Phonemic Restoration: Insights From a New Methodology

SUMMARY Phonemic restoration is a powerful auditory illusion in which listeners "hear" parts of words that are not really there. In earlier studies of the illusion, segments of words (phonemes) were replaced by an extraneous sound; listeners were asked whether anything was missing and where the extraneous noise had occurred. Most listeners reported that the utterance was intact and mislocalized the noise, suggesting that they had restored the missing phoneme. In the present study, a second type of stimulus was also presented: items in which the extraneous sound was merely superimposed on the critical phoneme. On each trial, listeners were asked to report whether they thought a stimulus utterance was intact (noise superimposed) or not (noise replacing). Since this procedure yields both a miss rate />(intact|replaced), and a false alarm rate P(replaced|intact), signal detection parameters of discriminability and bias can be calculated. The discriminability parameter reflects how similar the two types of stimuli sound; perceptual restoration of replaced items should make them sound intact, producing low discriminability scores. The bias parameter measures the tendency of listeners to report utterances as intact; it reflects postperceptual decision processes. This improved methodology was used to test the hypothesis that restoration (and more generally, speech perception) depends upon the bottom-up confirmation of expectations generated at higher levels. Perceptual restoration varied greatly with the phone class of the replaced segment and its acoustic similarity to the replacement sound, supporting a bottom-up component to the illusion. Increasing listeners' expectations of a phoneme increased perceptual restoration: missing segments in words were better restored than corresponding pieces in phonologically legal pseudowords; priming the words produced even more restoration. In contrast, sentential context affected the postperceptual decision stage, biasing listeners to report utterances as intact. A limited interactive model of speech perception, with both bottom-up and top-down components, is used to explain the results. Phonemic restoration is a powerful audi- In the decade since this experiment was tory illusion in which listeners "hear" parts reported, phonemic restoration has been very of words that are not really there. In the first widely cited and very little .studied. Only a report of the illusion, Warren (1970) re- handful of studies of the illusion have applaced the first /s/ in "legislatures" with a peared in the literature. The most thorough cough or a tone in the sentence, "The state of these was done by Warren and Obusek governors met with their respective legisla- (1971). Using the same sentence as Warren tures convening in the capital city." Listen- (1970), Warren and Obusek varied the naers were given typewritten versions of the ture of the replacement sound, the inforsentence and asked to circle the location of mation given to the subjects, and the size of the cough or tone and say whether it had the replaced chunk of speech. As in the first replaced a speech sound. Localization of the study, two measures of restoration were extraneous sound was poor, and essentially used: the distance metric (how accurately all of the subjects reported that the sentence subjects located the replacement sound) and was intact; they apparently restored the de- the hit rate (how often subjects accurately leted /s/.