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Chapter 12

Sensation and Perception Psych 367 Chapter 12.docx

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Douglas Wylie

Chapter 12 sound localization and the auditory scene Auditory localization - When you perceive objects located at different positions based on their sounds, you are experiencing auditory space - Auditory localization: feats of locating objects in space based on their sound - Locate the position of a sound in three dimensions: o the azimuth which extends from left to right o elevation extends up and down o distance of the sound source form the listener - sounds directly in front are easiest to localize and sounds off to the side and behind the head are header - the place that is activated in the cochlea provides information that determines the sounds pitch and timbre - information it uses involves location cues that are created by the way sound interacts with the listeners head and ears binaural cues for sound location - there are two binaural cues o interaural time difference o interaural level difference - both based on a comparison of the sounds signals reaching the left and right ears interaural time difference - is based on the fact that there is a difference in when a sound reaches the left and right ears - if the source is located directly in front of the listener, the distance to each ear is the same - because the ITD becomes larger as sound source are located more to the side, the magnitude of the ITD can be used as a cue to determine a sounds location - indicate that ITD is an effective cue for location of low frequency sounds interaural level difference - interaural level difference ILD is based on the difference in the sound pressure level of the sound - a difference in level between the two ears occurs because the head creates a barrier that reduces the intensity of sounds that reaches the far ear - this reduction of intensity at the far ear occurs for high frequency sounds but not for low frequency sounds - high frequency sound waves (which are small compared to the size of the head) are disrupted by the head but that low frequency waves are not - this disruption of high frequency sound waves created a decrease in sound intensity on the far side of the head called acoustic shadow using binaural cues for perceiving azimuth locations - ITD and ILD together we see that they complement each other - ITD and ILD provide information that enable people to judge location along the azimuth coordinate, but provide ambiguous information about the elevation of a sound - These places of ambiguity are illustrated by the cone of confusion - Monaural cue – a cue that depends on information from only one ear only Monaural cue for localization - Primary monaural cue for localization is called spectral cue because the information for localization is contained in differences in the distribution (or spectrum) of frequencies that reach the ear from different locations - These difference are caused by the fact that before the sound stimulus enters the auditory canal, it is reflected from the head and within the various folds of the pinnae - Differences in the way the sounds bounce around within the pinna create different frequency spectra for the two locations - The idea that localization can be affected by using mould to change the inside contours of the pinnae was also demonstrated by Hofman o They determined how localization changes when the mould is warn for several weeks, and then what happens when the mould is removed o The overlap between two grids indicates that localization was accurate o After measuring initial performance, they subjects were fitted with mould that altered the shape of the pinna and therefore changed the spectral cue o Localization performance is poor for the elevation coordinate immediately after the mould is inserted but location can still be judged by azimuth o Until 19 days localization had become reasonably accurate o A person had learned to associate new spectral cues it different directions in space o Localization remained excellent immediately after removal of the ear mould - Each type of cue works best for different frequencies and different coordinates - ITDs and ILDs work for judging azimuth locations - ITD best for judging low frequencies - ILD best for judging high frequencies - Spectral cues work best for judging elevation, especially a higher frequency - We also move our heads to provide additional information The physiology of auditory localization Narrowly tuned ITD neurons - Neurons in the inferior colliculus and superior olivary nuclei that respond to narrow range of ITDs - The neurons associated with the curves on the left fire when sound reaches the left ear first, and the ones on the right fire when sound reaches the left ear first - This type of responding in which specific ITD activates neurons tuned to that ITD is a form of specificity coding - Jeffress model starts with the idea that there are a series of neurons that each respond best to a specific ITD - These neurons are wired so that they each receive signals from the two ears, - Signals from the left ear arrive along the blue axon, and signals from the right arrive along the red - If the sound sources is directly in from of the listener, so the sound reaches the left and right ears simultaneously, then signals from the left and right ears start out together - As each signal travels along its axon, it stimulates each neuron in turn - Neurons receive signals from only left ear or the right ear but not both and they do not fire - But when both signals reach the middle neuron it fires o This neuron and the others in this circuit are called coincidence detectors because they only fire when both signals arrive at the neuron simultaneously o The firing of the middle neurons indicates that ITD = 0 Broadly tuned ITD neurons - There are neurons in the gerbils right hemisphere that respond best when sound is coming from the left and neurons in the left hemisphere that respond when sound is coming from the right - The location of a sound is indicated by the ratio of responding of these two types of broadly turned neurons - This type of coding resembles distributed coding model in which information in the nervous system is based on the pattern of neural responding - There is evidence for both narrowly tuned ITD neurons and broadly tuned ITD neurons - Both types provide information about low frequency sounds Perceptually organizing sounds in the environment Auditory scene analysis - Auditory scene: The array of sound sources in the environment - Auditory scene analysis: the process by which you separate the stimuli produced by each of the source in the scene into separate perceptions - The auditory systems problem is deciding which frequency components belong together to form each sound in the auditory scene - Principles of auditory grouping o The sound sources position in space can potentially help you separate the sources from one another, but the fact that you can still hear separate instruments when you listen to a recording played through a single speaker mean that the auditory system must also use other information to analyze an auditory scene into separate sound sources Principles of auditory grouping - There are a number of heuristics that help us perceptually organize elements of an auditory scene, and these heuristics are based on how sound usually originate in the environment - Onset time: If two sounds start at different times, it is likely that they came from two different sources Location - Anytime two sounds are separated in space, the cue of location helps us separate them perceptually Similarity of timbre and pitch -
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