Motor theory of speech perception

<h2 id="origins-and-development">Origins and development</h2>

The hypothesis has its origins in research using <a href="/facts/Pattern_playback/FCn4fxS1">pattern playback</a> to create reading machines for the <a href="/facts/Blindness/sFuqKj3n">blind</a> that would substitute sounds for orthographic letters.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> This led to a close examination of how spoken sounds correspond to the acoustic <a href="/facts/Spectrogram/Jt8zHI0H">spectrogram</a> of them as a sequence of auditory sounds. This found that successive <a href="/facts/Consonant/81X2SGS6">consonants</a> and <a href="/facts/Vowel/BNhCeFtH">vowels</a> overlap in time with one another (a phenomenon known as <a href="/facts/Coarticulation/gDbw2dVR">coarticulation</a>).<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a><a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> This suggested that speech is not heard like an acoustic "alphabet" or "cipher," but as a "code" of overlapping speech gestures.

<h3>Associationist approach</h3>
Initially, the theory was associationist: infants mimic the speech they hear and that this leads to <a href="/facts/Behaviorism/8DHljqrh">behavioristic</a> associations between articulation and its sensory consequences. Later, this overt mimicry would be short-circuited and become speech perception.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> This aspect of the theory was dropped, however, with the discovery that prelinguistic <a href="/facts/Infant/gaEL1QL4">infants</a> could already detect most of the phonetic contrasts used to separate different speech sounds.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>

<h3>Cognitivist approach</h3>
The behavioristic approach was replaced by a <a href="/facts/Cognitivism_(psychology)/5KQRDrsS">cognitivist</a> one in which there was a <a href="/facts/Language_module/8w9Kk1f0">speech module</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> The module detected speech in terms of hidden <a href="/facts/Distal_object/xLK0n2q4">distal objects</a> rather than at the proximal or immediate level of their input. The evidence for this was the research finding that speech processing was special such as <a href="/facts/Duplex_perception/Np9qozhC">duplex perception</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>

<h3>Changing distal objects</h3>
Initially, speech perception was assumed to link to speech objects that were both

<ul><li>the invariant movements of speech articulators<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a></li>
<li>the invariant motor commands sent to muscles to move the vocal tract articulators<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a></li></ul>
This was later revised to include the phonetic gestures rather than motor commands,<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a> and then the gestures intended by the speaker at a prevocal, linguistic level, rather than actual movements.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a>

<h3>Modern revision</h3>
The "speech is special" claim has been dropped,<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a> as it was found that speech perception could occur for nonspeech sounds (for example, slamming doors for <a href="/facts/Duplex_perception/Np9qozhC">duplex perception</a>).<a class="footnote-ref" id="fnref:21" href="#fn:21">21</a>

<h3>Mirror neurons</h3>
The discovery of <a href="/facts/Mirror_neuron/xfbp6k0S">mirror neurons</a> has led to renewed interest in the motor theory of speech perception, and the theory still has its advocates,<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a> although there are also critics.<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a>

<h2 id="support">Support</h2>
<h3>Nonauditory gesture information</h3>
If speech is identified in terms of how it is physically made, then nonauditory information should be incorporated into speech <a href="/facts/Percept/xLK0n2q4">percepts</a> even if it is still <a href="/facts/Subjectivity/XVvftgCU">subjectively</a> heard as "sounds". This is, in fact, the case.

<ul><li>The <a href="/facts/McGurk_effect/aP6opabm">McGurk effect</a> shows that seeing the production of a spoken <a href="/facts/Syllable/Ylt7pyJe">syllable</a> that differs from an auditory cue synchronized with it affects the perception of the auditory one. In other words, if someone hears "ba" but sees a video of someone pronouncing "ga", what they hear is different—some people believe they hear "da".</li>
<li>People find it easier to hear speech in <a href="/facts/Noise/6ueEsAG0">noise</a> if they can see the speaker.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a></li>
<li>People can hear syllables better when their production can be felt <a href="/facts/Haptic_communication/IkXD4hHr">haptically</a>.<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a></li></ul>
<h3>Categorical perception</h3>
Using a <a href="/facts/Speech_synthesizer/ceTczTv9">speech synthesizer</a>, speech sounds can be varied in <a href="/facts/Place_of_articulation/exCzaxeF">place of articulation</a> along a continuum from /bɑ/ to /dɑ/ to /ɡɑ/, or in <a href="/facts/Voice_onset_time/z2g14KQx">voice onset time</a> on a continuum from /dɑ/ to /tɑ/ (for example). When listeners are asked to discriminate between two different sounds, they perceive sounds as belonging to discrete categories, even though the sounds vary continuously. In other words, 10 sounds (with the sound on one extreme being /dɑ/ and the sound on the other extreme being /tɑ/, and the ones in the middle varying on a scale) may all be acoustically different from one another, but the listener will hear all of them as either /dɑ/ or /tɑ/. Likewise, the English consonant /d/ may vary in its acoustic details across different phonetic contexts (the /d/ in /du/ does not technically sound the same as the one in /di/, for example), but all /d/'s as perceived by a listener fall within one category (voiced alveolar plosive) and that is because "linguistic representations are abstract, canonical, phonetic segments or the gestures that underlie these segments."<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a> This suggests that humans identify speech using <a href="/facts/Categorical_perception/pY6dxUnT">categorical perception</a>, and thus that a specialized module, such as that proposed by the motor theory of speech perception, may be on the right track.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a>

