Academic journal article Psychonomic Bulletin & Review

The Motor Theory of Speech Perception Reviewed

Academic journal article Psychonomic Bulletin & Review

The Motor Theory of Speech Perception Reviewed

Article excerpt

More than 50 years after the appearance of the motor theory of speech perception, it is timely to evaluate its three main claims that (1) speech processing is special, (2) perceiving speech is perceiving gestures, and (3) the motor system is recruited for perceiving speech. We argue that to the extent that it can be evaluated, the first claim is likely false. As for the second claim, we review findings that support it and argue that although each of these findings may be explained by alternative accounts, the claim provides a single coherent account. As for the third claim, we review findings in the literature that support it at different levels of generality and argue that the claim anticipated a theme that has become widespread in cognitive science.

The motor theory of speech perception (see, e.g., Liberman, Cooper, Shankweiler, & Studdert-Kennedy, 1967; Liberman & Mattingly, 1985) is among the most cited theories in cognitive psychology.1 However, the theory has had a mixed scientific reception. On the one hand, it has few proponents within the field of speech perception, and many authors cite it primarily to offer critical commentary (e.g., Sussman, 1989). On the other hand, it is perhaps the only theory of speech perception recognized outside the field of speech, and there, its reception has been considerably more positive (e.g., Rizzolatti & Arbib, 1998; Williams & Nottebohm, 1985).

With the deaths of Alvin Liberman and Ignatius Mattingly, the two investigators who contributed most to the development of the theory, it is timely to review its main claims in order to determine which of them should be set aside and which deserve further consideration. The three main claims of the theory are the following: (1) Speech processing is special (Liberman & Mattingly, 1989; Mattingly & Liberman, 1988); (2) perceiving speech is perceiving vocal tract gestures2 (e.g., Liberman & Mattingly, 1985); (3) speech perception involves access to the speech motor system (e.g., Liberman et al., 1967).

We will argue that the first claim is difficult to evaluate because it has several readings: ( 1 a) that speech perception is special with respect to audition, in that its objects are not the proximal acoustic patterns but the distal gestures that generated the acoustic patterns; (2a) that speech is special with respect to audition, in that it implies recruitment of the motor system in perception; and (3a) that speech is produced and processed by a piece of neural circuitry that represents a specialization in the biological sense. We will argue that unless (1a) and (2a) are interpreted very narrowly, they are disconfirmed by the available evidence and that evidence for (3a) is difficult to obtain. However, we will argue that (2) and (3), the most radical claims of the theory, should not be dismissed.

As for (2), we will review some of the evidence relevant to the claim and argue that although each piece of evidence can be individually explained by alternative accounts, these accounts differ for each piece of evidence, whereas (2) provides a single coherent account of all of the findings.

As for (3), the core claim of the theory, we will review an extensive body of evidence compatible with the claim. Our review will cover findings of perception-motor links, both at the neural and at the behavioral levels, in domains of increasing generality. We will begin with the specific domain of speech perception, then will consider the more general domain of animal communication, and finally, will move beyond communication, covering the domain of perception in general. In this context, we will review both findings that motor competence3 is accessed in perception and findings that the motor system itself is involved in perceptual tasks. Our conclusion will be that the evidence supports (3) in its most general sense.

This article has five sections. In the first section, we will describe the successive versions of the motor theory of speech perception, illustrating how the theory has undergone progressive abstraction and has progressively placed more emphasis on speech as an evolutionary adaptation special to humans. …

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