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Chapter 12: Localization Scene Analysis

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Western University
Psychology 2115A/B
Christine Tsang

Chapter 12 Review: Sound Localization and Auditory Scenes By the end of this section, you should know:  Basic cues to sound localization  Principles of auditory stream segregation  The effect of room context on auditory perception Sound Localization  Processes by which we detect direction and distance that sound is coming from  Sounds in the environment are perceived by us to emanate by particular source – a location in space  We use a variety of cues to judge where those sounds are coming from  Localization: judgments of direction and distance of any particular sound source  Having 2 ears is important for detection Auditory Space  The space that surrounds the listener  Azimuth coordinates: position left to right o Horizontal  Elevation coordinates: position up and down o Sounds coming from above and below you  Distance coordinates: position from observer o Reflect a sense of how far away that sound source is  People are good at localizing sounds that are directly in front of them, we are not great at localizing sounds that are behind us or really far to the side  Problem for the brain in sound localization – because we are good at localizing sounds in front of us, we think sound localization is an easy process for the brain but in fact it is quite difficult  Localization cells for auditory cues at the level of the receptor are not coding for spatial location, they are coding for frequency, so doesn’t tell us anything about where that sound source is  Coding theory tells us more about pitch than were it is in space, so the auditory system must be using another set of cues other than place theory to determine sound location  The fact that we have 2 ears is a critical part to sound localization – binaural cues Binaural Cues: Horizontal (Azimuth) Localization  These cues provide us location cues that are based on the comparison between stimuli coming form the right side vs. left side  You have to have 2 sides for these cues to work  Interaural time difference (ITD) o The calculation between the difference in time of the arrival of the signal from one ear to the other o If sound arrives first to the right ear, then there is a delay and then it arrives to the other side o The brain is calculating the time difference in its arrival time from one side to the other o Whichever one it arrives to first indicates the location of the sound o Sounds that come from directly in front of you means the time of arrival to both ears is the same o Sounds on one side of the ear arrive to one faster than the other because it has to travel around the head  Interaural level difference (ILD) o Loudness of sound from one side to the other o When sound is low in frequency, interaural level cues are not reliable o High frequency sounds cast a shadow on the sound because they have wave forms that are very close together – the head is a big shadow in that pressure wave – muffling some of the frequencies coming out o For low frequency sounds, because these are longer wavelengths of sound (high are tightly compressed together) the head creates no shadow because it goes in between the pulses of the pressure wave o The sound waves can go around the head and be unaffected by the head shadow in low frequency, so the sound is the same for both ears o But with high frequency, the more the heat attenuates the sound o Level cues unreliable for low frequency – vary greatly, affected by location for high frequencies The Cone of Confusion  Any sound source on the surface of the cone would give rise to the same interaural time difference o There is a theoretical cone, imagine it coming out of your ear, in which any sound source that could be represented on the surface of this cone provides the same interaural differences o All the theoretical points on this cone have the same time and level difference – if the sound is on that cone somewhere, you don’t actually know where the sound is coming from, it could be coming from a number of possible directions  All points on the cone have the same ITD and ILD o Brains are constantly computing differences in time of arrival and intensity from one ear to the other – it is relying on cues to figure it out o Motor movements – moving our hear or walking around sources Monoaural Cue: Vertical Localization (Elevation)  Elevation based coordinates – trying to figure out if sound is coming from above or below you  ITD and ILD are very unreliable in trying to figure out sounds coming form above or below you so we have to rely on other kinds of cues to figure that out  One of the most primary is the pinna o An asymmetrical flesh around the hole of the ear o It is very useful for figuring out where sounds are coming from in the up/down dimension o Affect how loud sound is perceived – helps to amplify sound o Amplification of sound depends on the directionality of the sound (where it is coming from) o Pinna is filtering some sounds preferentially over other sounds, altering the spectral frequency of the sound arriving at the cochlea o Over time, we figure out using our pinna how direction of sound is correlated with the shadowing/amplification of certain frequencies o Localization in the vertical, up/down plane is largely learned over time  Participants asked to locate sounds at different elevations o Pre test has to locate where the sound source was, and then each of the participants was fitted with a mold that changed the shape of their pinna o You find that right after you insert the mold, their performance which was near perfect declines to terrible – bad at figuring out where the sound source is coming from and very bad in the up/down plane but more accurate in the left/right place  shows that the pinna does not affect intraaural time or level cues o Trained using this new pinna and found that they show gradual improvement and by day 19, they are almost the same as they were before the mold o When they remove the mold, performance remains quite high o Conclusions:  Clear that the pinna is very involved in elevation cues  Practice with the new ear, you can re-learn the sound cues that allow you to judge in the elevation coordinate  Even without the molds, after learning we are still pretty good – we have not replaced our original sound cues with the mold, we are now keeping two sets of learned coordinates How is Localization Information Represented in the Brain?  2 mechanisms proposed: distributed coding or specificity coding in cortex are similar  Time cues are most clear  Narrowly tuned ITD neurons: tuned to particular differences in time  Broadly tuned ITD neurons: tuned to basic differences between right vs. left but not individual neurons that do that  It would appear that both of these representations exist in the brain at different levels of brain processing  Narrowly tunes neurons are represented in lower brain areas – probably midbrain and inferior/superior colliculus  Narrowly tuned: work done in owls o Specific neurons that respond to intraaural time differences o Activity from cells in the subcortical regions suggest specificity coding – prefer information coming form one ear to the other and looking at time differences between the 2 ears o When i
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