Infants attend to distributional characteristics of their input (Maye et al., 2002(Maye et al., , 2008, leading to the hypothesis that phonetic categories could be acquired on the basis of bottom-up distributional learning alone (de Boer and Kuhl, 2003;Vallabha et al., 2007;McMurray et al., 2009).
this would require sound categories to be well separated, which often is not the case-for example, see Figure 1, which shows the English vowel space that is the focus of this paper.