How the Brain Perceives Speech

How the Brain Perceives Speech1

How the Brain Perceives Speech

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Table of Contents

How the brain perceives speech; A paper on the physiological and psychological attributes of

Speech perception......................................................................................................................1

1.0 Abstract................................................................................................................3

2.0 Introduction..........................................................................................................3

3.0 Human Ear and Auditory Processing...................................................................4

4.0 Speech Perception; the phonological aspects.......................................................7

4.1 Acoustic cues....................................................................................................7

4.2 Linearity and Segmentation Defect..................................................................8

4.3 Constancy and Normalization..........................................................................8

5.0 Psychological aspects of Speech perception........................................................9

5.1 Top-down influences........................................................................................9

5.2 Second Language or Foreign Language speech perception...........................10

5.3 Speech Perception in infants...........................................................................11

6.0 Conclusion..........................................................................................................11

7.0 References..........................................................................................................12


Abstract

This paper is a brief inquiry into speech perception in humans with an approach that

incorporates the physiological as well as the psychological aspects of speech perception.

Throughout this paper various concepts involved in speech perception as evident from

scientific as well as language studies and researches have been elaborated in order to

understand the topic better. Lastly this paper also appeals for more and more research to be

done in speech perception by human brain as it has got vast scope as well as benefits intended

for hearing impaired persons as well as for technology advancements.

Introduction

Incredibly the human brain is endowed with the innate ability to control human

behavior. Ipso-facto human brain is the primary source of human behavior. Under this broad

domain of human behavior, speech perception is a part. The whole process of speech

perception involves many complex aspects and this paper is partly an attempt to understand

speech perception in terms of its physiological and psychological attributes.

The hearing process is controlled by human ear which is also a complex organ; the

auditory information captured by the human ear is delivered via a neural network to the

human brain where it is processed and perceived. The brain also interprets sounds of

language which leads to perceiving sounds in a particular manner. According to (Joanne L.

Miller, 1995) the function of hearing, interpreting and understanding sounds characteristic of

a particular language constitutes speech perception. To understand speech perception, one

must first of all have an overview of the process of hearing or audition at the physiological

level from where the actual process of speech perception begins.


Human Ear and Auditory Processing

The auditory information gathered by the human ear, i.e. by both the ears is

transmitted through a network of neurons to the brain where it is processed. However, to

understand speech perception, it would be useful to have an overview of the physiology of

the human ear and the process of hearing. The organization of the human ear can be divided

into three units, the outer ear, the middle ear and the inner ear which together carry out the

auditory processing. Externally, as one can see, the outer ear has a flap of skin which is

attached to the head with the help of a cartilage. Just after the cartilage, the ear canal begins

and is also a part of the outer ear; the ear canal is a tubular structure which has dimensions of

0.5 cm diameter and is 3 cm long. The purpose of the outer ear mechanism is to capture

sounds from the surroundings and to carry such auditory information to the middle and inner

parts of the ear from where it is transmitted to the brain. At the end of the ear canal, the ear

drum or the tympanic membrane is located.

The tympanic membrane receives the auditory information, i.e. the sound waves

striking it and starts vibrating. Such vibrations reach the middle ear which structurally a set of

small bones which transmit the auditory information to the inner ear (cochlea). Now, the

cochlea converts the auditory vibrations into neural impulses which are in turn transmitted to

the brain. The function of the cochlea is scientifically intriguing. Cochlea typically looks like

a snail and has dimensions of 2mm diameter and is 3 cm long. The tubular form of cochlea is

filled with liquid which actually transmits the auditory information received to the brain via

network of neurons.


It is a common observation in physics that the transmission of sound from air to liquid

is quite low as most the sound energy is reflected. The main reason for such low transmission

of sound is because of low mechanical impedance of air as against high mechanical

impedance of liquid. Practical and everyday example that explains the above stated

phenomenon is that it requires a person to use more energy to wave hands in water than in

air. It is the middle ear which acts as the medium for increasing the sound energy that enters

into the liquid inner ear so that the sound energy transmitted is maximized. The cochlea

within the inner ear has a basilar membrane which in turn supports cochlear nerve consisting

of about 12,000 sensory cells (Smith, 2002) . The cochlear nerve has fibers which respond

and cause the vibration of nerve cells. These fibers respond to particular sound frequencies

only and this is called the place principle which continues all along the path of travel of

auditory information to the brain.

There is another phenomenon that takes place within the process of hearing which is

very important; it is the volley principle phenomenon. In simple words volley principle is

nothing but the transmission of information by the neurons (nerve cells) which do it by

producing short electrical pulses which are known as the ‘action potentials’. The basilar

membrane supports nerve cells each of which have the ability to encode sound waves via

action potentials. As for instance, a 100Hz sound signal is encoded by 100 action potentials

generated per second by a nerve cell. However, 500Hz is the limit for the nerve cells to

produce action potentials (Smith, 2002). Therefore, the mechanism within the human ear

distributes the task of encoding higher frequency sounds by distributing the task to many

nerve cells and not just one; this is just amazing.


