  If this is indeed how phonological learning works, it is clear that  while experience clearly plays a role, the inborn contribution to  that process is quite substantial. For discriminating phonemes  — however those discriminations might be shaped by subsequent  experience — is no simple matter. It involves what is called  ‘categorical perception, that is, the segmenting of a signal  that varies continuously along a number of physical dimensions (e.g., voice onset time and formant frequency) into discrete categories, so  that signals within the category are counted as the same, even though acoustically, they may differ from one another more than do two  signals in different categories (see Fig. 5). (Harnad 1987 is a  useful collection of work on categorical perception to the  mid-1980s.)