Vowel judgements and adaptation level

By means of artificially synthesized speech it has been shown that the acoustic structure of a sentence alters the perception of a succeeding vowel sound. Normally the vowel in bet has a higher frequency for its first resonance than the vowel in bit. If, however, a sound usually heard as bet is preceded by a sentence with a generally high first resonance, the sound may be heard as bit. This suggests that some sort of adaptation level for vowels is formed when any particular speaker is being heard. A number of experiments are described which partially support and partially disagree with a theory based on adaptation level. In favour of the theory are such facts as the disappearance of the effect if the test-word precedes the sentence, and the greater effect produced by a longer sentence. Opposed to the theory are the difficulty of producing the effect by altering the words used in the introductory sentence rather than altering the acoustic stimulus corresponding to each vowel, and the disappearance of the effect with repeated listening. These disagreements with theory are noted as comparable to findings in other psycho-physical situations and a revised theory still using the conception of adaptation level is suggested.