  One of the central aspects of contextualised language models is that they
should be able to distinguish the meaning of lexically ambiguous words by their
contexts. In this paper we investigate the extent to which the contextualised
embeddings of word forms that display multiplicity of sense reflect traditional
distinctions of polysemy and homonymy. To this end, we introduce an extended,
human-annotated dataset of graded word sense similarity and co-predication
acceptability, and evaluate how well the similarity of embeddings predicts
similarity in meaning. Both types of human judgements indicate that the
similarity of polysemic interpretations falls in a continuum between identity
of meaning and homonymy. However, we also observe significant differences
within the similarity ratings of polysemes, forming consistent patterns for
different types of polysemic sense alternation. Our dataset thus appears to
capture a substantial part of the complexity of lexical ambiguity, and can
provide a realistic test bed for contextualised embeddings. Among the tested
models, BERT Large shows the strongest correlation with the collected word
sense similarity ratings, but struggles to consistently replicate the observed
similarity patterns. When clustering ambiguous word forms based on their
embeddings, the model displays high confidence in discerning homonyms and some
types of polysemic alternations, but consistently fails for others.
