The Centre for Speech Technology Research, The university of Edinburgh

01 Feb 2005

Andrew Wedel (University of Arizona)

Effective contrast and alternation

Functionally, contrast can be thought of as a property that describes the extent to which a lexical entry can be quickly and accurately accessed over another in a given context. A large psycholinguistic literature has shown that the speed and accuracy of lexical access is dependent on a number of factors, three of which will be of interest here. First, access of a lexical entry is more efficient when the lexical entry shows high phonemic contrast, where phonemic contrast is defined in terms of the actual number of other phonemically similar entries in the lexicon (Luce and Pisoni 1998). For example, in English, 'cat' is an example of a word with many such similar lexical neighbors, e.g., 'hat', 'pat', 'fat', 'cut', 'kit', 'cot', 'cap', 'can', etc. 'Orange', on the other hand, is an example of a lexical entry with no highly similar lexical neighbors. Within this model then, high phonemic contrast is synonymous with having few near lexical neighbors.

Second, high frequency of a lexical entry relative to its lexical neighbors strongly enhances the efficiency with which it can be accessed. For example, the highly frequent word 'cat' is accessed more readily than the less frequent word 'cot', even though they exhibit a similar degree of phonemic contrast. Third, lexical access of a morpheme has been shown to be more efficient if it does not exhibit alternation, that is, if it surfaces in the same form in all contexts (Tsapkini et al. 1996). We use the term effective contrast to refer to the net effect of the composite influences on lexical access efficiency. Here, I'll argue that the hypothesis that lexicons evolve under pressure to optimize effective contrast, rather than actual or potential phonemic contrast, allows us to better understand two otherwise apparently disparate contrast patterns: (i) the root-affix contrast asymmetry, and (ii) the suppression of alternation in small roots.

i) Typological evidence suggests that affixes tend to alternate more often than roots, and also tend to be drawn from a more restricted inventory (Macarthy and Prince 1995 and references therein). Therefore, from the point of view of lexical access, affixes are at a disadvantage relative to roots both through their greater tendency to alternate, and through restricted access to elements of phonemic contrast. Within OT, both of these phenomena have been accounted for by positing a universal 'meta-constraint' that Root-Faith must outrank Affix-Faith. However, under a model in which systems evolve to optimize effective contrast, the fact that affixes are much more frequent than roots (Segalowitz and Lane 2000) suggests a more deeply explanatory account. Under this model, we expect the very high frequency of affixes to mitigate the negative effects on lexical access of alternation or low phonemic contrast. For roots, with their lower average frequency, uniform surface forms and higher phonemic contrast should be relatively more important. If a effective contrast is optimized within the lexicon, the difference in frequency between roots and affixes should lead to a pattern in which roots tend to show greater phonemic contrast and less alternation than affixes.

(ii) A survey of the distribution of final devoicing, vowel reduction, and palatalization alternations across nouns and adjectives in Turkish, Catalan and Czech show that in these languages, alternation is significantly less common in roots of fewer than 4 phonemes. Unlike the case of roots versus affixes, this asymmetry cannot be explained on the basis of frequency, because smaller morphemes tend to be more frequent than larger morphemes, which would predict the opposite pattern. However, the number of close lexical neighbors is strongly, inversely correlated with root size. On average then, small roots are less phonemically contrastive than larger roots. Given the negative impact of low phonemic contrast on effective contrast, alternation may be relatively more costly in small relative to large roots.

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