Speech perception Totally Explained

Everything about Speech Perception totally explained

Speech perception refers to the processes by which humans are able to interpret and understand the sounds used in language. The study of speech perception is closely linked to the fields of phonetics and phonology in linguistics and cognitive psychology and perception in psychology. Research in speech perception seeks to understand how human listeners recognize speech sounds and use this information to understand spoken language. Speech research has applications in building computer systems that can recognize speech, as well as improving speech recognition for hearing- and language-impaired listeners.

Basics of speech perception

The process of perceiving speech begins at the level of the sound signal and the process of audition. (For a complete description of the process of audition see Hearing.) After processing the initial auditory signal, speech sounds are further processed to extract acoustic cues and phonetic information. This speech information can then be used for higher-level language processes, such as word recognition.

Acoustic cues

The speech sound signal contains a number of acoustic cues that are used in speech perception. The cues differentiate speech sounds belonging to different phonetic categories. For example, one of the most studied cues in speech is voice onset time or VOT. VOT is a primary cue signaling the difference between voiced and voiceless stop consonants, such as "b" and "p". Other cues differentiate sounds that are produced at different places of articulation or manners of articulation. The speech system must also combine these cues to determine the category of a specific speech sound. This is often thought of in terms of abstract representations of phonemes. These representations can then be combined for use in word recognition and other language processes.
It isn't easy to identify what acoustic cues listeners are sensitive to when perceiving a particular speech sound:
ť At first glance, the solution to the problem of how we perceive speech seems deceptively simple. If one could identify stretches of the acoustic waveform that correspond to units of perception, then the path from sound to meaning would be clear. However, this correspondence or mapping has proven extremely difficult to find, even after some forty-five years of research on the problem.

If a specific aspect of the acoustic waveform indicated one linguistic unit, a series of tests using speech synthesizers would be sufficient to determine such a cue or cues. However, there are two significant obstacles:

One acoustic aspect of the speech signal may cue different linguistically relevant dimensions. For example, the duration of a vowel in English can indicate whether or not the vowel is stressed, or whether it's in a syllable closed by a voiced or a voiceless consonant, and in some cases (like American English /ɛ/ and /æ/) it can distinguish the identity of vowels. Some experts even argue that duration can help in distinguishing of what is traditionally called short and long vowels in English.
One linguistic unit can be cued by several acoustic properties. For example in a classic experiment, Alvin Liberman (1957) showed that the onset formant transitions of /d/ differ depending on the following vowel (see Figure 1) but they're all interpreted as the phoneme /d/ by listeners.

Linearity and the segmentation problem

Although listeners perceive speech as a stream of discrete units (phonemes, syllables, and words), this linearity is difficult to be seen in the physical speech signal (see Figure 2 for an example). Speech sounds don't strictly follow one another, rather, they overlap. A speech sound is influenced by the ones that precede and the ones that follow. This influence can even be exerted at a distance of two or more segments (and across syllable- and word-boundaries) Or, the VOT values marking the boundary between voiced and voiceless stops are different for labial, alveolar and velar stops and they shift under stress or depending on the position within a syllable.

Variation due to differing speech conditions. One important factor that causes variation is differing speech rate. Many phonemic contrasts are constituted by temporal characteristics (short vs. long vowels or consonants, affricates vs. fricatives, stops vs. glides, voiced vs. voiceless stops, etc.) and they're certainly affected by changes in speaking tempo. (see Figure 3 for an illustration of this). Dialect and foreign accent cause variation as well.

Perceptual constancy and normalization

Categorical perception is involved in processes of perceptual differentiation. We perceive speech sounds categorically, that's to say, we're more likely to notice the differences between categories (phonemes) than within categories. The perceptual space between categories is therefore warped, the centers of categories (or 'prototypes') working like a sieve or like magnets for in-coming speech sounds.
   Let us consider an artificial continuum between a voiceless and a voiced bilabial stop where each new step differs from the preceding one in the amount of VOT. The first sound is a pre-voiced [b], for example it has a negative VOT. Then, increasing the VOT, we get to a point where it's zero, for example the stop is a plain unaspirated voiceless [p]. Gradually, adding the same amount of VOT at a time, we reach the point where the stop is a strongly aspirated voiceless bilabial [pʰ]. (Such a continuum was used in an experiment by Lisker and Abramson in 1970. A two-alternative identification (or categorization) test will yield a discontinuous categorization function (see red curve in Figure 4).
   If we test the ability to discriminate between two sounds with varying VOT values but having a constant VOT distance from each other (20 ms for instance), listeners are likely to perform at chance level if both sounds fall within the same category and at nearly-100% level if each sound falls in a different category (see the blue discrimination curve in Figure 4).
   The conclusion to make from both the identification and the discrimination test is that listeners will have different sensitivity to the same relative increase in VOT depending on whether or not the boundary between categories was crossed. Similar perceptual adjustment is attested for other acoustic cues as well.

