The
English vowel nasalisation and devoicing rules and the Japanese devoicing rule
change feature specifications. That is, in English the [ - nasal] value of
phonemic vowels is changed to [+ nasal] phonetically when they occur before
nasals through a spreading process. Vowels in Japanese are phonemically voiced,
and the rule changes vowels that occur in the specified environment into
phonemically voiceless segments.
The
rule we have discussed are phonetically plausible, as are other assimilation
rules, and can be explained by natural phonetic process. This fact does not
mean that all these rules have to be learned at all; they would apply
automatically and universally, and therefore would not have to be included in
the grammar of any particular language. They are not, however, universal.
There
is a nasal assimilation rule in Akan that nasalises voiced stops when they
follow nasal consonants, as shown in the following example :
The
/b/ of the verb ‘come’ becomes an [m] when it follows the negative /m/. This
assimilation rule also has a phonetic explanation; the velum is lowered to
produce the nasal consonant and remains down during the following stop.
Although it is phonetically ‘natural’ assimilation rule, it does not occur in
the grammar of English; the word amber,
for example, shows an [m] followed by a [b]. A child learning Akan must learn
this rule, just as a child learning English learns to nasalise all vowels
before nasal consonants, a rule that does not occur in the grammar of Akan.
Assimilation
rules such as those we have discussed in English, Japanese, and Akan often have
the function of changing the value of phonetic features. They are
feature-changing or feature-spreading rules. Although nasality is
non-distinctive for vowels in English, it is a distinctive features of
consonants, and the nasalisation rule therefore changes a feature value.
The
Akan rule is a feature-changing rule tht states that [m] is an allophone of /b/
as well as an allophone of /m/.
There
is no one-to-one relationship between phonemes and their allophones. This fact
can be illustrated in another way :
We
will provide more examples of this one-to-many or many-to-one mapping between
phonemes and allophones below.
