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  1. Frontiers | Speech perception as an active cognitive process | Frontiers in Systems Neuroscience
  2. Recognizing Speech Commands Using Recurrent Neural Networks with Attention
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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 plosives, 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.


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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 is not easy to identify what acoustic cues listeners are sensitive to when perceiving a particular speech sound:. 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:. Although listeners perceive speech as a stream of discrete units [ citation needed ] phonemes , syllables , and words , this linearity is difficult to see in the physical speech signal see Figure 2 for an example.

Speech sounds do not strictly follow one another, rather, they overlap.

This influence can even be exerted at a distance of two or more segments and across syllable- and word-boundaries. Because the speech signal is not linear, there is a problem of segmentation. It is difficult to delimit a stretch of speech signal as belonging to a single perceptual unit. The research and application of speech perception must deal with several problems which result from what has been termed the lack of invariance. Reliable constant relations between a phoneme of a language and its acoustic manifestation in speech are difficult to find.

There are several reasons for this:. Phonetic environment affects the acoustic properties of speech sounds. One important factor that causes variation is differing speech rate. Many phonemic contrasts are constituted by temporal characteristics short vs. The resulting acoustic structure of concrete speech productions depends on the physical and psychological properties of individual speakers.

Men, women, and children generally produce voices having different pitch. Because speakers have vocal tracts of different sizes due to sex and age especially the resonant frequencies formants , which are important for recognition of speech sounds, will vary in their absolute values across individuals [8] see Figure 3 for an illustration of this. Research shows that infants at the age of 7.

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Frontiers | Speech perception as an active cognitive process | Frontiers in Systems Neuroscience

Despite the great variety of different speakers and different conditions, listeners perceive vowels and consonants as constant categories. It has been proposed that this is achieved by means of the perceptual normalization process in which listeners filter out the noise i. This may be accomplished by considering the ratios of formants rather than their absolute values. Similarly, listeners are believed to adjust the perception of duration to the current tempo of the speech they are listening to — this has been referred to as speech rate normalization.

Whether or not normalization actually takes place and what is its exact nature is a matter of theoretical controversy see theories below. Perceptual constancy is a phenomenon not specific to speech perception only; it exists in other types of perception too. Categorical perception is involved in processes of perceptual differentiation. People perceive speech sounds categorically, that is to say, they are 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 [14] or like magnets [15] for incoming speech sounds. In an artificial continuum between a voiceless and a voiced bilabial plosive , each new step differs from the preceding one in the amount of VOT. The first sound is a pre-voiced [b] , i. Then, increasing the VOT, it reaches zero, i. Such a continuum was used in an experiment by Lisker and Abramson in 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.

Recognizing Speech Commands Using Recurrent Neural Networks with Attention

In a classic experiment, Richard M. Warren replaced one phoneme of a word with a cough-like sound. Perceptually, his subjects restored the missing speech sound without any difficulty and could not accurately identify which phoneme had been disturbed, [17] a phenomenon known as the phonemic restoration effect.


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Therefore, the process of speech perception is not necessarily uni-directional. Another basic experiment compared recognition of naturally spoken words within a phrase versus the same words in isolation, finding that perception accuracy usually drops in the latter condition. When put into different sentences that each naturally led to one interpretation, listeners tended to judge ambiguous words according to the meaning of the whole sentence [18]. It may be the case that it is not necessary and maybe even not possible for a 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 can compensate for missing or noise-masked phonemes using their knowledge of the spoken language. Compensatory mechanisms might even operate at the sentence level such as in learned songs, phrases and verses, an effect backed-up by neural coding patterns consistent with the missed continuous speech fragments, [20] despite the lack of all relevant bottom-up sensory input. The first ever hypothesis of speech perception was used with patients who acquired an auditory comprehension deficit, also known as receptive aphasia.

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Since then there have been many disabilities that have been classified, which resulted in a true definition of "speech perception". It consists of many different language and grammatical functions, such as: features, segments phonemes , syllabic structure unit of pronunciation , phonological word forms how sounds are grouped together , grammatical features, morphemic prefixes and suffixes , and semantic information the meaning of the words.

In the early years, they were more interested in the acoustics of speech. In recent years, there has been a model developed to create a sense of how speech perception works; this model is known as the dual stream model. This model has drastically changed from how psychologists look at perception. The first section of the dual stream model is the ventral pathway. This pathway incorporates middle temporal gyrus, inferior temporal sulcus and perhaps the inferior temporal gyrus.

The ventral pathway shows phonological representations to the lexical or conceptual representations, which is the meaning of the words. The second section of the dual stream model is the dorsal pathway. This pathway includes the sylvian parietotemporal, inferior frontal gyrus, anterior insula, and premotor cortex.

Its primary function is to take the sensory or phonological stimuli and transfer it into an articulatory-motor representation formation of speech. There are two different kinds of aphasic patients: expressive aphasia also known as Broca's aphasia and receptive aphasia also known as Wernicke's aphasia. There are three distinctive dimensions to phonetics: manner of articulation, place of articulation, and voicing. Expressive aphasia : Patients who suffer from this condition typically have lesions on their left inferior frontal cortex.

These patients are described with having severe syntactical deficits, which means that they have extreme difficulty in forming sentences correctly. Expressive aphasic patients suffer from more regular rule governed principles in forming sentences, which is closely related to Alzheimer patients.

For instance instead of saying the red ball bounced, both of these patients would say bounced ball the red. This is just one example of what a person might say; there are of course many possibilities. Receptive aphasia : The patients suffer from lesions or damage located in the left temporoparietal lobe. Receptive Aphasic patients mostly suffer from lexical-semantic difficulties, but also have difficulties in comprehension tasks.

Though they have difficulty saying things or describing things, these people showed that they could do well in online comprehension tasks. This is closely related to Parkinson's disease because both of the diseases have trouble in distinguishing irregular verbs. For instance, a person suffering from expressive aphasia or Parkinson's disease would say "the dog goed home" instead of "the dog went home". This disease attacks the brain and makes the patients unable to stop shaking. The effects could be difficulty in walking, communicating, or functioning.

Over time the symptoms go from mild to severe, which can cause extreme difficulties in a person's life. Many psychologists relate Parkinson's disease to progressive nonfluent aphasia , which would cause a person to have comprehension deficits and being able to recognize irregular verbs. Agnosia is "the loss or diminution of the ability to recognize familiar objects or stimuli usually as a result of brain damage".

Speech agnosia : Pure word deafness, or speech agnosia, is an impairment in which a person maintains the ability to hear, produce speech, and even read speech, yet they are unable to understand or properly perceive speech.

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These patients seem to have all of the skills necessary in order to properly process speech, yet they appear to have no experience associated with speech stimuli. Patients have reported, "I can hear you talking, but I can't translate it". Phonagnosia : Phonagnosia is associated with the inability to recognize any familiar voices. In these cases, speech stimuli can be heard and even understood but the association of the speech to a certain voice is lost. This can be due to "abnormal processing of complex vocal properties timbre, articulation, and prosody—elements that distinguish an individual voice".

A group of psychologists conducted a study to test the McGurk effect with aphasia patients and speech reading. Then after they completed the first part of the experiment, the experimenters taught the aphasic patients to speech read, which is the ability to read lips. The experimenters then conducted the same test and found that the people still had more of an advantage of audio only over visual only, but they also found that the subjects did better in audio-visual than audio alone. The patients also did improve their place of articulation and their manner of articulation.