Title of the article

1.1.Motivations for a Phonotactic Device

This section will review standard arguments that demonstrate the cognitive and practical importance of phonotactics. English phonotactic rules such as:
‘/s/ may precede, but not follow /t/ syllable-initially’
(ignoring loanwords such as `tsar' and `tse-tse') may be adduced by judging the well-formedness of sequences of letters/phonemes, taken as words in the language, e.g. /stp/ vs. */tsp/. There may also be cases judged to be of intermediate acceptability. So, even if all of the following are English words:
/m/ `mother', /f/ `father', /sst/ `sister'
None of the following are, however:
*/m/, */f/, */tss/
None of these sound like English words. However, the following sequences:
/m/, /fu/, /snt/
"sound" much more like English, even if they mean nothing and therefore are not genuine English words. We suspect that, e.g., /snt/ 'santer', could be used to name a new object or a concept.

This simple example shows that we have a feeling for word structure, even if no explicit knowledge. Given the huge variety of words, it is more efficient to put this knowledge into a compact form - a set of phonotactic rules. These rules would state which phonemic sequences sound correct and which do not. In this same vein, second language learners experience a period when they recognize that certain phonemic combinations (words) belong to the language they learn without knowing the meaning of these words.

Convincing psycholinguistic evidence that we make use of phonotactics comes from studying the information sources used in word segmentation (McQueen, 1998). In a variety of experiments, this author shows that word boundary locations are likely to be signaled by phonotactics. The author rules out the possibility that other sources of information, such as prosodic cues, syllabic structure and lexemes, are sufficient for segmentation. Similarly, Treiman & Zukowski (1990) had shown earlier that phonotactics play an important role in the syllabification process. According to McQueen (1998), phonotactic and metrical cues play complementary roles in the segmentation process. In accordance with this, some researchers have elaborated on a model for word segmentation: the Possible Word Constraints Model (Norris, McQueen, Cutler & Butterfield, 1997), in which likely word-boundary locations are marked by phonotactics, metrical cues, etc., and in which they are further fixed by using lexicon-specific knowledge.

Exploiting the specific phonotactics of Japanese, Dupoux, Pallier, Kakehi & Mehler (2001) conducted an experiment with Japanese listeners who heard stimuli that contained illegal consonant clusters. The listeners tended to hear an acoustically absent vowel that brought their perception into line with Japanese phonotactics. The authors were able to rule out lexical influences as a putative source for the perception of the illusory vowel, which suggests that speech perception must use phonotactic information directly.

Further justification for the postulation of a neurobiological device that encodes phonotactics comes from neurolinguistic and neuroimaging studies. It is widely accepted that the neuronal structure of Broca’s area (in the brain's left frontal lobe) is used for language processing, and more specially that it represents a general sequential device (Stowe, Wijers, Willemsen, Reuland, Paans & Vaalburg, 1994; Reilly, 2002). A general sequential processor capable of working at the phonemic level would be a plausible realization of a neuronal phonotactic device.

Besides cognitive modeling, there are also a number of practical problems that would benefit from effective phonotactic processing. In speech recognition, for example, a number of hypotheses that explain the speech signal are created, from which the impossible sound combinations have to be filtered out before further processing. This exemplifies a lexical decision task, in which a model is trained on a language L and then tests whether a given string belongs to L. In such a task a phonotactic device would be of use. Another important problem in speech recognition is word segmentation. Speech is continuous, but we divide it into psychologically significant units such as words and syllables. As noted above, there are a number of cues that we can use to distinguish these elements - prosodic markers, context, but also phonotactics. Similarly to the former problem, an intuitive strategy here is to split the phonetic/phonemic stream at the points of violation of phonotactic constraints (see Shillcock et al. (1997) and Cairns, Shillcock, Chater & Levy (1997) for connectionist modeling). Similarly, the constraints of the letters forming words in written languages (graphotactics) are useful in word processing applications, for example, spell-checking.

There is another, more speculative aspect to investigating phonotactics. Searching for an explanation of the structure of the natural languages, Carstairs-McCarthy presented in his recent book (1999) an analogy between syllable structure and sentence structure. He argues that sentences and syllables have a similar type of structure. Therefore, if we find a proper mechanism for learning the syllabic structures, we might apply a similar mechanism to learning syntax as well. Of course, syntax is much more complex and more challenging, but if Carstairs-McCarthy is right, the basic principles of both devices might be the same.