There is a great deal of evidence that many phonological contrasts are perceptually available from a very early stage (Eimas et al., 1971;Moffitt, 1971;Trehub, 1973;Jusczyk and Derrah, 1987;Eimas et al., 1987).
but to prevent the acquisition process from being circular, the learner cannot operate solely on top-down information -the acoustic signal must provide some evidence for the phonemic categories.