• Perceived loudness remains constant beyond 300ms

Psychoacoustics of Pitch

• Pure tone research indicates human ability to detect small frequency differences
• Sensitivity to frequency changes is greatest in the middle of the auditory range, detecting a 0.1% change
• Sensitivity declines slightly at the extremes of the frequency range

Pitch Sensitivity

• Humans can typically hear frequencies from 20-20,000 Hz
• Sensitivity declines with age, mainly at higher frequencies
• Upper range is reduced to 15,000 Hz around post-secondary age
• Upper range decreases to 12,000 Hz by the 50s
• Teenagers can hear sounds around 17,400 Hz
• High-pitched sounds around 17,400 Hz were used to deter teens from loitering in the US during the 90s
• In the 2000's high pitched sounds around 17,400 Hz were used to create ringtones teachers couldn't hear

Ambient Room Noise

• Room ambient noise consists of sounds recorded inside an empty room
• Empty spaces have background noise levels of approximately 20db
• Sound-making factors consist of air currents creating pressure waves, noises from systems, hums from electronics, etc
• All frequencies combine to create “noise"

Types of Noise

• Noise consists of a wide range of frequencies at specific energies
• White noise is every frequency in the auditory range (20-20,000Hz) at equal energy
• Pink noise is every frequency in the auditory range (20-20,000Hz) with higher energies at lower frequencies
• Brown noise is every frequency in the auditory range (20-20,000Hz) with even higher energies at lower frequencies
• Blue noise is every frequency in the auditory range (20-20,000Hz) with higher energies at higher frequencies and little low-frequency energy
• Green noise contains every frequency in the auditory range (20-20,000Hz) with higher energies at middle frequencies

Sound Localization

• Locating a sound presents a dilemma similar to determining the distance of a visual object
• Information from two ears is critical to localizing a sound

Vision versus Audition

• Eyes are sensitive across a visual field; different retinal areas receive signals from different environmental areas
• Ears receive signals from the same place, and source is determined by timing/amplitude differences between the two ears

Interaural Cues

• Interaural time differences (ITD) is the difference in time between a sound arriving at one ear versus the other
• Azimuth measures a source angle on the horizontal plane relative to the midpoint between the ears
• Azimuth is measured in degrees, with 0 being straight ahead, and increasing clockwise behind with 180 degrees

Physiology of ITD

• The Medial Superior Olive (MSO) is a brainstem relay station where input from both ears contributes to detecting ITDs
• MSO is where the two ears first converge in the auditory pathway
• ITD detectors form connections from inputs coming from two ears during the first few months of life

Interaural Level Difference

• Interaural level difference (ILD) describes the intensity level difference between sound arriving at each ear
• The head blocks sound energy when the source isn't centered, creating a sound shadow

ILD and Head Shadow

• For frequencies above 1000 Hz, the head blocks higher-frequency sound energy from reaching the opposite ear
• ILD is greatest at 90 and -90 degrees, and is nonexistent directly in front (0 degrees) or behind (180 degrees)
• ILD generally correlates with the angle of the sound source, but not as strongly as with ITDs

Physiology of ILDs

• The Lateral Superior Olive (LSO) is a relay station in the brainstem where input from both ears contribute to detecting ILDs
• Excitatory connections to LSO come from the ipsilateral (same side) ear
• Inhibitory connections to LSO come from the contralateral (opposite side) ear
• Inhibitory and excitatory signals compete in sound localisation

Cone of Confusion

• The cone of confusion is a spatial region where sounds produce the same ITDs and ILDs, hindering localization
• Sounds coming from in front and behind will produce the same ILD and ITD information making it hard to localize
• Turning your head helps resolve the cone of confusion

Ear Shape

• Directional transfer function (DTF) measures how the pinna, canal, head, and torso change sound intensity
• DTF depends on sounds with different frequencies that arrive at each ear from different locations
• DTF helps locate sounds based on azimuth and elevation
• Each person has their unique DTF based on their own body

Ear Modification Study

• Wightman and Kistler researched whether adults adjust to changing pinnae through radical piercings, damaged ears, or prosthetics
• They created molds to fit over ears, changing pinna shape, then measuring sound localization abilities
• Immediate sound localization was very poor after inserting molds
• Adaptation improved localization but didn't reach baseline at 6 weeks
• Localization was near baseline after mold removal, and returned to baseline in less than 2 weeks

Fun Fact

• On the set of Lord of the Rings, the production crew made odd noises to watch Orland Bloom struggle to localize the sound while adapting to his ear prosthetics

