Chapter 11: Audition

In this chapter, we examine auditory perception, the sense that allows us to perceive and interpret sound. Auditory perception plays a crucial role in our daily lives, from communicating with others to enjoying music and being aware of our environment. We will start by exploring the basics of sound, sound waves, and how our ears are finely tuned to detect and process auditory information.

Understanding Sound Waves

The Nature of Sound

Sound is a phenomenon that originates from vibrations or disturbances in the air, creating waves of air pressure that our ears can detect. These waves can be caused by various sources, from the vibration of a tuning fork to the rustling of leaves in the wind. Understanding the fundamentals of sound is essential to grasp the intricacies of auditory perception.

Frequency and Pitch

Frequency measures the number of cycles per second in a sound wave and determines the pitch of the sound. Human hearing is typically sensitive to frequencies ranging from approximately 20 hertz (Hz) to 20,000 Hz. Different animals have varying audible ranges based on their physiology.

Amplitude and Loudness

Amplitude refers to the height of a sound wave, representing the change in air pressure. This characteristic is closely related to our perception of loudness. The greater the amplitude, measured in decibels (dB), the louder the sound. However, loudness is not solely determined by amplitude, as frequency also plays a significant role. As frequency increases the threshold for detecting the sound follows a curved path, first lowering (making it easier to detect the sound) and later increasing (making very high pitch sounds harder to detect). For this reason, it is possible for a tone that is presented at the same amplitude (measured in decibels) to fall below this threshold (going undetected) for very low frequencies, be easily heard for somewhat higher frequencies, and then go unnoticed again for very high frequency sounds.

Auditory Anatomy

The Outer Ear

The outer ear consists of the visible external portion known as the pinna and the auditory canal. Its primary function is to capture and funnel sound waves towards the middle ear.

The Middle Ear

The middle ear is home to the tympanic membrane (eardrum) and the ossicles, which include the malleus, incus, and stapes. These components work together to amplify and transmit sound vibrations from the eardrum to the inner ear.

The Inner Ear

Deep within the inner ear lies the cochlea, a coiled, fluid-filled structure responsible for transducing sound vibrations into neural signals. The cochlea contains specialized hair cells and membranes, including the basilar membrane and tectorial membrane, which are essential for sound perception.

The Process of Auditory Transduction

Cochlear Function

Sound vibrations transmitted through the ossicles eventually reach the oval window. When the stapes presses on the oval window this causes fluid within the cochlea to move. This movement stimulates hair cells along the basilar membrane, leading to auditory transduction.

Figure 11.3

Hair Cells

The inner hair cells are responsible for converting mechanical vibrations into neural signals, which are then transmitted via the auditory nerve to the brain. Outer hair cells

Georg von Békésy

Georg von Békésy, born in Budapest, Hungary in 1899, was a renowned scientist whose groundbreaking research in auditory physiology earned him the Nobel Prize in Physiology or Medicine in 1961. His work revolutionized our understanding of how humans perceive sound.

Early Life and Academic Pursuits

Békésy came from a family of musicians and artists, but he chose a different path, focusing on science. He initially studied engineering at the Budapest Technical University but soon shifted his interest towards physics, which led him to study at the University of Bern in Switzerland. There, he worked under the guidance of the eminent physicist Auguste Piccard, who encouraged his scientific endeavors.

Auditory Research: The Place Theory and the Cochlea

Békésy's most significant contributions came through his research on hearing and auditory perception. In the early 20th century, there was much debate about how humans perceive different frequencies of sound. Békésy's pioneering work in the 1920s and 1930s shed light on this mystery.

His famous "place theory" challenged the prevailing "frequency theory" of hearing. Békésy proposed that different frequencies of sound are detected at specific locations along the cochlea, the spiral-shaped, fluid-filled structure in the inner ear. He developed an innovative technique that allowed him to observe the motion of the basilar membrane inside the cochlea in response to different frequencies of sound. According to this theory low frequency sounds will maximally displace the basilar membrane closer to the apex (the end of the unrolled cochlea) while high frequency sounds will maximally displace the basilar membrane closer to the base (the side closer to where the stapes presses on the oval window).

The Glue Factory and the Elephant

One intriguing anecdote from Békésy's life involves an elephant skull. In his quest to understand hearing, he needed to study the cochlea of different animals to draw comparative conclusions. However, obtaining suitable specimens proved challenging.

Legend has it that Békésy had heard about an elephant skull at a glue factory in Zurich. He believed that studying the elephant cochlea could provide valuable insights since this would be larger and might be easier to study and photograph when sounds are played. Undeterred, he reportedly sent a graduate student to the glue factory with saws, knives, and other tools and in the end, Békésy obtained the elephant skull.

