Lexical parsability and morphological structure

A classical tenet in the psycholinguistic literature on the mental lexicon is that a parsed affix presents high activation levels (and thus contributes to activation spreading to other words with the same affix), and that such levels are tightly correlated with the affix productivity. In a number of influential papers, it has been suggested that parsability criteria interact with frequency to define morphological productivity in the lexicon. For example, the frequency of a derivative (e.g. government) relative to its base (govern) is shown to be a good predictor for parsability/productivity. The higher the frequency ratio, the more likely the morphological structure to be perceived, and the associated affix to be used productively. The present contribution intends to offer a computational explanatory basis for this correlational evidence, and assess its applicability to the acquisition of complex inflectional paradigms. In those languages, like Italian and German, whose inflection is stem-based rather than word-based, there is often no single paradigmatic form which can act as a base by being properly contained in all other inflected variants. Yet, it seems intuitive to suggest that verbs that are inflected for one paradigm cell only (e.g. neighbouring), are learned earlier and more easily but exhibit lower levels of perceived inflectional structure than verbs with richer paradigms. This appears to be in good accord with experimental evidence of time latencies in lexical decision, which are shown to correlate negatively with token frequency, paradigm size and paradigm entropy. Our simulations, based on Temporal Self-Organizing Maps (TSOMs) allow us to establish an interesting connection between inflectional parsability, frequency-based paradigm structure, and acquisitional constraints on the interaction between the human processor and working memory. Self-organising topological models of the mental lexicon can mimic the spatial and temporal organization of memory structures supporting the processing of symbolic sequences, and can provide an interesting framework for testing integrative accounts of lexical processing/acquisition as the complex result of general-purpose operations on word stimuli (e.g. working memory, long-term storage, sensory-motor mapping, rehearsal, unit integration, unit analysis, executive control, time-series processing), in line with recent acquisitions on the neuro-functional architecture of the perisylvian language network in the left hemisphere of human brain. Simulations of the incremental acquisition of "mini-paradigms" (small islands of morphological contrast encompassing up to three different forms for the same verb support the hypothesis that perception of structure (parsability) and morphological productivity strongly correlate in the inflectional lexica of German and Italian. In particular, by monitoring longitudinal progress in storage and generalisation of differently distributed inflectional paradigms in the two languages, we show that: i) high-frequency forms are stored and accessed significantly earlier than low-frequency forms; ii) deeply entrenched but paradigmatically isolated forms tend to block usage of other forms in the same paradigm; iii) low-frequency evenly distributed (highly entropic) intra-paradigmatic forms are acquired later but are easily extended. Our investigation credits the proposed computational framework with psycholinguistic plausibility, and grounds parsability-based models of morphological productivity on a specific, explicit proposal of lexical architecture. This provides an explanatory basis for both psycholinguistic and linguistic accounts of morphological structure, and offers an intermediate framework for scientific inquiry bridging the gap between linguistic units and functional units in neurosciences. Finally, it makes the interesting suggestion that principles of morpheme-based organisation of the mental lexicon are compatible with a learning strategy requiring memorisation of full forms.