This study examines the relationship between patterns of variation and speech perception using two English prefixes: 'in-'/'im-' and 'un-'. In natural speech, 'in-' varies due to an underlying process of phonological assimilation, while 'un-' shows a pattern of surface variation, assimilating before labial stems. In a go/no-go lexical decision experiment, subjects were presented a set of 'mispronounced' stimuli in which the prefix nasal was altered (replacing [n] with [m], or vice versa), in addition to real words with unaltered prefixes. No significant differences between prefixes were found in responses to unaltered words. In mispronounced items, responses to 'un-' forms were faster and more accurate than to 'in-' forms, although a significant interaction mitigated this effect in labial contexts. These results suggest the regularity of variation patterns has consequences for the lexical specification of words, and argues against radical under-specification accounts which argue for a maximally sparse lexicon