Auditory Analysis

According to the place theory, our auditory system can analyze complex sounds, breaking them down into their component frequencies. This process, akin to Fourier analysis, allows us to perceive the various elements of music and environmental sounds.

Causes of Hearing Loss

Noise-Induced Hearing Loss

One of the primary causes of hearing loss is exposure to loud sounds, such as concerts or prolonged use of headphones at high volumes. Noise-induced hearing loss can result in permanent damage to the auditory system.

Genetic Factors

A family history of hearing loss can increase the risk of inheriting hearing-related issues. Understanding one's genetic predisposition to hearing loss can help in adopting preventative measures.

Other Factors

Hearing loss can also occur due to factors like ear trauma, illness, or damage to the auditory system's delicate components. In some cases, hearing loss can be accompanied by tinnitus, a persistent ringing in the ears.

Gestalt Grouping

Auditory Grouping and Perceptual Processes

Auditory information can be grouped in ways that are similar to what has been found with visual information processing.

Similarity and Proximity in Auditory Grouping

Just as visual information can be grouped based on the similarity of color or spatial proximity, auditory information can be grouped based on similarity of pitch and temporal proximity.

Temporal Proximity: Manipulating Auditory Grouping

Temporal proximity, in particular, is a key factor in auditory grouping. To illustrate this concept, consider hearing tones that gradually alternate between high and low pitches. In this situation you may perceive one stream of information. However, as we speed up the sequence slightly, you might perceive two separate sequences, each consisting of high-low pairs. When the sequence is further accelerated, you may notice that the tones are now grouped together in four streams that do not rise and fall in pitch.

Figure 11.5

Much like spatial proximity influences visual grouping, temporal proximity shapes how we perceive auditory information. When sounds occur close together in time, we tend to group them into a single auditory stream.

This phenomenon, known as auditory stream segregation, occurs when we separate or segregate streams of auditory information based on their pitch. However, this segregation only happens when sounds are in close temporal proximity to one another.

Think of it as organizing the sounds into distinct "streams" based on their similarities, just as you might group objects together visually based on their color or spatial closeness.

Good Continuation in Auditory Perception

Another critical aspect of auditory perception is good continuation. This concept mirrors what happens in visual perception when our brains fill in gaps to create a continuous pattern or form.

In one auditory example, you hear a tone followed by silence, and the cycle repeats. Strangely, if static noise is added to the gaps that had been silent people tend to hear the tone as continuing behind the static. It feels as though it continues behind the static and isn't interrupted in much the same way as a visually presented line appears to continue even when it is blocked by an occluding object.

Shepard scale and the Tritone Paradox

The Shepard scale is an auditory illusion where a series of tones seem to continuously ascend or descend in pitch, yet never actually get higher or lower. This creates a sense that the tones are rising in pitch or falling in pitch when played in a sequence. So, in the image shown here time is on the x axis and frequency in on the y axis. Darker bands indicate louder sounds (i.e., greater amplitude) so each tone (from 1 to 12 on the x axis) in a mix of multiple frequencies. If these tones were played from left to right people would hear an ascending sequence. Notice that this sequence is simply repeated again and again (i.e., tones 1 to 12 are repeated).

The tritone paradox involves two of these Shepard (e.g., 1 and 6 or 3 and 8, etc.). It is called the tritone paradox because a tritone is a musical interval that's composed of two notes that are six semitones apart. What is interesting is that if people are presented with two of these tones that are separated by an interval of a tritone (e.g., getting tone 1 followed by tone 6) people will disagree about whether the tones were increasing in frequency or decreasing in frequency. Some listeners may perceive the same tone pair as ascending while other people may perceive them as descending. This is called the tritone paradox. Diana Deutsch (1991) has made the claim that the tritone paradox differs based on the language a person is exposed to in childhood.

Auditory Perception of Language

Finally, let's touch very briefly on how we perceive language. When we listen to spoken language, we tend to perceive individual words as distinct units. However, in reality, there aren't always clear pauses or silences between words.

Newborns have the remarkable ability to distinguish between various phonemes (the smallest units of sound in language) from all possible languages. However, as they grow, they start focusing on the specific phonemes relevant to their native language, filtering out those that aren't essential. This filtering process begins with vowels and later extends to consonants.

Language perception is complex, and our brains develop the ability to parse and understand spoken language over time. In essence, what seems like a series of isolated words is actually a continuous stream of sound, seamlessly processed by our brains.

Wernicke's area is a critical region in the human brain that plays a pivotal role in the understanding of spoken language. Located in the posterior part of the left hemisphere, Wernicke's area is an integral component of the broader language processing network. It was first identified by the German neurologist Carl Wernicke in the late 19th century and has since become a focal point for research into language comprehension. The primary function of Wernicke's area is to process the auditory information received from the ears and convert it into meaningful linguistic representations. This region is especially important for the comprehension of spoken language, allowing individuals to decipher and make sense of the sounds and words they hear during conversations, lectures, or any form of oral communication.

