Wednesday, August 11, 2010

Importance of phonemes in speech production

In a previous post I have questioned whether we need to explicitly represent phonemes in speech perception. Massaro and others have raised this issue in the past. Phonemes, the line of thinking goes, are only really important for production. There are linguistic arguments for this that I won't detail here. There is also well-known speech error data which shows that phoneme size units can break off and dislocate themselves. Here I want to highlight some evidence from aphasia. A reviewer of one of my papers pointed me to this study by Lindsey Nickels and David Howard.

A group of aphasics who exhibited speech production errors were asked to repeat words that varied in terms of the number of phonemes, number of syllables, or syllable complexity (defined in terms of consonant clusters). These variables are, of course, highly correlated, but the stimuli were carefully designed so that the contribution of each of these factors could be examined using logistic regression analyses.

The main result was that only number of phonemes in a word predicted correct repetition (see graph below derived from their Table 4) and once this variable was taken into account, the number of syllables or syllable complexity did not explain any additional variance.

Phonemes seem to matter in speech production. I have to say, though, that I'm not fully convinced that the others factors aren't also important.

Nickels, L., & Howard, D. (2004). Dissociating Effects of Number of Phonemes, Number of Syllables, and Syllabic Complexity on Word Production in Aphasia: It's the Number of Phonemes that Counts Cognitive Neuropsychology, 21 (1), 57-78 DOI: 10.1080/02643290342000122


Anonymous said...

It is actually known in speech production that even when "phoneme-size" units appear to "break off and dislocate themselves", closer inspection (e.g., EMG, kinematics) reveals that what APPEARED to be "phoneme-size" errors are in fact not compatible with a phoneme view (phonemes are abstract units, not production units, and thus should dislocate completely, not partially) but with a motor error view. See among others:

Mowrey RA, MacKay IR. Phonological primitives: electromyographic speech error evidence. J Acoust Soc Am. 1990 Sep;88(3):1299-312. PubMed PMID: 2229664.

Goldstein L, Pouplier M, Chen L, Saltzman E, Byrd D. Dynamic action units slip in speech production errors. Cognition. 2007 Jun;103(3):386-412. Epub 2006 Jul 5. PubMed PMID: 16822494; PubMed Central PMCID: PMC2394196.

Greg Hickok said...

Thank you for your comment. It is fair to point out that I'm using the term phoneme a bit sloppily. We should probably be talking about features specified over articulatory parameters.

Matt Goldrick said...

I think the speech error data mentioned in the previous post show that errors are not limited to segment sized units. However, it's also important to note that Goldstein et al.'s results provide evidence that errors involve the coordination of multiple gestures (see experiment 2). Furthermore, it should be noted that several authors (myself included) have interpreted these patterns as reflecting the gradient co-activation of multiple phonological representations (rather than displacement of individual gestural representations; Goldrick & Blumstein, 2006; McMillan, Corley, & Lickley, 2009).

More broadly, showing that errors can occur at these levels doesn't mean that the production system doesn't include more abstract levels of processing. I think there's several lines of evidence (behavioral and imaging-based) that support this claim. (I've been working on a review article so this is right on the tips of my fingers…)

1) Priming can target whole segments without transferring to featurally related segments (Roelofs, 1999)

2) The repeated phoneme effect (where repetition of a segment increases error rates on adjacent sounds) is, with some minor exceptions, contingent on repetition of an entire segment (not related segments; Stemberger, 1990, 2009 )

3) Errors arising in inner speech [under conditions minimizing covert articulation] show lexical bias effects but no influence of featural similarity (Oppenheim & Dell, 2008, in press)

4) Distinct deficits occur to sound-based processes in speech production. One set targets more abstract representations involved in lexically-driven sound retrieval; performance for these individuals is insensitive to phonological complexity. Others targeting more speech motor planning processes are strongly influenced by complexity (Goldrick & Rapp, 2007; Romani & Galluzzi, 2005; Romani, Olson, Semenza, & GranĂ , 2002)

[Greg, I'd suggest reviewing the Romani & Galluzzi, 2005, for a critical take on the Nickels & Howard results]

5) Consistent with the behavioral dissociations above, imaging studies have suggested that activity in posterior superior temporal regions is not modulated by phonological complexity; this modulates activity in more frontal regions (Graves, Grabowski, Mehta, & Gupta, 2008; Papoutsi, de Zwart, Jansma, Pickering, Bednar, & Horwitz, 2009)

Matt Goldrick said...

References for preceding comment…

Goldrick, M., & Rapp, B. (2007). Lexical and post-lexical phonological representations in spoken production. Cognition, 102, 219-260.
Graves, W. W., Grabowski, T. J., Mehta, S., & Gupta, P. (2008). The left posterior superior temporal gyrus participates specifically in accessing lexical phonology. Journal of Cognitive Neuroscience, 20, 1698-1710.
Oppenheim, G. M., & Dell, G. S. (2008). Inner speech slips exhibit lexical bias, but not the phonemic similarity effect. Cognition, 106, 528-537.
Oppenheim, G. M., & Dell, G. S. (in press). Motor movement matters: the flexible abstractness of inner speech. Memory & Cognition.
Papoutsi, M., de Zwart, J. A., Jansma, J. M., Pickering, M. J., Bednar, J. A., & Horwitz, B. (2009). From phonemes to articulatory Codes: An fMRI study of the role of Broca’s area in speech production. Cerebral Cortex, 19, 2156-2165.
Roelofs, A. (1999). Phonological segments and features as planning units in speech production. Language and Cognitive Processes, 14, 173-200.
Romani, C., & Galluzzi, C. (2005). Effects of syllabic complexity in predicting accuracy of repetition and direction of errors in patients with articulatory and phonological difficulties. Cognitive Neuropsychology, 22, 817-850.
Romani, C., Olson, A., Semenza, C., & GranĂ , A. (2002). Patterns of phonological errors as a function of a phonological versus an articulatory locus of impairment. Cortex, 38, 541-567.
Stemberger, J. P. (1990). Wordshape errors in language production. Cognition, 35, 123-157.
Stemberger, J. P. (2009). Preventing perseveration in language production. Language and Cognitive Processes, 24, 1431-1470.

Greg Hickok said...

Wow, thanks for the detailed comment Matt. Very helpful! I'm looking forward to your review paper.

I love being able to throw an issue out on the blog and get back a range of perspectives on it. As always, if anyone wants to highlight an issue or a paper on the blog, please send me a note and I'll post it!

CEJ said...

Your discussion seems to 'beg the question'--phonemes exist, or so you assume in order to come up with the idea that they are integral to speech production. And yet for the most part the phoneme has been repeatedly posited as a static discrete unit while articulation for speech is remarkable for its complex, dynamic co-articulation.

What if you assume the phoneme is but a fiction created by phonologists and phoneticians? Does that change your discussion?

CEJ said...

Two papers to consider: