Teaching and Learning Guide for: Mirror Neurons, the Motor System, and Language – From the Motor Theory to Embodied Cognition and Beyond
Authors: Venezia Jonathan H. , Hickok Gregory 
Published: August 2010

This guide accompanies the following article(s): Jonathan H. Venezia and Gregory Hickok, ‘Mirror Neurons, the Motor System, and Language – From the Motor Theory to Embodied Cognition and Beyond’, Language and Linguistics Compass 3 (2009) pp. 1403–1416. DOI: 10.1111/j.1749‐818x.2009.00169.x

Author’s Introduction

Mirror neurons have been invoked to explain a huge number of psychological phenomena including action understanding, imitation, empathy, theory of mind, consciousness, and many more. Indeed, mirror neurons have provided a physiological substrate for motor theories of cognition, which appear elegant, simple, and easy to accept at first glance. A computationally specific motor theory predating the discovery of mirror neurons is the motor theory of speech perception (MTSP). Decades after MTSP had fallen out of favor among speech scientists it enjoyed a renaissance in the age of mirror neurons. However, mirror neurons have done nothing to overcome the arguments leveled against MTSP years ago. We offer this guide as a reminder that we must be cautious when interpreting and drawing inference from new discoveries that seem to explain complex cognitive processes in attractively simple terms. Speech and language draw upon a myriad of sophisticated cognitive processes, distributed throughout cortex, which combine to solve extraordinarily difficult computational problems. However, there is a large body of evidence suggesting that speech perception, specifically, relies heavily on auditory representations in the superior temporal lobes, and we must interpret motor contributions to speech perception in light of this fact.

Key Readings

1. Rizzolatti & Craighero. 2004. The mirror‐neuron system. Annual Review of Neuroscience 27. 169–92.

doi:10.1146/annurev.neuro.27.070203.144230

This highly cited review provides appropriate background and motivation for a discussion on motor system contributions in speech perception, as mirror neurons have fueled the emergence of modern motor theories of cognition. The article covers the discovery and basic properties of mirror neurons in the macaque monkey, and outlines a proposed role for mirror neurons in action understanding. Homologies between monkeys and humans are discussed, and neurophysiological evidence for a human mirror neuron system is reviewed. Finally, evidence concerning a role for the human mirror system in speech is highlighted, including a proposed evolutionary connection between gestural communication and verbal speech. Thus, we recommend this article as a comprehensive introduction to the relation between mirror neurons, action, and speech.

2. Hickok, G. 2009. Eight problems for the mirror neuron theory of action understanding. Journal of Cognitive Neuroscience 21. 1229–43.

doi:10.1162/jocn.2009.21189

We recommend this article as an alternative to the typical interpretation of evidence concerning the mirror neuron theory of action understanding. Critical review of the relevant neurophysiological data in monkeys and humans is offered. Also, a discussion on the relation between mirror neurons and the motor theory of speech is contained. We feel that close examination and careful scrutiny of the mirror neuron theory of action understanding are of critical importance given the purported link between action‐based gestural communication and verbal speech.

3. Galantucci, Fowler, & Turvey. 2006. The motor theory of speech perception reviewed. Psychonomic Bulletin & Review 13. 361–77.

http://pbr.psychonomic‐journals.org/content/13/3/361.abstract

This is an excellent recent review of the motor theory of speech perception. The major tenets of the motor theory are concisely summarized along with the original findings that have motivated the theory throughout its several iterations. The evidence for each central claim is reviewed and evaluated, and a thorough discussion of the link between perceptual and motor speech systems is offered, including a section on neural evidence that gives mention to mirror neurons.

