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chap 12 textbook notes

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Kristie Dukewich

CH12 – Sound Localization and the Auditory Scene  ACOUSTIC SHADOW – decrease in sound intensity on the Auditory Location far side of the head, away from the sound source due to  AUDITORY SPACE – when perceive objects located at different positions the disruption of high-freq. sound waves based on their sound, w/o visual cues o project from the head in all directions, exists whenever there’s sound o AUDITORY LOCALIZATION – feats of locating objects in space based on their sound o People can locate the position of a sound in three directions:  AZIMUTH – extends from left to right  ELEVATION – extends up and down  DISTANCE – the sound source from the listener o Major problems w/ auditory system in locating sound source, in comparison w/ vision  Visual information for relative locations contained in the image on the surface of the retina  Audio information stimulate the cochlea based on sound frequencies  determine sound’s pitch and timbre  Does not contain information for relative locations o LOCATION CUES – used by the auditory system to determine sound location, created by the way sound interacts w/ listener o Using Binaural Cues for Perceiving Azimuth Locations  ITD and ILT complement each other  ITD  low freq. ILT  high freq.  Enable location along azimuth coordinate  Provide ambiguous information about elevation  CONE OF CONFUSION – when sound source its extremely one sided, all points (A & B) on this cone have same ILD and ITD  Binaural Cues for Sound Location o BINAURAL CUES – cue that depends on information from both ears  2 types: interaural time difference/level difference  both based on comparing sound signals reaching left/right ears  Monaural Cue for Localization o MONAURAL CUE – cue that depends on info from only one ear, o INTERAURAL TIME DIFFERENCE (ITD) – based on there’s a difference in a sound reaches the left and right ears important for sound location on elevation coordinates  If sound in front/behind o SPECTRAL CUE – primary monaural cue for localization listener, distance to each  Information for localization contained in differences in the ear same distribution (spectrum) of frequencies that reach ear from  If sound off to one side, different locations will reach one ear faster  Differences caused by sound stimulus reflected from the head  ITD larger as sound and w/i various folds of pinnae before entering auditory canal source located more to o Ex. 2 sounds sources 15° above head, 15° below  same ITD, ILD  Differences in way sounds bounce around within pinna create one side  ITD an effective different frequency spectra for the 2 location cue for locating low-frequency sounds o INTERAURAL LEVEL DIFFERENCE (ILD) – based on the difference in the sound pressure level of the sound reaching the two ears  Difference in level b/c head blocks sound reaching the other ear  ILD larger as sound source located more to one side  Occurs greater for higher frequencies than for lower frequencies  Higher freq. sound waves disrupted by the head, low freq. sound waves are not b/c period btwn waves long enough to compensate o Paul Hofman demonstrated localization affected by using mold to o Explanation by Lloyd Jeffress change inside contours of pinnae when mold worn for several weeks  Series of neurons that each respond best to specific ITD, wired  Also measurable effect when mold removed so that they each receive signal from the two ears  Ex. blue grid = positions of sound stimuli presented  left ear signal arrive along blue axon, left along red  Red grid = average localization performance  Sound directly in front  reaches left & right ear simultaneously  Localization performance poor for elevation coordinates  Signals from left/right ear starts together immediately after mold inserted  As signal travels through (red/blue) axon, stimulates each  Localization gradually improved  person learned new neuron sequentially  but neurons don’t fire unless: spectral cues associations to different directions in space  COINCIDENCE DETECTORS – these neurons only fire when  Localization remained excellent after mold remove  b/c both signals arrive at neuron simultaneously training w/ mold created new set of correlation btwn  Finally, fires neuron 5  indicate ITD = 0 spectral cues and location, old correlation still present  Explanation: different set of neurons involved in responding to set of spectral cues, like separate brain areas for processing different languages  Sound from right ear  reach neuron 3 simultaneously  fire  Note, the side neuron fires is opposite to the ear which received sound first, but proportional to ITD magnitude  Moving head provide additional ITD, ILD, and spectral information  BROADLY TUNED ITD NEURONS – There are neurons that are broadly o Help minimize the effect of Cone Of Confusion o Vision also helpful in sound localization tuned to ITD  Easier to determine the source o Recent research indicates localization can also be based on neurons that are broadly tuned The Physiology of Auditory Localization  Ex. gerbil’s right hemisphere respond best when sound is  How information in these cues is represented in various system for ITD? coming from left, and vice versa  NARROWLY TUNED ITD NEURONS – neurons narrowly tuned to respond  Location of sound indicated by ratio of responding of best to a narrow range of ITDs these two types of broadly tuned neurons o In the Inferior Colliculus and Superior Olivary Nuclei  Form of distributed coding, similar to color vision o ITD tuning curves for narrowly tuned neurons  Diagrams:  Neurons associated w/ cures on the left (blue) fire when sound  Left = ITD tuning curves for broadly tuned neurons in the reaches left year first, ones on the right (red) fire when sound two hemisphere reaches the right ear first  form of specificity coding  Right = patterns of response of the broadly tuned neurons for stimuli coming from left, front, right  there’s evidence for both narrowly & broadly tuned ITD neurons o unsure exactly which mechanism/combination works Perceptually Organizing Sounds in the Environment  rarely hear isolated single tones in environment o usually experience number of sounds simultaneously o How can auditory system separate
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