Response Organization in Selective Adaptation to Speech Sounds

Previous experiments in speech perception using the selective adaptation procedure have found a shift in the locus of the category boundary for a series of speech stimuli following repeated exposure to an adapting syllable. The locus of the boundary moves toward the category of the adapting syllable. Most investigators have interpreted these findings in terms of feature detector models in which specific detectors are reduced in sensitivity through repeated adaptation. The present experiment was conducted to determine whether the adaptation results might be due to changes in response organization as a consequence of the labeling instructions presented to subjects in selective adaptation experiments. A perceptually ambiguous speech stimulus was selected from the middle of a [bi]-[di] test series and used as an adaptor under two different sets of instructions. One group of subjects was told that the adapting stimulus was the syllable [bi], while another group was told that the stimulus was the syllable [di]. The acoustically ambiguous adaptor failed to produce a shift in the locus of the category boundary in the direction predicted on the basis of the labeling instructions presented to subjects. These results indicate that the acoustic attributes and perceived quality of the adapting stimulus determine the direction and magnitude of the adaptation effects rather than the labels provided by the experimenter.