Top-down influences on speech perception

The process of speech perception isn't necessarily uni-directional. That is, higher-level language processes connected with morphology, syntax, or semantics may interact with basic speech perception processes to aid in recognition of speech sounds. It may be the case that it isn't necessary and maybe even not possible for listener to recognize phonemes before recognizing higher units, like words for example. After obtaining at least a fundamental piece of information about phonemic structure of the perceived entity from the acoustic signal, listeners are able to compensate for missing or noise-masked phonemes using their knowledge of the spoken language.
In a classic experiment, Richard M. Warren (1970) replaced one phoneme of a word with a cough-like sound. His subjects restored the missing speech sound perceptually without any difficulty and what is more, they were not able to identify accurately which phoneme had been disturbed. This is known as the phonemic restoration effect. Another basic experiment compares recognition of naturally spoken words presented in a sentence (or at least a phrase) and the same words presented in isolation. Perception accuracy usually drops in the latter condition. Garnes and Bond (1976) also used carrier sentences when researching the influence of semantic knowledge on perception. They created series of words differing in one phoneme (bay / day / gay, for example). The quality of the first phoneme changed along a continuum. All these stimuli were put into different sentences each of which made sense with one of the words only. Listeners had a tendency to judge the ambiguous words (when the first segment was at the boundary between categories) according to the meaning of the whole sentence.

Research topics

Infant speech perception

Infants begin the process of language acquisition by being able to detect very small differences between speech sounds. They are able to discriminate all possible speech contrasts (phonemes). Gradually, as they're exposed to their native language, their perception becomes language-specific, for example they learn how to ignore the differences within phonemic categories of the language (differences that may well be contrastive in other languages - for example, English distinguishes two voicing categories of stop consonants, whereas Thai has three categories; infants must learn which differences are distinctive in their native language uses, and which are not). As infants learn how to sort incoming speech sounds into categories, ignoring irrelevant differences and reinforcing the contrastive ones, their perception becomes categorical. Infants learn to contrast different vowel phonemes of their native language by approximately 6 months of age. The native consonantal contrasts are acquired by 11 or 12 months of age. Some researchers have proposed that infants may be able to learn the sound categories of their native language through passive listening, using a process called statistical learning. Others even claim that certain sound categories are innate, that is, they're genetically-specified (see discussion about innate vs. acquired categorical distinctiveness).
   If day-old babies are presented with their mother’s voice speaking normally, abnormally (in monotone), and a stranger’s voice, they react only to their mother’s voice speaking normally. When a human and a non-human sound is played, babies turn their head only to the source of human sound. It has been suggested that auditory learning begins already in the pre-natal period.
   How do researchers know if infants can distinguish between speech sounds? One of the techniques used to examine how infants perceive speech, besides the head-turn procedure mentioned above, is measuring their sucking rate. In such an experiment, a baby is sucking a special nipple while presented with sounds. First, the baby’s normal sucking rate is established. Then a stimulus is played repeatedly. When the baby hears the stimulus for the first time the sucking rate increases but as the baby becomes habituated to the stimulation the sucking rate decreases and levels off. Then, a new stimulus is played to the baby. If the baby perceives the newly introduced stimulus as different from the background stimulus the sucking rate will show an increase.
   Best (1995) proposed a Perceptual Assimilation Model which describes possible cross-language category assimilation patterns and predicts their consequences. Flege (1995) formulated a Speech Learning Model which combines several hypotheses about second-language (L2) speech acquisition and which predicts, in simple words, that an L2 sound that isn't too similar to a native-language (L1) sound will be easier to acquire than an L2 sound that's relatively similar to an L1 sound (because it'll be perceived as more obviously ‘different’ by the learner).

Speech perception in language or hearing impairment

Research in how people with language or hearing impairment perceive speech isn't only intended to discover possible treatments. It can provide insight into what principles underlie non-impaired speech perception. Two areas of research can serve as an example:

Listeners with aphasia. Aphasia affects both the expression and reception of language. Both two most common types, Broca's and Wernike's aphasia, affect speech perception to some extent. Broca’s aphasia causes moderate difficulties for language understanding. The effect of Wernike’s aphasia on understanding is much more severe. It is agreed upon, that aphasics suffer from perceptual deficits. They are usually unable to fully distinguish place of articulation and voicing. As for other features, the difficulties vary. It hasn't yet been proven whether low-level speech-perception skills are affected in aphasia sufferers or whether their difficulties are caused by higher-level impairment alone.

Noise

One of the basic problems in the study of speech is how to deal with the noise in the speech signal. This is shown by the difficulty that computer speech recognition systems have with recognizing human speech. These programs can do well at recognizing speech when they've been trained on a specific speaker's voice, and under quiet conditions. However, these systems often do poorly in more realistic listening situations where humans are able to understand speech without difficulty.

