On the nature and organisation of morphological categories: verbal aspect through the lens of associative learning

The process by which awareness and/or knowledge of linguistic categories arises from exposure to patterns in data alone, known as emergence, is the corner stone of usage-based approaches to language. The present paper zooms in on the types of patterns that language users may detect in the input to determine the content, and hence the nature, of the hypothesised morphological category of aspect.

The large-scale corpus and computational studies we present focus on the morphological encoding of temporal information as exemplified by aspect (imperfective/perfective) in Polish. Aspect is so heavily grammaticalized that it is marked on every verb form, yielding the practice of positing infinitival verb pairs (‘do’ = ‘robićimpf/zrobićpf’) to represent a complete aspectual paradigm. As has been shown for nominal declension, however, aspectual usage appears uneven, with 90% of verbs strongly preferring one aspect over the other. This makes the theoretical aspectual paradigm in practice very gappy, triggering an acute sense of partialness in usage. Operationalising emergence as learnability, we simulate learning to use aspect from exposure with a computational implementation of the Rescorla-Wager rule of associative learning. We find that paradigmatic gappiness in usage does not diminish learnability; to the contrary, a very high prediction accuracy is achieved using as cues only the verb and its tense; contextual information does not further improve performance. Aspect emerges as a strongly lexical phenomenon. Hence, the question of cognitive reality of aspectual categories, as an example of morphological categories in general, should be reformulated to ask which continuous cues must be learned to enable categorisation of aspectual outcomes. We discuss how the gappiness of the paradigm plays a crucial role in this process, and how an iteratively learned, continuously developing association presents a possible mechanism by which language users process their experience of cue-outcome co-occurrences and learn to use morphological forms, without the need for abstractions.