Menn and Matthei (hereafter MM) begin with some information about the historical development of the two-lexicon model. They quote a paper by Ferguson, Peizer, and Weeks (1973), who noted a general human tendency to know more words than are typically said. That is, both children and adults know words that they rarely or never say. Thus, there seems to be a set of lexical representations for which the details of production are either murky or nonexistent, and we might hypothesize a split between input and output representations (Ingram, 1974), in other words, two separate lexicons.
So long as there is a consistency in children’s pronunciations, however, separate lexicons are unnecessary. If there is a regular mapping between the input representation (presumed to be identical to the adult forms) and the output representation, then a set of rewrite rules that capture the mapping are sufficient, and no output lexicon is needed. However, children are rarely consistent, and MM provide the example of two words (“down” and “stone”) that move in and out of a nasal harmony rule: They start out with no harmony ([dawn] and [don], resp.); the harmony rule then applies to other words (/binz/ –> [minz] and /dæns/ –> [næns]); finally, the harmony rule overtakes “down” and “stone”. With inconsistent mapping across similar words, rewrite rules are not helpful, or at least require arbitrary exceptions. Granted, two-lexicon models must also have lexical exceptions, but there are other advantages.
One of these advantages is that arbitrary exceptions in a one-lexicon system lead to more serious problems. The example is from Smith (1973) as interpreted by Macken (1980). The data comes from the child, Amahl, who displayed a pattern of velar harmony (/tr^k/ –> [kr^k]). Eventually, the pattern gave way to accurate production of alveolars, but one word, “took”, persisted as a regressive idiom, [gUk].
Macken assumes that this is possible because Amahl must have learned /gUk/ as the underlying form. Thus, when the harmony rule disappeared, /gUk/ would still surface as if harmony applied. As MM point out, however, this assumes that the child perceives “took” as /gUk/, which would lead us to expect that Amahl would not understand “took” as produced correctly. This seems highly unlikely, especially given our present-day understanding of children’s perceptual abilities. Furthermore, the example above with “down” and “stone” resisting a nasal harmony rule does not make sense if we assume exceptions are cases where the child has learned his own productions as underlying forms. At the very least, it would suggest that the underlying forms of words where nasal harmony does apply are perceived as if they had initial nasals. That defeats the advantage of the one-lexicon model, however, where we assume child and adult underlying forms are the same.
An output lexicon is helpful in this case because it provides a space for pronunciation representations that may be linked by a rule that operates across words or by arbitrary connections between input and output forms. Just as importantly, the output lexicon still allows children to be able to accurately perceive those words. That is, the output lexicon provides a storage facility for consistent or variable output representations while allowing for stable and accurate perception.
Despite the advantages, MM detail several problems they see with the two-lexicon model. First, it appears that selection rules—or the rules that lead to childlike forms in the output lexicon—sometimes operate over two words. This is problematic, however, if we take up the very standard assumption that combining words is done by the syntax and word combinations do not exist in the lexicon.
Another problem is that selection rules may sometimes be in competition with one another for a given word. MM give the example of productions by the child Daniel (also discussed by Menn in previous papers, I believe) of “boot” and “boat”, which are variably produced as [bup-dut] and [bop-dot] respectively. Thus, there appear to be separate labial harmony and alveolar harmony rules that compete in terms of realization of the same word. MM point out that there isn’t any sort of formalism in the two-lexicon model that allows for rule competition.
Other problems are given through the examination of daily changes in a couple of diary studies. For example, a child Jacob exhibited something like a vowel convergence, where [i] was produced like [ε]. So “tea” is first produced as [di] and then as [dεi]. “Key” was produced first as [ki], then as [xiε], and finally as [xε]. At the same time words with a mid front vowel switched between a low and high specification: “tape” was produced with both [i] and [e]. Ultimately, MM conclude that these similar words must be influencing each other in terms of production, but in a very unruly way. Similar cases are given for stress placement on two-syllables words beginning with [k] and over-application of the plural/3rd singular/possessive morpheme.
I’ll stop here for now. My next post will summarize what MM want to explain and then review the connectionist model that MM propose as a revised two-lexicon system.
Ferguson, C. A., Peizer, D. B., & Weeks, T. A. (1973). Model-and-replica phonological grammar of a child’s first words. Lingua, 31, 35-65.
Ingram, D. (1974). Phonological rules in young children. Journal of Child Language, 1, 49-64.
Macken, M. A. (1980). The child’s lexical representation: The ‘puzzle-puddle-pickle’ evidence. Journal of Linguistics, 16, 1-17.
Smith, N. V. (1973). The Acquisition of Phonology: A Case Study. Cambridge: Cambridge University Press.