This paper examines the perception of the low front vowel /ae/ which has been found to be more centralized by younger speakers of some varieties of Canadian English (Labov, Ash & Boberg 2006, De Decker 2006). The results of an experiment are presented here revealing that centralized variants of /ae/ are categorized and evaluated differently by members of the same speech community. This suggests that vocalic drift is not merely a mechanical operation but a process mediated by variation in perceptual analysis. In the experiment, subjects listened to different pronunciations of the word sack. The second formant of the vowel was manipulated using Praat (Boersma & Weenink 2009) to produce a 19-step continuum of forms ranging between canonical sack and sock . 39 Subjects were asked to categorize each variant as sounding like “sack”, “sock" or "either sack or sock". Results reveal differences in categorization of vowel stimuli along the lines of gender and age. Male respondents were found to assign more of the continuum the label sock while female respondents heard a higher percentage of the continuum as sack. That is, when presented with the exact same continuum of forms, males exhibit a narrower range for the vowel /ae/ whereas females showed an extended tolerance for /æ/ further along the continuum. A similar result is found for age: younger speakers exhibited more centralized perceptual boundaries than older speakers. It is argued that these results are consistent with a view that considers phonetic changes like /ae/-retraction are facilitated via perceptual re-analysis. This model of sound change is contrasted with others like Ohala's (1981, 1993) misperception theory which states that sound changes result from the under- or over-application of phonetic reconstruction rules.