Transcript Chapter 12 (new window)

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
Figure 12-6 p293
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.
Figure 12-14 p298
Figure 12-15 p298
Figure 12-16 p299
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
Figure 12-18 p300
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
Figure 12-32 p311
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.
Figure 12-33 p312
Figure 12-34 p312
Figure 12-35 p313