4. Lotto, A., Hickok, G. & Holt, L. 2009. Reflections on mirror neurons and speech perception. Trends in Cognitive Sciences 13. 110–4.

doi:10.1016/j.tics.2008.11.008

Lotto et al. provide a thorough discussion of the relation between mirror neurons and the motor theory of speech perception. The article establishes that mirror neurons do nothing to solve the computational problems that originally gave rise to the motor theory, nor do they account for decades‐old behavioral and neuropsychological challenges. In fact, the authors argue that the involvement of mirror neurons in speech perception would contradict the central tenets of the theory.

5. Iacoboni, M. 2008. The role of premotor cortex in speech perception: evidence from fMRI and rTMS. Journal of Physiology – Paris 102. 31–34.

doi:10.1016/j.jphysparis.2008.03.003

Iacoboni provides a nice review of the recent evidence for motor system involvement in speech perception, including a historical perspective linking this evidence with the motor theory of speech and with mirror neurons.

6. Wilson, S. & Iacoboni, M. 2004. Listening to speech activates motor areas involved in speech production. Nature Neuroscience 7. 701–2.

doi:10.1038/nn1263

This fMRI study provides unequivocal evidence that passively listening to speech sounds activates the speech motor system. Motor theorists often cite the results as evidence that speech perception is grounded in motor circuits, though extensive activation was observed in superior temporal cortex as well. Moreover, activation measured during speech production extended into regions of premotor cortex that were not active during perception of the same sounds. Indeed, even non‐speech stimuli (noise bursts) activated a premotor region that was active during speech perception and production. Should we conclude that perception of noise bursts, which our motor system cannot produce, is grounded in motor circuits? Thus, we recommend this study as a reminder that we must be careful in interpreting neuroimaging results.

7. Meister, I., Wilson, M., Deblieck, C., Wu, A., & Iacoboni, M. 2007. The essential role of premotor cortex in speech perception. Current Biology 19. 1692–6.

doi:10.1016/j.cub.2007.08.064

This repetitive transcranial magnetic stimulation (rTMS) study demonstrates that acute disruption of ventral premotor cortex produces a reduction in performance on a syllable identification task. As is obvious in the title, this finding is taken as evidence that the motor system is necessary for speech perception. However, syllable identification requires several cognitive processes including auditory‐perceptual categorization, cross‐modal comparison, and response selection. The latter processes may engage frontal‐motor systems preferentially and do not reflect speech perception per se. Further, the stimuli in this study were embedded in noise, and discrimination in noise may tax frontal systems preferentially. Again, we stress the importance of careful interpretation when the data appear straightforward.

8. Sato, M., Tremblay, P., & Gracco, V. 2009. A mediating role of the premotor cortex in phoneme segmentation. Brain and Language 111. 1–7.

doi:10.1016/j.bandl.2009.03.002

In this study, rTMS applied to ventral premotor cortex produces no effect on phoneme identification or syllable discrimination under normal listening conditions (without background noise). However, when phoneme discrimination required phonemic segmentation, a significant slowing of the behavioral response was observed (though accuracy was not affected). It is likely that phoneme discrimination relies on frontal circuits involved in the maintenance and manipulation of information in verbal working memory. Thus, the motor system may play a modulatory role in speech perception tasks that require complex operations, even under normal listening conditions. Indeed, what exactly are normal listening conditions? Perhaps a modulatory role for the motor system is an important component in everyday speech comprehension, but it is clearly not necessary to perform this function.

9. Binder, J., Liebenthal, E., Possing, E., Medler, D., & Ward, B. 2004. Neural correlates of sensory and decision processes in auditory object identification. Nature Neuroscience 7. 295–301.

doi:10.1038/nn1198

Here, fMRI activation during a syllable discrimination task is correlated with behavioral performance. The results indicate that percent correct correlates with activation in bilateral superior temporal cortices, while reaction time correlates with activation in bilateral inferior frontal cortices. The authors argue that percent correct (which they corrected for response bias) reflects perceptual processing, while reaction time reflects the decision‐making component of the task. While the distinction may not be quite so clear, this study seems to support the notion that frontal circuits involved in articulation may preferentially influence response selection components of tasks commonly used to assess speech perception ability.