<h3>Speech imitation</h3>
If people can hear the gestures in speech, then the imitation of speech should be very fast, as in when words are repeated that are heard in <a href="/facts/Headphone/hLd5e4Kv">headphones</a> as in <a href="/facts/Speech_shadowing/qve5j8TU">speech shadowing</a>.<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a> People can repeat heard syllables more quickly than they would be able to produce them normally.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a>

<h3>Speech production</h3>
<ul><li>Hearing speech activates vocal tract muscles,<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a> and the <a href="/facts/Motor_cortex/oQ2w0g3S">motor cortex</a><a class="footnote-ref" id="fnref:31" href="#fn:31">31</a> and <a href="/facts/Premotor_cortex/M3H1DWnM">premotor cortex</a>.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a> The integration of auditory and visual input in speech perception also involves such areas.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a></li>
<li>Disrupting the premotor cortex disrupts the perception of speech units such as <a href="/facts/Plosives/okC9pztc">plosives</a>.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a></li>
<li>The activation of the motor areas occurs in terms of the phonemic features which link with the vocal track articulators that create speech gestures.<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a></li>
<li>The perception of a speech sound is aided by pre-emptively stimulating the motor representation of the articulators responsible for its pronunciation .<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a></li>
<li>Auditory and motor cortical coupling is restricted to a specific range of neuronal firing frequency.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a></li></ul>
<h3>Perception-action meshing</h3>
Evidence exists that perception and production are generally coupled in the motor system. This is supported by the existence of <a href="/facts/Mirror_neuron/xfbp6k0S">mirror neurons</a> that are activated both by seeing (or hearing) an action and when that action is carried out.<a class="footnote-ref" id="fnref:38" href="#fn:38">38</a> Another source of evidence is that for <a href="/facts/Common_coding_theory/8d0fg2cu">common coding theory</a> between the representations used for perception and action.<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a>

<h2 id="criticisms">Criticisms</h2>
The motor theory of speech perception is not widely held in the field of speech perception, though it is more popular in other fields, such as <a href="/facts/Theoretical_linguistics/15wbqaY1">theoretical linguistics</a>. As three of its advocates have noted, "it has few proponents within the field of speech perception, and many authors cite it primarily to offer critical commentary".<a class="footnote-ref" id="fnref:40" href="#fn:40">40</a>p. 361 Several critiques of it exist.<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a>
<a class="footnote-ref" id="fnref:42" href="#fn:42">42</a>

<h3>Multiple sources</h3>
Speech perception is affected by nonproduction sources of information, such as context. Individual words are hard to understand in isolation but easy when heard in sentence context. It therefore seems that speech perception uses multiple sources that are integrated together in an optimal way.<a class="footnote-ref" id="fnref:43" href="#fn:43">43</a>

<h3>Production</h3>
The motor theory of speech perception would predict that speech motor abilities in infants predict their speech perception abilities, but in actuality it is the other way around.<a class="footnote-ref" id="fnref:44" href="#fn:44">44</a> It would also predict that defects in speech production would impair speech perception, but they do not.<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a> However, this only affects the first and already superseded behaviorist version of the theory, where infants were supposed to learn all production-perception patterns by imitation early in childhood. This is no longer the mainstream view of motor-speech theorists.

<h3>Speech module</h3>
Several sources of evidence for a specialized speech module have failed to be supported. 

