Are selective adaptation and contrast effects really distinct?

Although there is evidence that selective adaptation and contrast effects in speech perception are produced by the same mechanisms, Sawusch and Jusczyk (1981) reported a dissociation between the effects and concluded that adaptation and contrast occur at separate processing levels. They found that an ambiguous test stimulus was more likely to be labeled b following adaptation with [pha] and more likely to be labeled p following adaptation with [ba] or [spa] (the latter consisting of [ba] preceded by [s] noise). In the contrast session, where a single context stimulus occurred with a single test item, the [ba] and [pha] contexts had contrastive effects similar to those of the [ba] and [pha] adaptors, but the [spa] context produced an increase in b responses to the test stimulus, an effect opposite to that of the [spa] adaptor. One interpretation of this difference is that the rapid presentation of the [spa] adaptor gave rise to "streaming," whereby the [s] was perceptually segregated from the [ba]. In our experiment, we essentially replicated the results of Sawusch and Jusczyk (1981), using procedures similar to theirs. Next, we increased the interadaptor interval to remove the likelihood of stream segregation and found that the adaptation and contrast effects converged.