10. Hickok, G. & Poeppel, D. 2007. The cortical organization of speech processing. Nature Reviews Neuroscience 8. 393–402.

doi:10.1038/nrn2113

This critical review of speech perception research proposes a dual stream model to account for findings spanning the behavioral, developmental, neuropsychological, and imaging literatures. Task effects often observed in speech perception studies are addressed, as is motor system activation during speech perception. The dual stream model proposes a sensory theory of speech production, whereby auditory speech information from the temporal lobe is used to guide articulation mediated by frontal‐motor systems. We believe that this model best accounts for the sensory‐motor links observed in speech perception.

Online Resources

The Talking Brains Blog:

http://talkingbrains.blogspot.com/

Sample Outline

Week 1: Introduction to mirror neurons; the mirror neuron theory of action understanding

The discovery of mirror neurons has motivated much of the recent interest in exploring the role of the motor system in speech perception. This lesson should focus on introducing mirror neurons, their basic properties, and the most widely cited explanation of mirror neuron function – the mirror neuron theory of action understanding. The notion that we understand action by generating an internal representation of how we produce that action ourselves bears striking resemblance to motor theories of cognition, including the motor theory of speech perception. This week’s readings explore the evidence in support of the mirror neuron theory of action understanding, as well as several empirical weaknesses and inconsistencies that emerge from the mirror neuron literature.

Suggested Readings

Rizzolatti & Craighero. 2004. The mirror‐neuron system. Annual Review of Neuroscience 27. 169–92.

Hickok, G. 2009. Eight problems for the mirror neuron theory of action understanding. Journal of Cognitive Neuroscience 21. 1229–43.

Week 2: Mirror neurons and the motor theory of speech perception

The motor theory of speech perception was originally devised to account for the problem of acoustic invariance in the speech signal. However, major empirical challenges from several domains including development, neuropsychology, and animal research demonstrated that the motor theory failed to provide a realistic solution to this problem. Despite several attempts to re‐characterize the role of the motor system in speech perception, the original motor theory has received much attention in wake of the recent discovery of mirror neurons. Thus, this week’s readings introduce the motor theory, its relationship to mirror neurons, and several arguments that demonstrate why the motor theory fails to solve the computational problems that motivated its construction, and why mirror neurons do nothing to change this state of affairs.

Suggested Readings

Galantucci, Fowler, & Turvey. 2006. The motor theory of speech perception reviewed. Psychonomic Bulletin & Review 13. 361–77.

Lotto, A., Hickok, G. & Holt, L. 2009. Reflections on mirror neurons and speech perception. Trends in Cognitive Sciences 13. 110–4.

Week 3: Neuropsychology and the motor theory of speech perception

A straightforward and logically consistent prediction arises from the motor theory of speech perception: damage to the cortical speech‐motor system should result in dramatic speech comprehension deficits. Previous weeks’ readings suggest that this is not the case. This week will highlight two papers demonstrating that speech comprehension remains intact following long‐term and/or acute disruption of the speech‐motor system (Hickok et al.; Moineau et al.). We have also included a paper that is often interpreted as neuropsychological evidence supporting a role for frontal motor systems in speech perception (Basso et al.).

Suggested Readings

Basso, A., Casati, G. & Vignolo, L.A. 1977. Phonemic identification deficit in aphasia. Cortex 13. 85–95.

Hickok, G et al. 2004. Bilateral capacity for speech sound processing in auditory comprehension: Evidence from Wada procedures. Brain and Language 107. 179–84.

Moineau, S., Dronkers, N.F. & Bates, E. 2005. Exploring the processing continuum of single‐word comprehension in aphasia. Journal of Speech, Language and Hearing Research 48. 884–96.

