I finally mustered the courage (i.e., sufficient control over my blood pressure) to read Pulvermuller & Fadiga's recent (2010) review paper in Nature Reviews Neuroscience. But I don't want to talk about their paper -- yet. I want to discuss a paper they cite. Here is the context in which they cite it: P & F are, of course, arguing for the importance of the motor system in receptive language. After arguing correctly that sensory and motor aspects of speech must interact and proposing (controversially) that this interaction is important not only for production but for perception/understanding, they write:
We acknowledge that, apart from action-perception learning, the human brain also supports the purely perceptual learning of small vocabularies of word forms in the absence of articulation, but note that monkeys also exhibit this type of perceptual learning. Notably, children with severe neurological motor deficits that affected articulation had reduced auditory vocabularies -- that is, they understood fewer words than children with similar deficits that did not affect articulation [Bishop et al. 1990] -- a finding consistent with the importance of motor links for vocabulary learning. -Pulvermuller & Fadiga, 2010, pp. 352-353
I was not aware of the Bishop et al. paper (embarrassingly), so I had a look. I'm glad I did because it shows (i) that the ability to produce speech does not affect the ability to perceive speech sounds and (ii) let me say it again: TASK MATTERS.
Bishop, Brown, & Robson studied 48 10-18 year olds, all with cerebral palsy. 12 were congenitally anarthric (A) "never having been able to produce articulate speech", 12 were severely dysarthric (D) "with labored, and often unintelligible, speech", and 24 were control (C) subjects with cerebral palsy but with normal speech. The controls are critical because cerebral palsy is associated with a general lowering of intellectual ability. Thus, to a first approximation, group differences could be attributed to differences motor speech control. (This is not entirely true because the anarthric patients in general had more severe motor problems, e.g., there were nonambulatory unlike most control subjects which as the the authors point out could affect health and learning generally.)
In a first set of experiments subjects were tested on
1. a test of non-verbal intelligence, Raven's Matrices, to ensure good matching between groups
2. a phoneme discrimination task (yes-no, nonword syllable discrimination using minimal pair phonemic contrasts) -- Yes they used d'! Woohoo!
3. a receptive vocabulary task (British Picture Vocabulary Scale, similar to the Peabody scale)
4. a test for receptive grammar (TROG), a sentence-picture matching test.
1. Groups did not differ on the non-verbal test (they are reasonable matched)
2. Speech impaired groups (anarthric and dysarthric) performed worse than controls on the phoneme discrimination test (d' = 1.6, 1.5, ~2.5, for A, D, and C groups respectively -- there were actually two C groups that I've combined here). No difference between the speech impaired groups.
3. Vocabulary was reduced in the speech impaired groups relative to controls. Vocabulary age equivalents were: A, 8:0; D, 8:5; C, ~10. No diff between the speech impaired groups.
4. No differences between any of the groups on the receptive grammar test.
What does this mean? It suggests that the ability to speak indeed affects speech sound discrimination and is associated with vocabulary reduction (although 8 year old vocabularies are probably better than a monkeys, cf, P&V quote above), but lack of motor speech does not impair receptive grammar. The latter finding is relevant (i.e., contradictory) to another of P&V's claims, but won't be discussed here.
Before all you motor theory/mirror neuron enthusiasts start celebrating there are two important caveats regarding the discrimination test. One is the fact that despite the a complete lack of speech development, anarthric patients are nonetheless able to discriminate minimal pair phonemic contrasts better than chance (remember discrimination threshold for d' measures = 1.0), have receptive vocabularies that afford everyday communication, and have relatively good receptive grammar skills. Therefore, motor speech ability is not necessary for basic receptive speech competence.
The other caveat is Bishop et al.'s second experiment involving the same population. They worried that the nonword syllable discrimination task may unnecessarily tax phonological working memory, which is dependent on motor articulatory ability, so they used another task: subjects were presented with a picture (e.g., a boy) and then a spoken syllable (e.g., "boy" or "voy"); they were asked to decide whether the syllable correctly named the picture or whether the syllable was incorrectly pronounced and therefore did not match. The matches and mismatches represented minimal pairs. A standard syllable discrimination task (boy-voy, same or different?) was also administered for comparison.
1. The standard discrimination task replicated what was found in Experiment 1: speech impaired subjects performed worse than controls (d'=1.72 vs. 2.24, respectively; A & D were pooled in this study).
2. The picture-syllable judgment task, which involved the *same* phonemic contrasts, came out differently: no difference between speech impaired and control subjects (d'=2.52 vs. 2.59 respectively).
Bishop et al. summarize the findings nicely:
The lack of impairment on the word [picture-syllable] judgment task rules out the possibility that the speech-impaired persons are operating with a reduced system of phoneme contrasts. An alternative explanation in terms of short-term memory seems the most plausible.... It may be that if one has to retain novel, meaningless phonological information then the process is facilitated by overtly or covertly generating an articulatory representation. Indeed, some of the normal speakers in our study were observed repeating nonword pairs to themselves in the same-different task before making a judgment. This strategy would be difficult or impossible for those with dysarthria or anarthria.... p. 218.
So when measured properly the ability to perceive speech is unimpaired, relative to controls, in individuals who never developed the ability to speak. The motor speech system is not necessary for speech perception.
But what about vocabulary? Is motor speech necessary for vocabulary development? It depends on what you mean by necessary. The Bishop et al. study showed that a vocabulary of an 8 year old is achievable -- which is not bad considering that the control group achieved an average vocabulary of a 10 year old -- but still below par. Why might this be?
Drawing the work of Gathercole & Baddeley (1989) which showed a correlation between vocabulary development and phonological STM, Bishop et al. suggest that it has to do with phonological short-term memory. Learning new words requires the retention of sequences of novel phoneme strings that can be associated with meanings. If the ability to internally rehearse such strings is impaired, one might be consistently behind the curve in vocabulary development, having to rely more on external repeated exposure to new vocabulary items.
Thus, the influence of the motor speech system on receptive language ability all boils down to its role in phonological short-term memory. Basic perceptual abilities are largely unaffected by even severe disruption of the motor speech system.
Bishop DV, Brown BB, & Robson J (1990). The relationship between phoneme discrimination, speech production, and language comprehension in cerebral-palsied individuals. Journal of speech and hearing research, 33 (2), 210-9 PMID: 2359262
GATHERCOLE, S., & BADDELEY, A.D. (1989). Evaluation of the role of phonological STM in the development of vocabulary in children: A longitudinal study Journal of Memory and Language, 28 (2), 200-213 DOI: 10.1016/0749-596X(89)90044-2
Pulvermüller F, & Fadiga L (2010). Active perception: sensorimotor circuits as a cortical basis for language. Nature reviews. Neuroscience, 11 (5), 351-60 PMID: 20383203