Chapter 11: Perception of SoundOverview of Questions

• If a tree falls in the forest and no one is there to hear it, is there a sound?

• What is it that makes sounds high pitched or low pitched?

• How do sound vibrations inside the ear lead to the perception of different pitches?

• How are sounds represented in the auditory cortex?

Pressure Waves and Perceptual Experience

• Two definitions of “sound”

– Physical definition - sound is pressure changes in the air or other medium

– Perceptual definition - sound is the experience we have when we hear

Sound Waves

• Loud speakers produce sound by

– The diaphragm of the speaker moves out, pushing air molecules together

– The diaphragm also moves in, pulling the air molecules apart

– The cycle of this process creates alternating high- and low-pressure regions that travel through the air

Sound Waves - continued

• Pure tone - created by a sine wave

– Amplitude - difference in pressure between high and low peaks of wave

• Perception of amplitude is loudness

• Decibel (dB) is used as the measure of energy in the moving air molecules

• Number of dB = 20 logarithm(p/po)

• The decibel scale relates the amplitude of the stimulus with the psychological experience of loudness

Figure 11.2 Sine-wave pressure changes for a pure tone.

Sound Waves - continued

• Frequency - number of cycles within a given time period

– Measured in Hertz (Hz) - 1 Hz is 1 cycle per second

– Perception of pitch is related to frequency

– Tone height is the increase in pitch that happens when frequency is increased

Musical Scales and Frequency

• Letters in the musical scale repeat

• Notes with the same letter name (separated by octaves) sound similar - called tone chroma

• Notes separated by octaves have frequencies that are multiples of each other

Range of Human Hearing

Sound Quality: Timbre

• All other properties of sound except for loudness and pitch constitute timbre

• Timbre is created partially by the multiple frequencies that make up complex tones

– Fundamental frequency is the first harmonic

– Musical tones have additional harmonics that are multiples of the fundamental frequency

Sound Quality: Timbre

• Additive synthesis - process of adding harmonics to create complex sounds

• Frequency spectrum - display of harmonics of a complex sound

• Attack of tones - buildup of sound at the beginning of a tone

• Decay of tones - decrease in sound at end of tone

Frequency Spectrum - The heights of the lines indicate the amplitude of each of the frequencies that make up the tone.

Figure 11.10 Frequency spectra for a guitar, a bassoon, and an alto saxophone playing a tone with a fundamental frequency of 196 Hz. (Note is a G)

• The Human Ear

Figure 11.12 The middle ear.

M

Oval Window

S

Round Window

Function of Ossicles

• Outer and inner ear are filled with air

• Inner ear filled with fluid that is much denser than air

• Pressure changes in air transmit poorly into the denser medium

• Ossicles act to amplify the vibration for better transmission to the fluid

Figure 11.13 Environments inside the outer, middle, and inner ears. The fact that liquid fills the inner ear poses a problem for the transmission of sound vibrations from the air of the middle ear.

Mechanical amplification of the sound message

The Inner Ear

• Main structure is the cochlea

– Fluid-filled snail-like structure set into vibration by the stapes

– Divided into the scala vestibuli and scala tympani by the cochlear partition

– Cochlear partition extends from the base (stapes end) to the apex (far end)

– Organ of Corti contained by the cochlear partition

The Organ of Corti

• Key structures

– Basilar membrane vibrates in response to sound and supports the organ of Corti

– Inner and outer hair cells are the receptors for hearing

– Tectorial membrane extends over the hair cells

• Transduction at the hair cells takes place due to the interaction of these structures

Neural Signals for Frequency

• There are two ways nerve fibers signal frequency

– Which fibers are responding

• Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers

– How fibers are firing

• Rate or pattern of firing of nerve impulses

Figure 11.18 Hair cells all along the cochlea send signals to nerve fibers that combine to form the auditory nerve. This figure indicates that these fibers can signal a tone’s frequency: (a) by which fibers fire in response to the tone; and (b) by the pattern of nerve impulses in response to the tone. See text for details.

Békésys’ Place Theory of Hearing

• Frequency of sound is indicated by the place on the organ of Corti that has the highest firing rate

• Békésy determined this in two ways

– Direct observation of the basilar membrane in a cadaver

– Building a model of the cochlea using the physical properties of the basilar membrane

Figure 11.21 Vibration of the basilar membrane, showing the position of the membrane at three instants in time, indicated by the blue, green, and red lines, and the envelope of the vibration, indicated by the black dashed line. P indicates the peak of the basilar membrane vibration. (From Experiments in Hearing, by G. von Békésy, 1960, figures 11.43. Copyright © 1960 by McGraw-Hill Book Company, New York. Reprinted by permission.)

Figure 11.22 The envelope of the basilar membrane’s vibration at frequencies ranging from 25 to 1,600 Hz, as measured by Békésy (1960). These envelopes were based on measurements of damaged cochleas. The envelopes are more sharply peaked in healthy cochleas.

Evidence for the Place Theory

• Tonotopic map

– Cochlea shows an orderly map of frequencies along its length

• Apex responds best to high frequencies

• Base responds best to low frequencies

Figure 11.23 Tonotopic map of the guinea pig cochlea.

