Because formant frequencies are a crucial cue in identifying English vowels, some pulsatile multichannel cochlear implants attempt to deliver formant frequency information by stimulating different electrodes depending on instantaneous formant values. For users of these cochlear implants, it may be possible to predict performance in vowel confusion studies based on the subject's ability to discriminate different electrodes. One possible confounding factor is that cochlear implants stimulate cochlear locations that are more basal than normal (i.e., associated with higher frequencies). To what extent is this more basal-than-normal stimulation a problem? Is it important to stimulate the ``physiologically correct'' places in the cochlea, or (to state a diametrically opposed hypothesis) is the place code infinitely plastic? A mathematical model of vowel perception with cochlear implants is proposed that explicitly tests both hypotheses. The model attempts to predict a postlingually deaf subject's vowel confusions based on his electrode discrimination and on his ``perceptual bias'' (meaning, in this case, the extent to which he has adapted to more-basal-than-normal stimulation). The conclusion is that more-basal-than-normal stimulation of the cochlea degrades speech perception shortly after implantation, but becomes less critical as subjects adapt to the new location.