In the previous post, I described Menn and Matthei’s assessment of progress on the two-lexicon model. They highlight several advantages of the model, but also note problems, including the apparent competition between children’s “selection rules” (or rules specific to the output lexicon), as well as non-deterministic cross-word patterns. To combat these and other problems, MM suggest that the formalism of the two-lexicon model migrate from a generative perspective to a more connectionist one. At this point, they make a very handy list of the key generalizations they would like to capture with a revised, connectionist two-lexicon model, or with any model of child speech production for that matter. I have restated them here, while keeping MM’s original groupings.

Reduction of Information

1. Children recognize more words than they can say
2. Children recognize more phonemic contrasts than they can realize in speech
3. Early productions tend to cluster together in terms of phonetic properties
4. Early productions also tend to contain a limited set of phonetic elements

Mapping

1. Children’s productions appear to be simplified (compared to adult forms) and often appear systematic (many words share a pattern)

Inertia of the System

1. Early, frequently produced words may retain a high level of fidelity, resulting in “phonological idioms” compared to more recently acquired production forms
2. Changes in systematic productions tend to happen to newly acquired words; more established words are more resistant to change

We could also add to this list MM’s frequent observation that imitated production forms tend to be much more like adult forms.

To provide a general feel for a connectionist model of early speech production, MM lay out the “initial settings” for such a model. With respect to connections, MM posit simultaneous and sequential connections. Simultaneous connections link the speech modalities of motor commands, auditory percepts, and kinesthetic sensation (of one’s own productions). The three modalities, motor/auditory/kinesthetic or MAK, must be wired together efficiently by learning. Sequential connections are within-modality connections that represent change over time. So, a simultaneous connection might link together the feeling, action plan, and acoustic record of a [b], while sequential acoustic connections might link together the [b] burst to the following formants of an [a] vowel in the syllable [ba]. Although MM do not make this explicit, it appears that sequences of connections also represent stored forms, or words.

Next, MM lay out a series of what I will call linking mechanisms. First, sequential auditory patterns can be stored and learned by attention to adult speech. Second, there is an internal feedback loop, which MM relate to babbling, which has a basic predictive property that allows the model to guess how a sequential motor pattern might sound and thereby modify it to observe whether the result is the same or different (essentially a supervised learning component provided by the stored, “correct” adult forms). Third, imitation will result in links between stored adult-produced auditory sequences and the child’s own MAK sequences. Fourth, stored adult sequences will be associated with real-word states (meanings), which then leads associations between the child’s own MAK sequences and real-world states.

MM give a fair amount of attention to the idea that adults might assist in the development of a child’s MAK sequences. The basic idea is that an adult mimics the phonetic properties of a child’s utterance (absolute pitch, formant values, etc.). Here’s an explanatory quote: “A purely sound-based imitation of the child by the adult…will produce links between the child’s internal MAK associations and the sound of the adult’s voice, the child’s innate normalization abilities should be enhanced.”

Once normalization is established (although I’m not sure why it needs to be established first in this proposal), the child might seek to produce words in a more adult-like fashion. MM propose that social factors like semantically contingent responding by parents (Snow, 1977) could provide such a mechanism. MM conclude by saying that their connectionist model is not fully developed, and that many attractive qualities of the old two-lexicon model, like the selection rules, have been replaced by vaguer concepts. However, they believe that the absolute boundaries of the input and output lexicons in the original model simply do not serve us, and we should abandon them.

My primary concern with the connectionist model that MM propose is that it seems to completely abandon the original problem that the two-lexicon model addresses. Looking back at their list of key generalizations, I would single out two, but the connectionist model does not clearly address either. First, how is it that children can recognize more words/sounds than they can produce? Second, why are children’s early productions both simplified and systematic?

It’s difficult to see how the proposed connectionist model makes headway on these problems. In fact, it seems as if they have been replaced with several other problems in the study of child speech. The discussion of speech normalization is a perfect example. Given general agreement that toddlers have a good understanding of the perceptual form of their native language, this problem could be assumed to be solved at the time that production begins. For example, I know of no evidence that children ever attempt to imitate the absolute values of any acoustic property of adult forms, which seems to be a major problem if we want to address normalization.

To conclude, I generally see the box-and-arrow iteration of the two-lexicon model as being preferable, if only for specificity. Athough I agree with MM that the box-and-arrow model could be replaced advantageously by a connectionist model, the advantages are simply not clear enough here. In the future, I will present a more recent attempt at a connectionist network by Menn and colleagues, which may address the perception-production disparity more directly.

REFERENCES

Snow, C. E. (1977). The development of conversation between mothers And babies. Journal of Child Language, 4, 1-13.