Auditory Distance

• The basic distance cue identifies relative intensity of sound
• Inverse-square law states that intensity decrease equals distance squared
• Intensity decreases faster as distance increases from the source
• Level decreases by half every time distance is doubled representing a factor of 4
• Decrease of of 6dB is equal to halving volume, and is a factor of 2

Auditory Distance Perception

• Spectral composition of sounds consists of higher frequencies decreasing in energy more than lower frequencies
• Spectral composition is related to how sound waves travel from the source to one ear.
• Auditory distance depends on relative amounts of direct versus reverberant energy

Spatial Hearing

• Auditory or other senses may heighten if you lose one sense depending on which senses routinely interact
• People with vision loss experience improved hearing
• One reason for spatial hearing enhancement is that the visual cortex gets recruited to process auditory input
• The spatial enhancement is the result of a lack of visual inputs

Hearing and Blindness

• Some blind people navigate by making clicks and listening to echoes
• fMRI studies show visual cortex activation in response to auditory clicks used for echolocation in blind people

Complex Sounds

• The Fundamental frequency is the lowest frequency of harmonic spectrum
• Auditory system is sensitive to natural harmonic relationships
• The missing-fundamental effect means listeners perceiving pitch that corresponds to the original fundamental frequency if it is missing

Missing Harmonics

• When given only three harmonics of the frequency, listeners still hear the note of the missing fundamental frequency
• Harmonics share a common energy fluctuation, which is every 4 ms for a 250 Hz signal (the fundamental)

Timbre

• Timbre is conveyed by the presence and volume of various harmonics, and context sound is heard
• Physical context and sound context are both factors in timbre perception
• Identical instruments in different physical sound spaces sound different
• Various instruments can create overtones and harmonics differently

Complex Sounds

• Attack refers to the part of a sounds that occurs at the beginning of the sound/onset
• Decay refers that the period of time when the sound amplitude decreases/the offset

Auditory Scene

• The auditory environment is typically composed of many sounds and is generally regarded "busy"
• The only truly quiet place - is an anechoic chamber
• Auditory scene analysis regards separating the component sounds from their various sources
• Auditory attention is a process of filtering background and non-salient sounds to focus on one or more salient sounds

Auditory Scene Analysis

• Sounds are spatially separated from other background auditory information
• Separation depends on sounds specific spectral and temporal qualities
• Grouping results from timbre or pitch
• Auditory stream segregation provides the perceptual organization of an acoustic signal leading to events heard separately

Grouping by Onset

• If sounds are heard at nearly the same time, the brain assumes they are coming from a shared origin
• Harmonic sounds are grouped into a single complex tone
• This is consistent with the Gestalt law of common fate

Grouping by Timbre

• Similar timbres are assumed to arise from the same source
• Each voice/instrument has a different timbre, even if producing identical melodies

Grouping by Pitch

• Tones with similar pitches are grouped together

Sound Recognition

• Participants can be trained to recognise completely novel sounds to a degree
• Listeners learn new sounds with few repetitions even when heard within other sounds

Auditory Continuity

• When processing sounds and distraction: follow key sounds despite the presence of other auditory information

• Principle of good continuation: despite interruptions, a sound is still heard

• Experiments reveal that missing sounds get restored and encoded as if actually present

Continuity and Restoration

• To restore complex sounds listeners use "higher-order" sources of information
• "Higher-order" restoration processes use memory and logic
• Gaps in a sound stream are less detrimental if filled with noise rather than with silence
• Listeners struggle discerning gaps even when they are filled with noise

Sound Restoration

• Starlings trained to peck when they heard the difference between two starling songs
• Testing songs are either intact or interrupted songs with different gaps
• The starlings will peck for the songs with silent gaps indicating they filled these gaps

Filling Auditory Gaps

• EEG was used to record neural activity as participants learned 'nozzle' and 'novel' as different sounds.
• The sounds were then played with the key sound blocked by noise.
• EEG readings predicted which word the participants would respond with
• Neural processing shows that when the brain fills in the information of missing sounds, EEG patterns become indistinct.

Dominance of vision

• Hearing can act as the primary sense for maintaining attentiveness/vigilance
• Acoustic startle reflex involves rapid motor response to sounds
• Very few neurons are involved in the startle reflex
• Activation of the sympathetic nervous system increases rate of acoustic startle

Auditory Attention

• Attention can shift in some sounds while not listening to others
• The brain can be overloaded when attempting to listen to more than one sound stream
• Inattentional deafness involves a failure to properly process a fully-audible but unexpected sound
• This results from the brain being engaged in processing other types of attention