The existence of two ears for hearing serve the purpose of giving directional

advantage to hearing sounds. Human beings can recognize the direction of the sound from it

has emerged as well as its source. Humans have the ability to find the difference between two

sounds and their sources as well and this is attributed to the existence of two ears. There are

two ways through which the directional audio information is captured by the two ears; one is

that the sounds with frequencies above 1 kHz are blocked by the head which does not allow

those sounds to reach the other ear (Smith, 2002). Therefore, the ear that is in proximity of

the sound wave receives it with better signal than the one that is opposite to it. The second

thing that takes place is that though the farther ear receives sound, it receives it later than the

ear that is near to such a sound. It also depends on a person’s ability to rotate his/her head in

order to align to the sound being produced and to detect the change in sound patterns.

Now, the question comes in mind as to how would the brain perceive if both the ears

are subject to a sound signal simultaneously; interestingly scientists observed that the brain of

a person will perceive that sound as emerging from the centre of his/her (Listener’s) head.

However, auditory perception in human brain is not adequate in determining the distance of

the source of sound waves. For instance often it is the case with human listeners who tend to

perceive high pitched frequencies as coming from a nearer source while they perceive low

frequency sounds as coming from a farther source. On the other hand, animals such as the

bats will find it quite easy to find the sources of nearby sounds and objects; they use their

active sonar facility for doing so. Some of the researchers recently concluded that blind

people will be able to trace out the distance of the source of the sound using the process of

‘active eco localization’.


Speech Perception; the phonological aspects

As we have seen from the above discussion, speech perception begins right from the

process of hearing which involves physiology of the ear and its related parts. When the ear

takes in the auditory signals from external environment, the auditory signal is processed in

order that acoustic cues and phonetic information is gathered from it. Later, the information is

processed via higher level language processes that are undertaken by human brain in order to

recognize linguistic units such as phonemes, words, patterns, etc.

Acoustic cues

Acoustic cues are those that are utilized in the process of speech perception in order to

categorize speech sounds into various phonetic groups. Allowed voice onset time (VOT) is

one such acoustic cue that allows listeners to interpret auditory information received by the

ears. As for instance, the VOT distinguishes voiced and voiceless stop consonants. Some of

the researchers have found out that whenever there is a change in the speaking rate, the

listeners tend to alter the overlap between audio signals and the phonetic categories for that

particular sound. As for instance, people change the perceived VOT for consonants in

accordance with speech rate of the audio signal. However, if a phoneme such as /p/ which has

rapid speech rate is delivered in a particular audio signal with slow speech rate, then it is

likely to be interpreted by the listener’s brain as /b/.

Further, the brain uses a combination of acoustic cues in order to recognize higher

order linguistic units starting from the basic level of recognizing phonemes, in fact, units such


as words are perceived as a combination of particular group of phonemes. However, there are

certain problems associated with speech perception which makes it difficult to understand as

to how the brain deciphers speech. One such defect is discussed in the following section.

Linearity and Segmentation Defect

Researchers broadly agree over the fact that the brain perceives speech by segmenting

it into its discrete linguistic units such as phonemes, syllables, words, etc, which is what, is

known as the linearity principle in speech perception, however, actual results by means of

physical experimentation indicate otherwise. Spectrograms taken on speech perception

indicate that various perceived linguistic units actually overlap with each other negating the

linearity notion of speech perception. Such overlapping follows a pattern which clearly

indicates that a particular unit is influenced by the ones that follow it and the ones that

precede it. The same pattern is extended to at least two or more segments, which is what is

known as segmentation problem. Hence, a particular linguistic unit such as the phoneme is

influenced by the one that follows and the one that precedes it.

Constancy and Normalization

Constancy and normalization taking place in the speech perception process are very

important processes that help our understanding of speech perception. Irrespective of the type

of speakers in English, listeners make the categorization between vowels and consonants by

differentiating them under constant categories. Such categorization takes place by means of

perceptual normalization wherein listeners tend to clear out noise or variation in order to

clearly distinguish. The system that processes speech perception in the listener tends to adjust

to a speaker’s variation in speech and his/her acoustic characteristics. Variation occurs


because of differences in vocal tracts of speakers and the process of adjustment explained

above is called vocal tract normalization. There is another process of normalization known as

the speech rate normalization wherein the listener adjusts to the tempo of the speaker. In fact

other forms of perceptions also incorporate the process of perceptual constancy.

Psychological aspects of Speech perception

Apart from the aspects of speech perception discussed above, i.e. the physiology of the

ear, certain fundamental phonological processes, etc it would give us a complete

understanding if we can switch on to throw light upon certain psychological tracts involved in

the process. One common and everyday observation is that listeners are more comfortable

while perceiving their first language and often find it hard to perceive a foreign language as

the phonological structures vary from one language to another. Other such aspects, as for

instance, Infant speech perception, the top-down influences, etc are some of the predominant

psychological aspects in speech perception. In following section some of the chosen aspects

mentioned above are discussed.

