Two-dimensional Wordlikeness Effects in Lexical Organisation
Authors: Vito Pirrelli, Claudia Marzi, Marcello Ferro (CNR-ILC, 2014)
Topic: Lexical and Semantic Resources and Analysis

Abstract
The main focus of research on wordlikeness has been on how serial pro-cessing strategies affect perception of similarity and, ultimately, the global net-work of associative relations among words in the mental lexicon. Compara-tively little effort has been put so far, however, into an analysis of the reverse relationship: namely, how global organi-sation effects influence the speakers’ per-ception of word similarity and of words’ internal structure. In this paper, we ex-plore the relationship between the two dimensions of wordlikeness (the “syn-tagmatic” and the “paradigmatic” one), to suggest that the same set of principles of memory organisation can account for both dimensions.

Introduction
The language faculty requires the fundamental ability to retain sequences of symbolic items, access them in recognition and production, find similarities and differences among them, and assess their degree of typicality (or WORDLIKE- NESS) with respect to other words in the lexicon. In particular, perception of formal redundancy appears to be a crucial precondition to morphol-ogy induction, epitomised by the so-called WORD ALI GNMENT problem. The problem arises when-ever one has to identify recurrence of the same pattern at different positions in time, e.g. book in handbook, or mach in both German macht and gemacht. Clearly, no “conjunctive” letter coding scheme (e.g., Coltheart et al. 2001; Harm & Seidenberg 1999; McClelland & Rumelhart 1981; Perry et al. 2007; Plaut et al. 1996), which requires that the representation of each symbol in a string be anchored to its position, would account for such an ability. In Davis’ (2010) SPATIAL ENCODING, the identity of the letter is described as a Gaussian activity function whose max value is centred on the letter’s actual position, enforcing a form of fuzzy matching, common to other models disjunctively encoding a symbol and its position (Grainger & van Heuven 2003; Henson 1998; Page & Norris 1998, among others). The role of specific within-word letter positions interacts with short-term LEXICAL BUFFERING and LEXICALITY effects. Recalling a stored representation requires that all symbols forming that representation are simultaneously activated and sustained in working memory, waiting to be serially retrieved. Buffering accounts for the comparative difficulty in recalling long words: more concurrently-activated nodes are easier to be confused, missed or jumbled than fewer nodes are. Notably, more frequent words are less likely to be confused than low-frequency words, since long-term entrenchment improves performance of immediate serial recall in working memory (Baddeley 1964; Gathercole et al. 1991). Serial (or syntagmatic) accounts of local ordering effects in word processing are often complemented by evidence of another, more global (or paradigmatic) dimension of word perception, based on the observation that, in the normal course of processing a word, other non-target neighbouring words become active. In the word recognition literature, there is substantial agreement on the inhibitory role of lexical neighbours (Goldinger et al. 1989; Luce & Pisoni 1998; Luce et al. 1990). Other things being equal, target words with a large number of neighbours take more time to be recognised and repeated, as they suffer from their neighbours’ competition in lexical buffering. This is particularly true when the target word is low-frequency. Nonetheless, there is contrasting evidence that dense neighbourhoods may speed up word reading time rather than delaying it (Huntsman & Lima 2002), and that high-entropy word families make their members more readily accessible than lowentropy families (Baayen et al. 2006). Marzi et al. (2014) provide clear computational evidence of interactive effects of paradigm regularity and type/token lexical frequency on the acquisition of German verb inflection. Token frequency plays a paramount role in item-based learning, with highly frequent words being acquired at comparatively earlier stages than lowfrequency words. Morphological regularity, on the other hand, has an impact on paradigm acquisition, regular paradigms being learned, on average, within a shorter time span than fully or partially irregular paradigms. Finally, frequency distribution of paradigmatically-related words significantly interacts with morphological regularity. Acquisition of regular paradigms depends less heavily on item-based storage and is thus less affected by differences in frequency distributions of paradigm members. Conversely, irregular paradigms are less prone to be generalised through information spreading and their acquisition mainly relies on itemised storage, thus being more strongly affected by the frequency distribution of paradigm members and by frequencybased competition, both intra- and interparadigmatically. We suggest that compounded evidence of wordlikeness and paradigm frequency effects can be accounted for within a unitary computational model of lexical memory. We provide here preliminary evidence in this direction, by looking at the way a specific, neuro-biologically inspired computational model of lexical memories, Temporal Self-Organising Maps (TSOMs), accounts for such effects.

Conclusion
Wordlikeness is a fundamental determinant of lexical organisation and access. Two quantitative measures of wordlikeness, namely n-gram probability and neighbourhood density, relate to important dimensions of lexical organisation: the syntagmatic (or horizontal) dimension, which controls the level of predictability and entrenchment of a serial memory trace, and the paradigmatic (or vertical) dimension, which controls the number of neighbours that are co-activated by the target word. The two dimensions are nicely captured by TSOMs, allowing the investigation of their dynamic interaction. In accessing and recalling a target word, a large pool of neighbours can be an advantage, since they tend to support patterns of activation that are shared by the target word. However, their help may turn out to interfere with recall, if the connection strength of one or more neighbours is overwhelmingly higher than that of the target. Deeply entrenched friends eventually become competitors. This dynamic establishes a nice connection with paradigm acquisition, where a uniform distribution of paradigm members is helpful in spreading morphotactic information and speed up acquisition, and paradigmatically- related forms in skewed distributions compete with one another (Marzi et al. 2014). We argue that both neighbourhood and morphological effects are the result of predictive (syntagmatic) activation and competitive (paradigmatic) co-activation of parallel processing nodes in densely interconnected networks. As a final qualification, our experiments illustrate the dynamic of activation and storage of letter strings, with no information about morphological content. They provide evidence of the first access stages of early lexical processing, where strategies of automatic segmentation are sensitive to possibly apparent morphological information (Post et al. 2008). Nonetheless, our data suggest that perception of wordlikeness and morphological structure can be accounted for by a common pool of principles governing the organisation of long-term memories for time series.