Chapter 12: Auditory Localization and Organization

Chapter 12: Auditory Localization and

Organization

Figure 12-1 p290

Auditory Localization

• Auditory space - surrounds an observer and exists wherever there is sound

• Researchers study how sounds are localized in space by using:

– Azimuth coordinates - position left to right

– Elevation coordinates - position up and down

– Distance coordinates - position from observer

Figure 12-3 p291

Auditory Localization - continued

• On average, people can localize sounds

– Directly in front of them most accurately

– To the sides and behind their heads least accurately.

• Location cues are not contained in the receptor cells like on the retina in vision; thus, location for sounds must be calculated.

Figure 12-2 p291

Binaural Cues for Sound Localization

• Binaural cues - location cues based on the comparison of the signals received by the left and right ears

– Interaural time difference (ITD)- difference between the times sounds reach the two ears

• When distance to each ear is the same, there are no differences in time.

• When the source is to the side of the observer, the times will differ.

Figure 12-4 p292

Binaural Cues for Sound Localization - continued

• Interaural level difference (ILD) - difference in sound pressure level reaching the two ear

– Reduction in intensity occurs for high frequency sounds for the far ear

• The head casts an acoustic shadow.

• This effect doesn’t occur for low frequency sounds.

• Cone of Confusion

Figure 12-5 p292

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Figure 12-7 p294

Monaural Cue for Sound Location

• Monaural cue – uses information from one ear

• The pinna and head affect the intensities of frequencies.

• Measurements have been performed by placing small microphones in ears and comparing the intensities of frequencies with those at the sound source.

– This is a spectral cue since the information for location comes from the spectrum of frequencies.

Figure 12-8 p294

Monaural Cue for Sound Location - continued

• ILD and ITD are not effective for judgments on elevation since in many locations they may be zero.

• Experiment investigating spectral cues

– Listeners were measured for performance locating sounds differing in elevation.

– They were then fitted with a mold that changed the shape of their pinnae.

Monaural Cue for Sound Location - continued

– Right after the molds were inserted, performance was poor for elevation but was unaffected for azimuth.

– After 19 days, performance for elevation was close to original performance.

– Once the molds were removed, performance stayed high.

– This suggests that there might be two different sets of neurons—one for each set of cues.

Figure 12-9 p295

The Physiological Auditory Location

• Auditory nerve fibers synapse in a series of subcortical structures– Cochlear nucleus– Superior olivary nucleus (in the brain

stem)– Inferior colliculus (in the midbrain)– Medial geniculate nucleus (in the

thalamus)– Auditory receiving area (A1 in the

temporal lobe)

Figure 12-10 p296

The Physiological Auditory Location - continued

• Hierarchical processing occurs in the cortex

– Neural signals travel through the core, then belt, followed by the parabelt area.

– Simple sounds cause activation in the core area.

– Belt and parabelt areas are activated in response to more complex stimuli made up of many frequencies.

Figure 12-11 p297

The Physiological Representation of Auditory Space - continued

• Jeffress Model for narrowly tuned ITD neurons

– These neurons receive signals from both ears.

– Coincidence detectors fire only when signals arrive from both ears simultaneously.

– Other neurons in the circuit fire to locations corresponding to other ITDs.

Figure 12-12 p297

Figure 12-13 p297

Broad ITD Tuning Curves in Mammals

• Broadly-tuned ITD neurons

– Research on gerbils indicates that neurons in the left hemisphere respond best to sound from the right, and vice versa.

– Location of sound is indicated by the ratio of responding for two types of neurons.

– This is a distributed coding system.

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Localization in Area A1 and the Auditory Belt Area

• Broadly-tuned ITD neurons

– Malhorta and Lomber (2007)

– Cooling and lesioning

Auditory Where (and What) Pathways

• What, or ventral stream, starts in the anterior portion of the core and belt and extends to the prefrontal cortex.– It is responsible for identifying sounds.

• Where, or dorsal stream, starts in the posterior core and belt and extends to the parietal and prefrontal cortices.– It is responsible for locating sounds.

• Evidence from neural recordings, brain damage, and brain scanning support these findings.

Figure 12-17 p300

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Figure 12-19 p301

Hearing Inside Rooms

• Direct sound - sound that reaches the listener’s ears straight from the source

• Indirect sound - sound that is reflected off of environmental surfaces and then to the listener

• When a listener is outside, most sound is direct; however inside a building, there is direct and indirect sound.