Week 4: Neuroimaging, the motor system, and speech perception

Though it has been clearly established that the motor system is not necessary for speech perception, recent functional neuroimaging evidence indicates that speech‐motor areas of cortex are active during speech listening. This evidence has provided a boon to recent efforts aimed at reviving the motor theory of speech perception. However, alternative explanations of motor activation during speech listening have been proposed. This week’s readings address both positions.

Suggested Readings

Buchsbaum, B., G. Hickok & C. Humphries. 2001. Role of left posterior superior temporal gyrus in phonological processing for speech perception and production. Cognitive Science 25. 663–78.

Pulvermuller, F., M. Huss, F. Kherif, F. Moscoso del Prado Martin, O. Hauk & Y. Shtyrov. 2006. Motor cortex maps articulatory features of sounds. Proceedings of the National Academy of Sciences of the United States of America 103. 7865–70.

Wilson, S. & Iacoboni, M. 2004. Listening to speech activates motor areas involved in speech production. Nature Neuroscience 7. 701–2.

Week 5: Transcranial Magnetic Stimulation, the motor system, and speech perception

Recent efforts at establishing a necessary role for the motor system in speech perception have employed transcranial magnetic stimulation (TMS) as a primary investigative tool. Several TMS studies provide evidence that the motor system contributes to some elements of low‐level speech perception tasks. However, these effects are observed when the speech stimuli are embedded in noise, and seem to disappear for clear speech. This week’s readings present the strongest evidence from the TMS literature.

Suggested Readings

D’Ausilio, A., F. Pulvermuller, P. Salmas, I. Bufalari, C. Begliomini & L. Fadiga. 2009. The motor somatotopy of speech perception. Current Biology 19. 381–5.

Meister, I. G., S. M. Wilson, C. Deblieck, A. D. Wu & M. Iacoboni. 2007. The essential role of premotor cortex in speech perception. Current Biology 17. 1692–6.

Sato, M., Tremblay, P. & Gracco, V. 2009. A mediating role of the premotor cortex in phoneme segmentation. Brain and Language 111. 1–7.

Week 6: Syllable identification vs. syllable discrimination; syllable vs. word comprehension

Last week’s readings focused on low‐level speech perception tasks involving comparison of sublexical speech units. In particular, D’Ausilio et al. (2009) and Meister et al. (2007) used a phoneme/syllable identification task, which involves perceiving a sound and matching the auditory representation of that sound to the appropriate visual representation (labeled keypad) or internal representation (memorized keypad mapping). Such cross‐modal mapping is likely to engage frontal executive systems, and it is difficult to disentangle motor contributions to executive function from motor contributions to perception. Syllable discrimination, on the other hand, involves an internal comparison of two sounds and can be performed entirely within the auditory modality. However, it remains difficult to disentangle perceptual from decision processes in syllable discrimination where controlled comparison and an overt response are required. The first of this week’s readings attempts to correlate perceptual and decision processes, respectively, with different brain regions using a syllable discrimination paradigm in fMRI. The second reading demonstrates that syllable discrimination dissociates from whole‐word comprehension in patients with lesions that produce language comprehension deficits.

Suggested Readings

Binder, J., Liebenthal, E., Possing, E., Medler, D. & Ward, B. 2004. Neural Correlates of sensory and decision processes in auditory object identification. Nature Neuroscience 7. 295–301.

Miceli, G., G. Gainotti, C. Caltagirone & C. Masullo. 1980. Some aspects of phonological impairment in aphasia. Brain and Language 11. 159–69.

Week 7: What is the motor system doing during speech perception? Forward models and analysis by synthesis

Previous weeks should have established that the motor system is not necessary for speech perception and that motor system contributions during speech listening may reflect higher‐order decision processes. Nonetheless, stimulation of the motor system affects speech perception under certain (typically adverse) conditions. Thus, the question, ‘what is the motor system doing during speech perception?’. This week we focus on two related mechanisms that have been postulated in the literature: forward models and analysis by synthesis.

