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.

The experiments reported employed nonspeech analogs of speech stimuli to examine the perceptual interaction between first-formant onset frequency and voice-onset time, acoustic cues to the voicing distinction in English initial stop consonants. The nonspeech stimuli comprised two pure tones varying in relative onset time, and listeners were asked to judge the simultaneity of tone onsets. These judgments were affected by the frequency of the lower tone in a manner that parallels the influence of first-formant onset frequency on voicing judgments. This effect was shown to occur regardless of prior learning and to be systematic over a wide range of lower tone frequencies including frequencies beyond the range of possible first-formant frequencies of speech, suggesting that the effect in speech is not attributable to (tacit) knowledge of production constraints, as some current theories suggest.

Two acoustic variables that correlate with the distinction between intervocalic [b] and [p] are closure duration and presence or absence of low-frequency glottal pulsing during the closure interval. These variables may be considered to exhibit a trading relation (Repp, 1982), to the extent that a longer closure is required to perceive the consonant as voiceless when glottal pulsing is present than when it is not. Such trading relations have been interpreted as reflecting a special speech mode of perception. In the present experiments, we demonstrated a trading relation between closure duration and closure pulsing for a set of [abaJ-[apa] stimuli. Next we showed that a similar effect could be obtained with square-wave analogue stimuli that mimicked the segment durations and peak amplitudes of the speech stimuli but that were not phonetically categorizable. This nonspeech trading relation depended on the degree of spectral continuity between the low-frequency pulsing and the adjacent portions of the square wave. The implications of these results for the speech mode hypothesis are discussed.

It is generally believed that selective adaptation effects in speech perception are due to a reduction in sensitivity of auditory feature detectors. Recent evidence suggest that these effects may derive instead from contrast. In a further test of the contrast hypothesis, we conducted two experiments each involving both adaptation and contrast sessions with matching stimulus sets. During the adaptation sessions of Experiment 1, subjects identified two series of velar stimuli varying in voice onset time, [ga]-[kha] and [gi]-[khi], before and after adaptation with of the following stimuli: [ga], [kha], [gi], and [khi]. In the contrast session, subjects identified either of two ambiguous test items (drawn from near the phonetic boundaries of the [ga]-[kha] and the [gi]-[khi] series) following a single presentation of [ga], [kha], [gi], or [khi]. For both the adaptation and contrast sessions, (a) the [--a] test items were more greatly affected (in a contrast direction) by the [--a] than by the [--i] adaptor/context stimuli, and (b) the [--i] test items were not differentially affected by the [--1] and [--i] adaptor/context stimuli. An analogous design was used in Experiment 2, except that the stimulus sets varied in pitch rather than vowel quality. For both the adaptation and contrast sessions, the test items were not differentially affected by the pitch of the adaptor/context stimulus. These parallel results provide further evidence that adaptation effects are actually a form of contrast.

It is generally believed that selective adaptation effects in speech perception are due to a reduction in sensitivity of auditory feature detectors. We have recently presented evidence that these effects may derive instead from response contrast. In a further test of the contrast hypothesis, we conducted both an adaptation and a contrast experiment with matching sets of stimuli. In the adaptation experiment, subjects identified two series of velar stimuli varying in VOT, [ga-kha] and [gi-khi], before and after adaptation with each of the following test stimuli: [ga], [kha], [gi], and [khi]. In the contrast experiment, subjects identified either of two ambiguous test items (drawn from near the phonetic boundaries of the [ga-kha] and the [gi-khi] series) following a single presentation of [ga], [kha], [gi], or [khi]. For both the adaptation and the contrast experiments, (1) the [Ca] test items were more greatly affected (in a contrast direction) by [Ca] than by the [Ci] adaptor/context stimuli, and (2) the [Ci] test items were not differentially affected by the [Ca] and [Ci] adaptor/context stimuli. These parallel results suggest that vowel contingent adaptation results may well be due to response contrast rather than sensory fatigue.