Deck 12: Auditory Localization and Organization

Question 1

We can perceive space visually, as we saw in the chapter on depth perception, and through the sense of hearing, as we have described in this chapter. How are these two ways of perceiving space similar and different?
Figure Coffee shop scene, which contains multiple sound sources. The most immediate sound source for the man in the middle is the voice of the woman talking to him across the table. Additional sources include speakers on the wall behind him, which are broadcasting music, and all the other people in the room who are speaking. The four problems we will consider in this chapter (1) auditory localization, (2) sound reflection, (3) analysis of the scene into separate sound sources, and (4) musical patterns that are organized in time-are indicated in this figure.

Answer 1

Depth perception and localization relate to knowing where the stimuli in the environment are relative to one another. Complexities arise as a result of different positions, timings and intensities of stimuli. Thus, it is important to have a robust, sensitive system for detecting multiple stimuli and their differences in order to form an accurate picture of the environment.
Depth perceptions from both visual and auditory perspectives involve localizing the stimuli in the environment. They both include mechanisms for accounting for the differences in relative positioning and timings of stimuli, and both of them generate electrical signals in the nervous system that are processed in the brain.
The main differences lie in the specifics of the mechanics used to receive and differentiate between stimuli and their locations. The visual system mainly relies on the images formed by the visual stimuli on the retina. The images formed on the retina often directly account for the relative distance and positioning of differing stimuli.
By contrast, the auditory system has to account for the varying positions indirectly, as stimuli of similar frequency will sound the same regardless of positioning. Two ways of differentiating stimuli along the azimuth (left-to-right) plane are binaural cures: the interaural time difference (ITD) , and the interaural level difference (ILD).
Based on behavioral studies, the interaural time difference is most effective when distinguishing between low frequency sounds. The ITD operates on the principle that sound sources located at different points on the azimuth plane reach the left and right ears at different time intervals. Hence, the observer can distinguish between them.
Additionally, the interaural level difference is an effective way of distinguishing between high-frequency sounds. The head serves as an acoustic barrier to high-frequency sound stimuli. Hence, the reduction in intensity, or "level", of the signals-which corresponds to the distance they are from the ears-allows the left and right ears to distinguish between them.
For differences in vertical level, the monoaural cue of the spectral cue helps in localizing stimuli. The spectral cue refers to the differing spectra of frequencies that enter the ear. This is because the sound waves are reflected off various folds of the pinnae as they enter the auditory canal. The pattern of these reflections is affected by the elevation level of the stimulus.
From the above explanations, it can be seen how, in contrast to the direct ways the visual system can account for location differences, the auditory system has several indirect and sophisticated means for localizing the stimuli it receives.

Question 2

How good are the acoustics in your classrooms? Can you hear the professor clearly? Does it matter where you sit? Are you ever distracted by noises from inside or outside the room?

Answer 2

The quality of acoustics in the classroom depends on a variety of factors. These factors mainly relate to the relationship between the "direct sounds", which reach the listener directly from its source, and the "indirect sounds", which are reflected off the indoor environment before reaching the observer.
The two major factors that affect the quality of sound in a lecture hall are the reverberation time and signal-to-noise ratio (SNR). The reverberation time is defined as the amount of time it takes for the level of indirect sound in the room to reach1/1000 th of its original pressure.
Alternatively, it can be defined as a decrease in level of indirect sound to 30dB. Ideally, the reverberation time for a small classroom should be between 0.4 to 0.6 seconds. In cases of higher reverberation times, the clarity of the spoken lecture will get distorted.
The signal-to-noise ratio (SNR) is the difference between the level of the teacher's voice and the level of the distracting background noises. Ideally, the SNR should be +10 to +15 dB or more.
In cases of lower signal-to-noise ratio, the students are more likely to be distracted by backgrounds noises. SNR is affected by the relative proximity of the listener to the lecturer and the source of the background noises, hence the student's seating positions should be arranged keeping this in mind.

Question 3

How is object recognition in vision like stream segregation in hearing?
Figure Each musician produces a sound stimulus, but these signals are combined into one signal, which enters the ear.

Answer 3

Object recognition and stream segregation in vision and hearing involve the integration of various sensations, or lack thereof, into the final "whole" that is perceived. There are several similarities regarding the broad principles on which these processes operate in vision and hearing.
Both auditory stream segregation and visual object recognition rely on various underlying processes to generate a "whole" perception. This includes various mechanisms for integrating the different stimuli into one comprehensive complete picture.
For instance, both the visual and auditory systems rely on mechanisms to differentiate between the locations of the sources. The auditory system uses cues such as the interaural time difference (ITD) and interaural level difference (ILD) to discern location, while the visual system uses the varying positions of the images on the retina for the same.
Additionally, both the visual and auditory systems have mechanisms in place to compensate for disruptions in stimuli. Hence, a continuous interpretation of a stimulus is possible despite distortions. This can also be exploited to create optical and auditory illusions.
The visual and auditory systems can also differentiate between neighboring stimuli based on differences in their intensity. For instance, when the auditory system perceives music of rapidly alternating high and low frequencies, it is interpreted as coming from separate sources.
Hence, both the visual and auditory systems have ways of integrating different stimuli and forming a complete whole, using mechanisms that go beyond simply summing up all the various sensations. Instead, they differentiate between the locations of various sources, and interpret the true nature of several stimuli based on underlying principles regarding intensity and onset time differences.