Figure 11.6

Pathway from the ear to A1

In the pathway from the ear to the primary receiving area for auditory information, several key structures are involved, including the superior olivary nucleus, inferior colliculus, medial geniculate nucleus (in the thalamus), and the auditory cortex (A1). This neural pathway serves to process and transmit auditory information from the external environment to the brain's primary center for sound perception.

1. Superior Olivary Nucleus (SON): The pathway begins with the superior olivary nucleus, which is crucial for localizing sounds. It assesses the time difference in sound arrival between the two ears and differences in sound intensity. This information helps individuals pinpoint the direction from which sounds originate.

2. Inferior Colliculus: From the superior olivary nucleus, auditory information is relayed to the inferior colliculus. This structure is responsible for filtering out self-generated sounds, such as one's own vocalizations, chewing, and respiration. It helps prioritize external auditory stimuli over internal noises.

3. Medial Geniculate Nucleus (Thalamus): The information then travels to the medial geniculate nucleus in the thalamus. The thalamus acts as a relay center for various sensory inputs, including auditory information. It serves as a crucial gateway, regulating the flow of sensory data to the cortex.

4. Auditory Cortex (A1): The final destination in this pathway is the primary receiving area for auditory information, known as the auditory cortex or A1. Located in the temporal lobe of the brain, A1 is responsible for processing and interpreting auditory stimuli. It plays a pivotal role in sound perception, speech comprehension, and music processing.

It's worth noting that this pathway is organized in a tonotopic manner, similar to how the basilar membrane in the cochlea is arranged. Different regions of A1 respond to specific ranges of sound frequencies. High-frequency sounds activate one region, while low-frequency sounds activate another. This tonotopic map allows the brain to process auditory information with spatial precision, just as in vision, where a retinotopic map helps process visual stimuli based on their location in the visual field.

Auditory Ambiguities and Synesthesia

In the realm of perception, auditory processes share some intriguing similarities with visual perception, and we can encounter ambiguities in both sensory modalities.

Auditory Ambiguities

Just as we discussed visual illusions like the infamous dress that could appear as black and blue or gold and white, auditory illusions can also confound our senses. One such auditory illusion made waves across the internet: the Laurel-Yanny auditory clip.

In the Laurel-Yanny clip, some individuals hear "Laurel" while others perceive "Yanny." Remarkably, this perception can vary from one person to another and even change depending on the equipment used, such as speakers or headphones. To dissect this illusion, researchers examined the spectrograph of the Laurel-Yanny clip. They found that the high-frequency components of the sound leaned more towards "Yanny," while the low-frequency elements sounded more like "Laurel." By altering the auditory input— either by boosting the high frequencies (or the low frequencies)—researchers could influence how individuals perceived the clip.

Synesthesia: A Blend of Senses

Moving beyond ambiguities, let's explore synesthesia, a fascinating phenomenon in which individuals experience a blending of their senses. While we all possess some degree of synesthetic ability, certain individuals, known as synesthetes, exhibit a unique and often unusual mingling of their sensory experiences.

One common form of synesthesia is color-grapheme synesthesia, where people perceive letters and numbers as having distinct colors. For instance, the letter "A" might appear as red, while "B" is blue. These color associations are consistent for synesthetes throughout their lives, and they experience them as a natural part of their perception.

Synesthesia comes in various forms, such as music-color synesthesia, where musical notes evoke specific colors. This blending of sensory experiences, while unusual, is not considered a disorder but rather a variation in perception.

The Neural Basis of Synesthesia

Research by V. S. Ramachandran has uncovered intriguing insights into the neural basis of synesthesia. It appears that certain areas of the brain, such as the fusiform gyrus, play a pivotal role in this phenomenon. In the case of color- grapheme synesthesia, the fusiform gyrus, responsible for color perception (V4), interacts with the adjacent area dedicated to processing letters and numbers.

Moreover, this peculiar blending of senses is hereditary, suggesting a genetic component. Individuals with synesthesia may possess specific gene mutations that result in the cross-wiring of sensory regions in the brain. This cross-wiring manifests as a greater propensity for metaphorical thinking and creativity, often found among artists, poets, and novelists.

Conclusion

In conclusion, the study of audition unveils a captivating world of sensory perception, where our ears serve as gateways to a rich and multifaceted auditory landscape. From the intricate mechanics of the ear to the neural pathways that decode sound, we've explored the remarkable processes that underlie our ability to hear and make sense of the world through sound.