Chapter 10.docx

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Mathias Niemier

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Chapter 10 – Hearing in the Environment Sound Localization  Having 2 ears is crucial for determining auditory locations  For most positions in space, sound source will be closer to one ear than to other  2 potential types of information for determining the source of a sound o Even though sound travels fast, pressure wave will not arrive at both ears at same time  Sounds arrive sooner, albeit very slightly, at ear closer to source o Intensity of sound is greater at ear closer to source Interaural Time Difference  Interaural time difference (ITD): difference in time between sound arriving at one ear vs the other o If source is to left, sound will reach left ear first; if to right, will reach right ear first  Terms used to describe locations on imaginary circle that extends around us – front, back, left and right  Azimuth: angle of sound source on horizontal plane relative to point in center of head between ears. Measured in degrees, with 0 degrees being straight ahead. Angle increases clockwise toward right, with 180 degrees being directly behind  IDTs are largest, about 640 microseconds when sounds come directly from left or right  Sound coming from directly in front of directly behind listener produces ITD of 0; sound reaches both ears simultaneously  For intermediate locations, ITD will be somewhere between 2 values – second source located at angle of 60 degrees will always produce ITD of 480 us and sound coming from -20 degrees will always produce ITD of -200 Physiology of ITD  As info moves upward through system, with every additional synapse timing between 2 ears is likely to become less precise  Medial superior olives (MSOs): relay station in brain stem where inputs from both ears contribute to detection of interaural time difference  ITD detectors form connections from inputs coming in from 2 ears during first few months of life  Developmental sequence is similar to one for formation of binocular neurons in visual cortex and probably has similar cuase  Interpretation of ITDs is critically dependent on size of head Interaural Level Difference  Interaural level difference: difference in level (intensity) between sound arriving at one ear vs other  Sounds are more intense at ear closer to sound source because head partially blocks sound pressure wave from reaching opposite ear  Properties of ILD relevant for auditory localization are similar to ITD o Sounds more intense at ear that is closer to sound source, and less intense at ear farther away o ILD is largest at 90 and -90 degrees, and is nonexistent at 0 degrees (directly in front) and 180 (directly behind o Between 2 extremes, ILD generally correlates with angle of sound source, but because of irregular shape of head, correlation isn’t quite as great as it is with ITDs  Important difference between 2 cues – head blocks high frequency sounds much more effectively than it does low frequency sounds because long wavelengths of low frequency sounds “bend around” head in much same way that large ocean waves crashes over piling near shore Physiology of ILDs  Neurons sensitive to intensity differences between 2 ears can be found in lateral superior olives (LSOs)  Lateral superior olives (LSOs): relay station in brain stem were inputs from both ears contribute to detection of interaural level difference  Excitatory connections to LSOs come from ipsilateral ear (excitatory connections to left SLO originate in left cochlea, and excitatory connections to right LSO come from right cochlea)  Inhibitory inputs come from contralateral ear (ear on opposite side of head) via medial nucleus of trapezoid body (MNTB)  What makes neurons in LSOs so sensitive to differences in intensity across 2 ears is competition between excitatory inputs from one ear (ipsilateral) in inhibitory inputs from other ear (contralateral)  When sound is more intense at one ear, connections from ear are both better at exciting LSO neurons on that side and better at inhibiting LSO neurons on other side Cones of Confusion  Cone of confusion: region of positions in space where all sounds produce same time and level (intensity) differences (ITDs and ILDs)  Infinite number of cones are nested inside one another – widest “cone is really circle extending from directly in front of you, up to directly over your head, back to directly behind back of head, and continuing directly below you  Widest cone is most confusing cone Pinna and Head Cues  Reason why cones of confusion aren’t major practical problem for auditory system is that time and intensity differences aren’t only cues for hearing location of sound sources  Shape of pinna is complex with lots of idiosyncratic nooks and crannies  Pinnae serve to funnel sound energy into ear canal  Because of shape, pinnae funnel certain sound frequencies more efficiently than others – intensity of each frequency varies slightly according to direction of sound  Head related transfer function (HRTF): function that describes how pinna, ear canal, head and torso change intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth and elevation)  Importance of HRTF in sound localization is easily understood if we consider difference between hearing concert live vs through set of headphones  Just as stereoscopes can be designed to simulate binocular disparity, possible to stimulate HRTFs – 2 microphones are placed near eardrums then sound source is recorded from 2 microphones  Just as heads grow and change during development and are often subject to mutilations of varying degrees  Research suggests that listeners learn about way HRTFs relate to places in environment through extensive experience listening to sounds, while other sources of info provide feedback about location Auditory Distance Perception  Simplest cue for judging distance of sound source is relative intensity of sound – because sounds become less intense with greater distance, listeners have little difficulty perceiving relative distances of 2 identical sound sources  Effectiveness of relative intensity decreases substantially as distance increases because sound intensity decreases according to inverse square law  Inverse square law: principle stating that as distance from source increases, intensity decreases faster such that decrease in intensity is distance squared. General law applies to optics and other forms of energy  Intensity works best as distance cue when sound source or listener is moving  Listeners also get info about how far away source is when they move through environment – in manner akin to motion parallax in perception of visual depth, sounds that are father away don’t seem to change direction in relation to listener as much as nearer sounds do  Possible cue for auditory distance is spectral composition of sounds – sound absorbing qualities of air dampen high frequencies more than low frequencies, so when sound sources are far away, higher frequencies decrease in energy more than lower frequencies as sound waves travel from source to ear  Final distance cue stems from fact that sound that arrives at ear is some combination for energy and reverberant energy o Relative amounts of direct vs reverberant energy inform listener about distance because when sound source is close to listener, most energy reaching ear is direct, whereas reverbera
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