Question 4

In the experiments on metrical structure, the stimulus was a steady, accent-free, series of beats, like the sound produced by a metronome. But in most music, specific beats are accented. Determine a number of ways that this accenting is achieved, by listening to a few different kinds of music.
Figure 1
Figure 1 Four measures of a composition by J. S. Bach (Choral Prelude on Jesus Christus unser Heiland , 1739). When played rapidly, the upper notes become perceptually grouped and the lower notes become perceptually grouped, a phenomenon called auditory stream segregation.

Figure 2
Figure 2 (a) When high and low tones are alternated slowly, auditory stream segregation does not occur, so the listener perceives alternating high and low tones. (b) Faster alternation results in segregation into high and low streams.

Question 5

What are some situations in which (a) you use one sense in isolation, and (b) the combined use of two or more senses is necessary to accomplish a task?
Figure (a) Subjects listened to sequences of short and long tones. On half the trials, the first tone was short; on the other half, long. The durations of the tones ranged from about 150 ms to 500 ms (durations varied for different experimental conditions), and the entire sequence repeated for 5 seconds. (b) English-speaking subjects (E) were more likely than Japanese-speaking subjects (J) to perceive the stimulus as short-long. (c) Japanese-speaking subjects were more likely than English-speaking subjects to perceive the stimulus as long-short.

Question 6

How is auditory space described in terms of three coordinates?

Question 7

What is the basic difference between determining the location of a sound source and determining the location of a visual object?

Question 8

Describe the binaural cues for localization. Indicate the frequencies and directions relative to the listener for which the cues are effective.

Question 9

Describe the monaural cue for localization.

Question 10

What happens to auditory localization when a mold is placed in a person's ear? How well can a person localize sound once he or she has adapted to the mold? What happens when the mold is removed after the person has adapted to it?

Question 11

Describe the auditory pathway from cochlea to auditory cortex.

Question 12

Describe the Jeffress model, and how neural coding for localization differs for birds and for mammals.

Question 13

Describe how auditory localization is organized in the cortex. What is the evidence that A1 is important for localization? That areas in addition to A1 are involved in localization? What is the evidence for what and where pathways in the auditory system?

Question 14

Why does music played outdoors sound different from music played indoors?

Question 15

What is the precedence effect, and what does it do for us perceptually?

Question 16

What are some basic principles of architectural acoustics that have been developed to help design concert halls? What are some special problems in designing classrooms?

Question 17

What is auditory scene analysis, and why is it a "problem" for the auditory system?

Question 18

What are the basic principles of auditory grouping that help us achieve auditory scene analysis? Be sure you understand the following experiments: Bregman and Campbell (Figure 1); "galloping" crossing streams (Figure 2); scale illusion (Figure 3); auditory continuity (Figure 4); and melody schema (Figure 5).
Figure 1 (a) When high and low tones are alternated slowly, auditory stream segregation does not occur, so the listener perceives alternating high and low tones. (b) Faster alternation results in segregation into high and low streams.

Figure 2 (a) Two sequences of stimuli: a sequence of similar notes (red), and a scale (blue). (b) Perception of these stimuli: Separate streams are perceived when they are far apart in frequency, but the tones appear to jump back and forth between stimuli when the frequencies are in the same range.

Figure 3 (a) These stimuli were presented to a listener's left ear (blue) and right ear (red) in Deutsch's (1975) scale illusion experiment. Notice how the notes presented to each ear jump up and down. (b) Although the notes in each ear jump up and down, the listener perceives a smooth sequence of notes. This effect is called the scale illusion, or melodic channeling.

Figure 4 A demonstration of auditory continuity, using tones.

Figure 5 "Three Blind Mice." (a) Jumping octave version. (b) Normal version.

Question 19

What is the difference between the rhythmic pattern and metrical structure?

Question 20

Why can we describe the beating of a metronome as an ambiguous metrical stimulus? Describe the experiments that demonstrate a connection between (a) movement and metrical grouping and (b) a person's language and metrical grouping.

Question 21

Describe the ways that (a) vision "dominates" hearing; (b) hearing dominates vision; (c) sound provides information that influences what we see.