Evidence for the Place Theory - continued

• Neural frequency tuning curves

– Pure tones are used to determine the threshold for specific frequencies measured at single neurons

– Plotting thresholds for frequencies results in tuning curves

– Frequency to which the neuron is most sensitive is the characteristic frequency

Evidence for the Place Theory - continued

• Auditory masking experiments

– First, thresholds for a number of frequencies are determined

– Then, a single intense masking frequency is presented at the same time that the thresholds for the original frequencies are re-determined

– The masking effect is seen at the masking tone’s frequency and spreads to higher frequencies more than lower ones

Figure 11.25 The procedure for a masking experiment. (a) Threshold is determined across a range of frequencies. Each blue arrow indicates a frequency where the threshold is measured. (b) The threshold is redetermined at each frequency (blue arrows) in the presence of a masking stimulus (red arrow).

Figure 11.26 Results of Egan and Hake’s (1950) masking experiment. The threshold increases the most near the frequencies of the masking noise and the masking effect spreads more to high frequencies than to low frequencies. (Adapted from “On the Masking Pattern of a Simple Auditory Stimulus,” by J. P. Egan and H. W. Hake, 1950, Journal of the Acoustical Society of America, 22, 622-630. Copyright © 1950 by the American Institute of Physics. Adapted by permission.)

Response of Basilar Membrane to Complex Tones

• Fourier analysis - mathematic process that separates complex waveforms into a number of sine waves

• Research on the response of the basilar membrane shows the highest response in auditory nerve fibers with characteristic frequencies that correspond to the sine-wave components of complex tones

• Thus the cochlea is called a frequency analyzer

Figure 11.32 A complex sound wave. Applying Fourier analysis to this wave indicates that it is made up of the three components in (b), (c), and (d).

Figure 11.33 The cochlea is called a frequency analyzer because it analyzes incoming sound into its frequency components and translates the components into separated areas of excitation along its length. In this example, the complex tone, which consists of frequency components at 440, 880, and 1,320 Hz, enters the outer ear. The shaded areas on the basilar membrane represent places of peak vibration for the tone’s three components, and the darkened hair cells represent the hair cells that will be most active in response to this tone.

Timing of Neural Firing and Frequency

• Phase locking

– Nerve fibers fire in bursts

– Firing bursts happen at or near the peak of the sine-wave stimulus

– Thus, they are “locked in phase” with the wave

– Groups of fibers fire with periods of silent intervals creating a pattern of firing

Figure 11.34 The response of three nerve fibers (a, b, and c) that are phase-locked to the stimulus. Notice that the fibers don’t fire on every cycle of the stimulus, but when they do fire, they fire only when the stimulus is at or near its peak.

Pathway from the Cochlea to the Cortex

• 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 11.36 Diagram of the auditory pathways. This diagram is greatly simplified, as numerous connections between the structures are not shown. Note that auditory structures are bilateral -- they exist on both the left and right sides of the body -- and that messages can cross over between the two sides. (Adapted from Wever, 1949.)

Auditory Areas in the Cortex

• 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 11.37 The three main auditory areas in the cortex are the core area, which contains the primary auditory receiving area (A1), the belt area, and the parabelt area. Signals, indicated by the arrows, travel from core, to belt, to parabelt. The temporal lobe is pulled back to show areas that would not be visible from the surface. (Adapted from J. H. Kaas, T. A. Hackett, and M. J. Trano, (1999). “Auditory processing in primate cerebral cortex,” Current Opinion in Neurobiology, 9, 164-170.)

What and Where Streams for Hearing

• 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 11.38 Areas in the monkey cortex that respond to auditory stimuli. The green areas respond to auditory stimuli, the purple areas to both auditory and visual stimuli. The arrows from the temporal lobe to the frontal lobe represent the what and where streams in the auditory system. (Adapted from Porembra, et al., 2003.)

Perceiving Pitch and Complex Sounds

• Tonotopic maps are found in A1

– Neurons that respond better to low frequencies are on the left and those that respond best to high frequencies are on the right

– However, early research did not show a direct relationship between pitch perception and the tonotopic map

Figure 11.41 The outline of the core area of the monkey auditory cortex, showing the tonotopic map in the primary auditory receiving area, A1, which is located within the core. The numbers represent the characteristic frequencies (CF) of neurons in thousands of Hz. Low CF’s are on the left, and high CF’s are on the right. (Adapted from Kosaki et al., 1997).

Effect of the Missing Fundamental

• The fundamental frequency is the lowest frequency in a complex tone

• When the fundamental and other lower harmonics are removed, the perceived pitch is the same, but the timbre changes

• The pitch perceived in such tones is called periodicity pitch

Figure 11.44 (a) Frequency spectra for a tone with a fundamental frequency of 400 Hz. (b) First harmonic (the fundamental) removed. (c) First three harmonics removed. All three of these tones are perceived to have the same pitch.

Periodicity Pitch

• Pattern of stimulation on the basilar membrane cannot explain this phenomenon since removing the fundamental and harmonics creates different patterns

• Periodicity pitch is perceived even when the tones are presented to two ears

• Thus, pitch must be processed in an area where the signal from both ears are combined -- a central pitch processor

Cochlear Implants

• Electrodes are inserted into the cochlea to electrically stimulate auditory nerve fibers

• The device is made up of

– A microphone worn behind the ear

– A sound processor

– A transmitter mounted on the mastoid bone

– A receiver surgically mounted on the mastoid bone

Figure 11.46 Cochlear implant device. See text for details.

Cochlear Implants - continued

• Implants stimulate the cochlea at different places on the tonotopic map according to specific frequencies in the stimulus

• These devices help deaf people to hear some sounds and to understand language

• They work best for people who receive them early in life or for those who have lost their hearing, although they have caused some controversy in the deaf community