<ul><li><a href="/facts/Duplex_perception/Np9qozhC">Duplex perception</a> can be observed with door slams.<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a></li>
<li>The <a href="/facts/McGurk_effect/aP6opabm">McGurk effect</a> can also be achieved with nonlinguistic stimuli, such as showing someone a video of a basketball bouncing but playing the sound of a ping-pong ball bouncing.</li>
<li>As for <a href="/facts/Categorical_perception/pY6dxUnT">categorical perception</a>, listeners can be sensitive to acoustic differences within single phonetic categories.</li></ul>
As a result, this part of the theory has been dropped by some researchers.<a class="footnote-ref" id="fnref:47" href="#fn:47">47</a>

<h3>Sublexical tasks</h3>
The evidence provided for the motor theory of speech perception is limited to tasks such as syllable discrimination that use speech units not full spoken words or spoken sentences. As a result, "speech perception is sometimes interpreted as referring to the perception of speech at the sublexical level. However, the ultimate goal of these studies is presumably to understand the neural processes supporting the ability to process speech sounds under ecologically valid conditions, that is, situations in which successful speech sound processing ultimately leads to contact with the mental lexicon and auditory comprehension."<a class="footnote-ref" id="fnref:48" href="#fn:48">48</a> This however creates the problem of " a tenuous connection to their implicit target of investigation, speech recognition".<a class="footnote-ref" id="fnref:49" href="#fn:49">49</a>