Research methods

The methods used in speech perception research can be roughly divided into three groups: behavioral, computational, and, more recently, neurophysiological methods. Behavioral experiments are based on an active role of a participant, for example subjects are presented with stimuli and asked to make conscious decisions about them. This can take the form of an identification test, a discrimination test, similarity rating, etc. These types of experiments help to provide a basic description of how listeners perceive and categorize speech sounds.
Computational modeling has also been used to simulate how speech may be processed by the brain to produce behaviors that are observed. Computer models have been used to address several questions in speech perception, including how the sound signal itself is processed to extract the acoustic cues used in speech, as well as how speech information is used for higher-level processes, such as word recognition.
Neurophysiological methods rely on utilizing information stemming from more direct and not necessarily conscious (pre-attentative) processes. Subjects are presented with speech stimuli in different types of tasks and the responses of the brain are measured. The brain itself can be more sensitive than it appears to be through behavioral responses. For example, the subject may not show sensitivity to the difference between two speech sounds in a discrimination test, but brain responses may reveal sensitivity to these differences.

Without the necessity of taking an active part in the test, even infants can be tested; this feature is crucial in research into acquisition processes. The possibility to observe low-level auditory processes independently from the higher-level ones makes it possible to address long-standing theoretical issues such as whether or not humans possess a specialized module for perceiving speech or whether or not some complex acoustic invariance (see lack of invariance above) underlies the recognition of a speech sound.

Theories

Research into speech perception (SP) has by no means explained every aspect of the processes involved. A lot of what has been said about SP is a matter of theory. Several theories have been devised to develop some of the above mentioned and other unclear issues. Not all of them give satisfactory explanations of all problems, however the research they inspired has yielded a lot of useful data.

Motor theory of SP

Some of the earliest work in the study of how humans perceive speech sounds was conducted by Alvin Liberman and his colleagues at Haskins Laboratories. Using a speech synthesizer, they constructed speech sounds that varied in place of articulation along a continuum from /bɑ/ to /dɑ/ to /gɑ/. Listeners were asked to identify which sound they heard and to discriminate between two different sounds. The results of the experiment showed that listeners grouped sounds into discrete categories, even though the sounds they were hearing were varying continuously. Based on these results, they proposed the notion of categorical perception as a mechanism by which humans are able to identify speech sounds.
More recent research using different tasks and methodologies suggests that listeners are highly sensitive to acoustic differences within a single phonetic category, contrary to a strict categorical account of speech perception.
In order to provide a theoretical account of the categorical perception data, Liberman and colleagues worked out the motor theory of speech perception, where “the complicated articulatory encoding was assumed to be decoded in the perception of speech by the same processes that are involved in production” and even later to intended articulatory gestures, thus "the neural representation of the utterance that determines the speaker’s production is the distal object the lister perceives" By claiming that the actual articulatory gestures that produce different speech sounds are themselves the units of speech perception, the theory bypasses the problem of lack of invariance.

Fuzzy-logical model of SP

The fuzzy logical theory of speech perception developed by Massaro proposes that people remember speech sounds in a probabilistic, or graded, way. It suggests that people remember descriptions of the perceptual units of language, called prototypes. Within each prototype various features may combine. However, features are not just binary (true or false), there's a fuzzy value corresponding to how likely it's that a sound belongs to a particular speech category. Thus, when perceiving a speech signal our decision about what we actually hear is based on the relative goodness of the match between the stimulus information and values of particular prototypes. The final decision is based on multiple features or sources of information, even visual information (this explains the McGurk effect). Computer models of the fuzzy logical theory have been used to demonstrate that the theory's predictions of how speech sounds are categorized correspond to the behavior of human listeners.

Acoustic landmarks and distinctive features

In addition to the proposals of Motor Theory and Direct Realism about the relation between phonological features and articulatory gestures, Kenneth N. Stevens proposed another kind of relation: between phonological features and auditory properties. According to this view, listeners are inspecting the incoming signal for the so-called acoustic landmarks which are particular events in the spectrum carrying information about gestures which produced them. Since these gestures are limited by the capacities of humans’ articulators and listeners are sensitive to their auditory correlates, the lack of invariance simply doesn't exist in this model. The acoustic properties of the landmarks constitute the basis for establishing the distinctive features. Bundles of them uniquely specify phonetic segments (phonemes, syllables, words).

Exemplar theory

Exemplar models of speech perception differ from the four theories mentioned above which suppose that there's no connection between word- and talker-recognition and that the variation across talkers is ‘noise’ to be filtered out.
The exemplar-based approaches claim listeners store information for word- as well as talker-recognition. According to this theory, particular instances of speech sounds are stored in the memory of a listener. In the process of speech perception, the remembered instances of for example a syllable stored in the listener’s memory are compared with the incoming stimulus so that the stimulus can be categorized. Similarly, when recognizing a talker, all the memory traces of utterances produced by that talker are activated and the talker’s identity is determined. Supporting this theory are several experiments reported by Johnson that suggest that our signal identification is more accurate when we're familiar with the talker or when we've visual representation of the talker’s gender. When the talker is unpredictable or the sex misidentified, the error rate in word-identification is much higher.
The exemplar models have to face several objections, two of which are (1) insufficient memory capacity to store every utterance ever heard and, concerning the ability to produce what was heard, (2) whether also the talker’s own articulatory gestures are stored or computed when producing utterances that would sound as the auditory memories.Further Information

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