A scene made of sounds

Question 1

Interaural time difference (ITD)

Answer 1

the difference in time between arrivals of sound at one ear versus the other. ITD allows us to locate a sound on the horizontal plane that extends from a center point betweeen the ears

Question 2

Azimuth

Answer 2

the angle of a sound source on the horizonal plane relative to a point in the center of the head between the ears. 0 degrees azimuth corresponds to a sound that is straight ahead.

Question 3

Binaural input

Answer 3

sound input from both ears

Question 4

Medial superior olives (MSOs)

Answer 4

the first places in auditory system where inputs from both ears converge. Firing rates of MSO neurons increase in response to very brief time differences between inputs from the 2 ears of cats.

Question 5

Jeffress neural coincidence model

Answer 5

there are neurons called coincidence detectors (or ITD detectors), which fire the most in response to a particular ITD (there is a place code for localization)

Question 6

ITD tuning curve

Answer 6

a curve that describes the neuron’s firing as a function of ITD. Recent evidence suggests that the brain uses frequency differences to measure time.
-> ITD tuning curves are very broad in mammals -> it has been suggested that localization is based on coding from many broadly tuned ITD-detecting neurons in both ears, whose relative responses indicate the location of a sound (population coding)

Question 7

Interaural level difference (ILD)

Answer 7

the difference between levels (intensities) of sound at one ear versus the other. ILD is also correlates with the angle of the sound source, but less precisely than ITD because of the irregular shape of the head.
-> best for high-frequency tones (almost nonexistent below 1000Hz)

Question 8

acoustic shadow

Answer 8

sounds are more intense at the ear closer to the sound source because the head partially blocks the sound pressure wave from reaching the opposite ear

Question 9

lateral superior olives

Answer 9

contain neurons sensitive to ILDs. The LSOs receive excitatory input from the ipsilateral cochlea. The LSOs receive inhibitory input from the contralateral ear via the meidla nucleus of the trapezoid body (MNTB)

Question 10

Cone of confusion

Answer 10

a region of positions in space where all sounds produce the same ITDs and ILDs. However, cones of confusion are not a major problem, because when you move your head, the ITDs and ILDs of a sound source shift

Question 11

spectral cues

Answer 11

cues in which information for sound localization is contained in the spectrum of frequencies that reach each ear from different locations. The shape of the pinnae allows funneling particular sound frequencies more efficiently than other into the ear canal. The intensity of each frequency varies slightly according to the direction of the sound

Question 12

Directional transfer function (DTF)

Answer 12

a measure that describes how the pinna, ear canal, head and torse of an individual change the intensity of sounds with different frequencies
-> computed by playing sounds and recording them with microphones inserted in both ears, close to the eardrums
-> a person’s DTF changes during development, due to the changing shape of the heads and ears, and due to learning to localize sounds through experience

Question 13

anterior belt area

Answer 13

responsible for perceiving complex sounds and patterns of sound. A ‘what’ auditory pathway extends from the anterior belt to the temporal pole and the PFC. It is associated with perceiving sounds

Question 14

posterior belt area

Answer 14

involved in localizing sounds. A “where” auditory pathway extends from the posterior belt to the parietal lobe and then to the PFC. It is associated with localizing sounds

Question 15

Study: plastic ear molds

Answer 15

Plastic ear molds are inserted in adults’ ears and their ability to localize sounds was tested
-> Listener immediately became much worse at localizing sounds
-> By 6 weeks with these molds in their ears, their localization abilities had greatly improved
-> Furthermore, listeners also remained quite good at localizing with their “old ears”, after the molds were removed

Question 16

Hwo does the brain measure time differences?

Answer 16

it uses frequency differences
-> For the ear that the sound reaches first, the travelling wave reaches a lower frequency further down the cochlea compared with the ear to which the sound arrives slightly later
-> Therefore, tiny time differences between the ears result in the travelling wave being at different places (frequencies) along the base membrane
! Neurons in the MSO were found to capture slight frequency differences across the ears => ITDs might actually be used for localization in the form of interaural frequency differences

Question 17

Relative intensity

Answer 17

listeners can perceive the relative distance of 2 identical sound sources, because sounds become less intense with greater distance
-> the effectiveness of relative intensity decreases quickly as distance increases
-> when the sound source or listener is moving, intensity works best as a distance cue

Question 18

Spectral composition

Answer 18

air dampens high frequencies more than low frequencies => when sound sources are far away, higher frequencies decrease in energy more than lower frequencies
-> this change in spectral composition is noticeable only for large distances, more than 1 km.

Question 19

Relative amounts of direct versus reverberant (indirect) energy

Answer 19

when a sound source is closer to a listener, most of the energy reaching the ear is direct, whereas reverberant energy (energy which has bounced off surfaces in the environment) is higher when the sound source is further away.

Question 20

Source segregration

Answer 20

the distinction of auditory events/objects in the broader auditory environment.

Question 21

Spatial segregration

Answer 21

segregating sounds on the basis of their spectral qualities
-> Sounds with the same pitch (or similar pitches) are more likely to be treated as coming from the same source

Question 22

Harmonicity

Answer 22

sound components that are related harmonically to each other (frequency of each is a multiple of the others’ frequencies) are grouped in a complex sound with the corresponding fundamental frequency.

Question 23

Acoustic startle reflex

Answer 23

a rapid motor response to a sudden sound
-> affected by emotional state
-> it takes only 10ms because the transmission goes through the brain stem to the spinal cord

Question 24

Inattentional deafness

Answer 24

miss out on hearing sounds because of selective auditory attention to one sound only.
-> Selective auditory attention can be affected by task-specific goals, not only be physical characteristics of sound