Suggested Readings

Poeppel, D., W. J. Idsardi & V. van Wassenhove 2008. Speech perception at the interface of neurobiology and linguistics. Philosophical Transactions of the Royal Society of London B Biological Sciences 363. 1071–86.

Skipper, J.I., van Wassenhove, V., Nusbaum, H.C. & Small, S.L. 2007. Hearing lips and seeing voices: How cortical areas supporting speech production mediate audiovisual speech perception. Cerebral Cortex 17. 2387–99.

Week 8: A sensory theory of speech production: the Dual Stream Model of Speech Perception

Much attention has been paid thus far to the influence of motor representations on speech perception. However, a reverse relation has been proposed that equally well explains the tight sensory‐motor links in observed in speech research. Namely, the dual stream model of speech perception proposes that auditory‐phonological representations in the bilateral superior temporal lobes are interfaced with the motor system to guide production. A feedback loop in this sensory‐motor network (the dorsal stream) is consistent with the architecture proposed in the dual stream model and may explain the neural mechanism by which motor‐speech systems are able to influence perception. This week’s readings review the dual stream model and provide a computational example of feed‐forward and feedback connections in a sensory‐motor speech network.

Suggested Readings

Guenther, F. H. 2006. Cortical interactions underlying production of speech sounds. Journal of Communication Disorders 39. 350–65.

Hickok, G. & Poeppel, D. 2007. The cortical organization of speech perception. Nature Reviews Neuroscience 8. 393–402.

Week 9: (Optional) The mirror neuron system and action word semantics

Up to this point, the exclusive focus of this guide has been on the role of the motor system in perceiving speech sounds. However, a large part of the mirror neuron literature has centered on the role of the motor system in linguistic representation of body‐centered actions (i.e., action semantics). These are logically independent claims to motor system function in language. This week we introduce the connection between mirror neurons and action word semantics, including arguments for and against a proposed role for mirror neurons in representing the meaning of action words. Some content will be a review of issues discussed in earlier weeks.

Suggested Readings

Pulvermu¨ ller, F., O. Hauk, V. Nikulin & R. Ilmoniemi. 2005. Functional links between motor and language systems. The European Journal of Neuroscience 21. 793–7.

Hickok, G (submitted). The role of mirror neurons in speech perception and action word semantics.

Week 10: (Optional) Sign language and the human mirror system

A final optional topic will be on the relation between mirror neurons and gesture‐based language, which relies on production and comprehension of manual actions. We know of a very good review on this topic that is suggested for this session’s reading.

Suggested Readings

Corina, D. & Knapp, H. 2006. Special issue: Sign language processing and the mirror neuron system. Cortex 4. 529–39.

Project Idea: Perform a literature search and identify a single piece of original research that argues for or against one of the following theories: (1) The mirror neuron theory of action understanding, (2) The motor theory of speech perception, (3) The mirror neuron theory of action word semantics. Summarize both positions, including background literature, and clearly state why you think the chosen article supports one position over the other. Optional: present your paper as a slide discussion in front of the class.

Focus Questions

 What practical issues arise when trying to connect the non‐human primate mirror neuron literature with research on the human mirror system?

 Describe the computational problem that arises from interpreting a speech signal with acoustic lack of invariance. How does the motor theory of speech perception propose to solve this problem?

 What are some weaknesses in using clinical (lesion) evidence to argue against a theory that makes specific claims to localization of function in the brain?

 Why is it difficult to make causal claims about localized brain function based on functional neuroimaging evidence?

 Why is the distinction between perceptual and decision processes in speech perception important? Are decision processes a fundamental part of interpreting the speech signal?

 Why is the distinction between syllable‐ and word‐level comprehension important in the discussion on the motor system and speech perception?

 What developmental function might depend on integrating auditory information with the motor system?

 How is the discussion on mirror neurons and speech perception (i.e., perception of speech sounds) logically independent from the discussion on mirror neurons and action word semantics? Why is this distinction important?