<h2 id="birds">Birds</h2>
It has been suggested that <a href="/facts/Bird/RJF6zwLP">birds</a> also hear each other's <a href="/facts/Bird_song/5A0oHvHo">bird song</a> in terms of vocal gestures.<a class="footnote-ref" id="fnref:50" href="#fn:50">50</a>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Cohort_model/XbEE7woK">Cohort model</a></li>
<li><a href="/facts/Speech_recognition/z7S7Pgk6">Speech recognition</a></li>
<li><a href="/facts/Auditory_phonetics/ej9p582E">Auditory phonetics</a></li>
<li><a href="/facts/Trace_(psycholinguistics)/oJWjNGDM">Trace (psycholinguistics)</a></li>
<li><a href="/facts/Haskins_Laboratories/BSyxGGKC">Haskins Laboratories</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.haskins.yale.edu/">Haskins Laboratories</a> <a href="https://web.archive.org/web/20190509003749/http://www.haskins.yale.edu/">Archived</a> 2019-05-09 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li>
<li><a href="http://www.haskins.yale.edu/pubs.html">Source of pdfs upon the motor theory of speech perception</a> <a href="https://web.archive.org/web/20090504100143/http://www.haskins.yale.edu/pubs.html">Archived</a> 2009-05-04 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Liberman, A. M.; Cooper, F. S.; Shankweiler, D. P.; Studdert-Kennedy, M. (1967). "Perception of the speech code". Psychological Review. 74 (6): 431–461. doi:10.1037/h0020279. PMID 4170865. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Liberman, A. M.; Mattingly, I. G. (1985). "The motor theory of speech perception revised". Cognition. 21 (1): 1–36. CiteSeerX 10.1.1.330.220. doi:10.1016/0010-0277(85)90021-6. PMID 4075760. S2CID 112932. <a href="/wiki/CiteSeerX_(identifier)" target="_blank">/wiki/CiteSeerX_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Liberman, A. M.; Mattingly, I. G. (1989). "A specialization for speech perception". Science. 243 (4890): 489–494. Bibcode:1989Sci...243..489L. doi:10.1126/science.2643163. PMID 2643163. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Liberman, A. M.; Whalen, D. H. (2000). "On the relation of speech to language". Trends in Cognitive Sciences. 4 (5): 187–196. doi:10.1016/S1364-6613(00)01471-6. PMID 10782105. S2CID 12252728. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Galantucci, B.; Fowler, C. A.; Turvey, M. T. (2006). "The motor theory of speech perception reviewed". Psychonomic Bulletin & Review. 13 (3): 361–377. doi:10.3758/bf03193857. PMC 2746041. PMID 17048719. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Galantucci, B.; Fowler, C. A.; Turvey, M. T. (2006). "The motor theory of speech perception reviewed". Psychonomic Bulletin & Review. 13 (3): 361–377. doi:10.3758/bf03193857. PMC 2746041. PMID 17048719. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Galantucci, B.; Fowler, C. A.; Turvey, M. T. (2006). "The motor theory of speech perception reviewed". Psychonomic Bulletin & Review. 13 (3): 361–377. doi:10.3758/bf03193857. PMC 2746041. PMID 17048719. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Liberman, A. M. (1996). Speech: A special code. Cambridge, MA: MIT Press. ISBN 978-0-262-12192-7 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Liberman, A. M.; Delattre, P.; Cooper, F. S. (1952). "The role of selected stimulus-variables in the perception of the unvoiced stop consonants". The American Journal of Psychology. 65 (4): 497–516. doi:10.2307/1418032. JSTOR 1418032. PMID 12996688. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Liberman, A. M.; Delattre, P. C.; Cooper, F. S.; Gerstman, L. J. (1954). "The role of consonant-vowel transitions in the perception of the stop and nasal consonants". Psychological Monographs: General and Applied. 68 (8): 1–13. doi:10.1037/h0093673. PDF Archived 2016-03-03 at the Wayback Machine <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Fowler, C. A.; Saltzman, E. (1993). "Coordination and coarticulation in speech production". Language and Speech. 36 ( Pt 2-3) (2–3): 171–195. doi:10.1177/002383099303600304. PMID 8277807. S2CID 7199908. PDF <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Liberman, A. M.; Delattre, P. C.; Cooper, F. S.; Gerstman, L. J. (1954). "The role of consonant-vowel transitions in the perception of the stop and nasal consonants". Psychological Monographs: General and Applied. 68 (8): 1–13. doi:10.1037/h0093673. PDF Archived 2016-03-03 at the Wayback Machine <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Liberman, A. M.; Cooper, F. S.; Shankweiler, D. P.; Studdert-Kennedy, M. (1967). "Perception of the speech code". Psychological Review. 74 (6): 431–461. doi:10.1037/h0020279. PMID 4170865. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Liberman, A. M.; Cooper, F. S.; Shankweiler, D. P.; Studdert-Kennedy, M. (1967). "Perception of the speech code". Psychological Review. 74 (6): 431–461. doi:10.1037/h0020279. PMID 4170865. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Liberman, A. M.; Isenberg, D.; Rakerd, B. (1981). "Duplex perception of cues for stop consonants: Evidence for a phonetic mode". Perception & Psychophysics. 30 (2): 133–143. doi:10.3758/bf03204471. PMID 7301513. <a href="https://doi.org/10.3758%2Fbf03204471" target="_blank">https://doi.org/10.3758%2Fbf03204471</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Liberman, A. M.; Delattre, P. C.; Cooper, F. S.; Gerstman, L. J. (1954). "The role of consonant-vowel transitions in the perception of the stop and nasal consonants". Psychological Monographs: General and Applied. 68 (8): 1–13. doi:10.1037/h0093673. PDF Archived 2016-03-03 at the Wayback Machine <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Liberman, A. M. (1970). "The grammars of speech and language" (PDF). Cognitive Psychology. 1 (4): 301–323. doi:10.1016/0010-0285(70)90018-6. Archived from the original (PDF) on 2015-12-31. Retrieved 2009-06-02. <a href="https://web.archive.org/web/20151231094548/http://www.haskins.yale.edu/Reprints/HL0099.pdf" target="_blank">https://web.archive.org/web/20151231094548/http://www.haskins.yale.edu/Reprints/HL0099.pdf</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">Liberman, A. M.; Cooper, F. S.; Shankweiler, D. P.; Studdert-Kennedy, M. (1967). "Perception of the speech code". Psychological Review. 74 (6): 431–461. doi:10.1037/h0020279. PMID 4170865. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Liberman, A. M.; Mattingly, I. G. (1985). "The motor theory of speech perception revised" (PDF). Cognition. 21 (1): 1–36. CiteSeerX 10.1.1.330.220. doi:10.1016/0010-0277(85)90021-6. PMID 4075760. S2CID 112932. Archived from the original (PDF) on 2021-04-15. Retrieved 2009-06-02. <a href="https://web.archive.org/web/20210415025120/http://www.haskins.yale.edu/Reprints/HL0519.pdf" target="_blank">https://web.archive.org/web/20210415025120/http://www.haskins.yale.edu/Reprints/HL0519.pdf</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Galantucci, B.; Fowler, C. A.; Turvey, M. T. (2006). "The motor theory of speech perception reviewed". Psychonomic Bulletin & Review. 13 (3): 361–377. doi:10.3758/bf03193857. PMC 2746041. PMID 17048719. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Fowler, C. A.; Rosenblum, L. D. (1990). "Duplex perception: A comparison of monosyllables and slamming doors". Journal of Experimental Psychology. Human Perception and Performance. 16 (4): 742–754. doi:10.1037/0096-1523.16.4.742. PMID 2148589. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Galantucci, B.; Fowler, C. A.; Turvey, M. T. (2006). "The motor theory of speech perception reviewed". Psychonomic Bulletin & Review. 13 (3): 361–377. doi:10.3758/bf03193857. PMC 2746041. PMID 17048719. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746041</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Massaro, D. W.; Chen, T. H. (2008). "The motor theory of speech perception revisited". Psychonomic Bulletin & Review. 15 (2): 453–457, discussion 457–62. doi:10.3758/pbr.15.2.453. PMID 18488668. S2CID 9266946. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">MacLeod, A.; Summerfield, Q. (1987). "Quantifying the contribution of vision to speech perception in noise". British Journal of Audiology. 21 (2): 131–141. doi:10.3109/03005368709077786. PMID 3594015. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
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Motor theory of speech perception open-in-new

Motor theory of speech perception