Top-down influences

The above discussed phonological aspects are not always technically necessary to be in

place for speech to be perceived, for instance, one may perceive words directly even before

recognizing the individual phonemes. Therefore, the technical step by step process exists only

for guidance and not be adhered always. The skipping of basic sounds happens when the

individual happens to possess knowledge of the fundamentals of the auditory information

he/she is about to receive. When listeners are aware of the subject or the idea they are about

to hear, they are likely to skip the fundamental phonemic structures while hearing. In view of


researching the above mentioned trait among listeners, (Warren, 1970) experimented by

replacing a phoneme in an auditory information, already known to the audience, with sound

pattern similar to cough and found that the audience were able to recognize and restore the

actual sound of that phoneme in it while recollecting the whole information. In fact, the

audience were unable to even recognize as to which phoneme was muddled up! The above

phenomenon came to be known as the restoration effect.

Second Language or Foreign Language speech perception

Considerable amount of research was conducted to investigate upon the aspect of

speech perception in second language as well as foreign language. Broadly, this research area

is known as cross-language speech perception. The difficulty in perceiving speech of a new

language arises mainly because different languages possess unique phonetic characteristics as

well as different fundamental structures. Therefore, as phonetic characteristics vary, the

sounds produced in different languages vary accordingly. There is also notion that a person is

very much used to his/her first language and therefore, the phonological attributes of that

language are etched in the brain of the person. Hence, the person will face difficulty while

receiving and a perceiving new language.

When two languages are closely similar, then the language learners who shift from one

language to another will face difficulty. According to (Best, 1995) perceptual assimilation

model, it is easier to perceive the sounds of language that is different from one’s first

language than one that is phonologically closer to one’s first language. Likewise various


other models were proposed by researchers to accumulate patterns in speech perception in

second language and foreign language as well. For example, (Flege, 1995) Speech Learning

Model covers several hypotheses in speech perceptions in context of second language

acquisition and speech perception. An example for differences speech perception; Japanese

learners of English find it difficult to pronounce phonetic sounds specifically liquid

consonants such as /r/ and /l/.

Speech Perception in infants

Infant speech perceptions are interesting to study. When infants begin to perceive their

mother tongue, they usually differentiate slightly between speeches. Infants begin to react

more towards their language or the language of their mothers and their perception becomes

specific with progression of time. Researchers have suggested that infants acquire perception

vowel sounds before consonant sounds. Also, as they grow up infants learn how to categorize

sounds from one another. Generally, infants learn to perceive vowel sounds at the age of six

months and consonants at the age of eleven or twelve months. While categorizing infants also

learn how to sort out unwanted sounds. Passive learning technique is believed to be used by

infants in learning to perceive sounds of their native languages. In fact, infants are more

specific towards the voice of their mothers; it has been proved in the experiments of some of

the researchers that infants on their first day react only to the voice of their mothers and not

any other sound that is perceived by them as being strange. Moreover, it has been observed

that infants react categorically towards the sounds of humans than non-human sounds. In


recent developments researchers are using infrared spectroscopy to capture speech perception

scenarios not only in infants but in other spheres of this research area as well.

Conclusion

This paper which is a brief inquiry into the physiological and psychological aspects of

perceptual functioning with regards to speech dealt with various concepts that captured the

scenario within speech perception. The main sources that were considered for the preparation

of this paper were mainly secondary academic resources. This paper would also recommend

that more and more research studies have to be encouraged to investigate into speech

perception studies because there are various benefits from the outputs of such studies.

In-depth research in this area would help teachers devise better teaching strategies to

teach students pursuing studies in foreign languages and their second languages. One major

advantage with research studies in this field is that they would benefit the hearing impaired

patients with the manifestation of updated aids. Benefits from speech perception studies also

include advancements in speech recognizing computer systems and digital signal processing.

There also exists the complexity in the idea of how the brain perceives speech although many

hypotheses have been proposed to capture the clear picture, therefore, the subject should be

dealt with more than one perspective from sciences’ point of view as well as from linguistics

point of view.


References

Best, C. T. (1995). Perceptual Assimilation Model of non‐native speech perception

(PAM). Yale University. New Haven: Haskins Laboratories.

Flege, J. E. (1995). Second-language Speech Learning: Theory, Findings, and. In W.

Strange, Speech Perception and Linguistic Experience: (pp. 229-273). Timonium: York

Press.

Joanne L. Miller, P. D. (1995). Speech, Language, and Communication (Handbook Of

Perception And Cognition). San Diego, California, United States: Academic Press.

Smith, S. W. (2002). The Scientist and Engineer's Guide to Digital Signal Processing.

San Diego, CA, United States: California Technical Publishing.

Warren, R. M. (1982). Auditory perception: a new synthesis. Oxford, United

Kingdom: Pergamon Press.

How the Brain Perceives Speech

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