Figure 12-20 p301

Perceiving Two Sounds That Reach the Ears at Different Times

• Experiment by Litovsky et al.– Listeners sat between two speakers: a lead

speaker and a lag speaker.

– When sound comes from the lead speaker followed by the lag speaker with a long delay, listeners hear two sounds.

– When the delay is decreased to 5 - 20 msec, listeners hear the sound as only coming from the lead speaker - the precedence effect.

Figure 12-21 p302

Architectural Acoustics

• The study of how sounds are reflected in rooms.

• Factors that affect perception in concert halls.

– Reverberation time - the time is takes sound to decrease by 1/1000th of its original pressure

• If it is too long, sounds are “muddled.”• If it is too short, sounds are “dead.”• Ideal times are around two seconds.

Architectural Acoustics - continued

• Factors that Affect Perception in Concert Halls – Intimacy time - time between when sound

leaves its source and when the first reflection arrives

• Best time is around 20 ms.

– Bass ratio - ratio of low to middle frequencies reflected from surfaces

• High bass ratios are best.

– Spaciousness factor - fraction of all the sound received by listener that is indirect

• High spaciousness factors are best.

Acoustics in Classrooms - continued

• Ideal reverberation time in classrooms is

– .4 to .6 second for small classrooms.

– 1.0 to 1.5 seconds for auditoriums.

– These maximize ability to hear voices.

– Most classrooms have times of one second or more.

• Background noise is also problematic.

– Signal to noise ratio should be +10 to +15 dB or more.

Figure 12-22 p304

Auditory Organization: Scene Analysis

• Auditory Scene - the array of all sound sources in the environment

• Auditory Scene Analysis - process by which sound sources in the auditory scene are separated into individual perceptions

• This does not happen at the cochlea since simultaneous sounds are together in the pattern of vibration of the basilar membrane.

Auditory Organization: Scene Analysis - continued

• Heuristics that help to perceptually organize stimuli

– Onset time - sounds that start at different times are likely to come from different sources

– Location - a single sound source tends to come from one location and to move continuously

– Similarity of timbre and pitch - similar sounds are grouped together

Separating the Sources

• Compound melodic line in music is an example of auditory stream segregation.

• Experiment by Bregman and Campbell

– Stimuli were alternating high and low tones

– When stimuli played slowly, the perception is hearing high and low tones alternating.

– When the stimuli are played quickly, the listener hears two streams; one high and one low.

Figure 12-23 p305

Figure 12-24 p305

Figure 12-25 p306

Separating the Sources - continued

• Experiment by Deutsch - the scale illusion or melodic channeling

– Stimuli were two sequences alternating between the right and left ears.

– Listeners perceive two smooth sequences by grouping the sounds by similarity in pitch.

– This demonstrates the perceptual heuristic that sounds with the same frequency come from the same source, which is usually true in the environment.

Figure 12-26 p306

Separating the Sources - continued

• Proximity in time - sounds that occur in rapid succession usually come from the same source

– This principle was illustrated in auditory streaming.

• Auditory continuity - sounds that stay constant or change smoothly are usually from the same source

Separating the Sources - continued

• Experiment by Warren et al.

– Tones were presented interrupted by gaps of silence or by noise.

– In the silence condition, listeners perceived that the sound stopped during the gaps.

– In the noise condition, the perception was that the sound continued behind the noise.

Figure 12-27 p307

Separating the Sources - continued

• Effect of past experience

– Experiment by Dowling

• Melody “Three Blind Mice” is played with notes alternating between octaves

• Listeners find it difficult to identify the song

• But after they hear the normal melody, they can then hear it in the modified version using melody schema

Figure 12-28 p307

Auditory Organization: Perceiving Meter

• Rhythmic pattern is a series of changes across time.

• Metrical structure is the underlying beat of music.

Figure 12-29 p308

Figure 12-30 p310

Connections Between Hearing and Vision

• Visual capture or the ventriloquist effect - an observer perceives the sound as coming from the visual location rather than the source for the sound

• Experiment by Sekuler et al.

– Balls moving without sound appeared to move past each other.

– Balls with an added “click” appeared to collide.

Figure 12-31 p311

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Hearing and Vision: Physiology

• The interaction between vision and hearing is multisensory in nature.

• Thaler et al (2011) – Used expert blind echolocators to create clicking sounds and observed these signals activated the bran.

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