Laura C. Dilley

Altering Context Speech Rate Can Cause Words to Appear or Disappear

Speech is produced over time, and this makes sensitivity to timing between speech events crucial for understanding language. Two experiments investigated whether perception of function words (e.g., or, are) is rate dependent in casual speech, which often contains phonetic segments that are spectrally quite reduced. In Experiment 1, talkers spoke sentences containing a target function word; slowing talkers’ speech rate around this word caused listeners to perceive sentences as lacking the word (e.g., leisure or time was perceived as leisure time). In Experiment 2, talkers spoke matched sentences lacking a function word; speeding talkers’ speech rate around the region in which the function word had been embedded in Experiment 1 caused listeners to perceive a function word that was never spoken (e.g., leisure time was perceived as leisure or time). The results suggest that listeners formed expectancies based on speech rate, and these expectancies influenced the number of words and word boundaries perceived. These findings may help explain the robustness of speech recognition when speech signals are distorted (e.g., because of a casual speaking style).

Inter-transcriber reliability for two systems of prosodic annotation: ToBI (Tones and Break Indices) and RaP (Rhythm and Pitch)

Abstract Speech researchers often rely on human annotation of prosody to generate data to test hypotheses and generate models. We present an overview of two prosodic annotation systems: ToBI (Tones and Break Indices) (Silverman et al., 1992), and RaP (Rhythm and Pitch) (Dilley & Brown, 2005), which was designed to address several limitations of ToBI. The paper reports two large-scale studies of inter-transcriber reliability for ToBI and RaP. Comparable reliability for both systems was obtained for a variety of prominence- and boundary-related agreement categories. These results help to establish RaP as an alternative to ToBI for research and technology applications.

Evidence for a rhythm perception deficit in children who stutter.

Stuttering is a neurodevelopmental disorder that affects the timing and rhythmic flow of speech production. When speech is synchronized with an external rhythmic pacing signal (e.g., a metronome), even severe stuttering can be markedly alleviated, suggesting that people who stutter may have difficulty generating an internal rhythm to pace their speech. To investigate this possibility, children who stutter and typically-developing children (n=17 per group, aged 6-11 years) were compared in terms of their auditory rhythm discrimination abilities of simple and complex rhythms. Children who stutter showed worse rhythm discrimination than typically-developing children. These findings provide the first evidence of impaired rhythm perception in children who stutter, supporting the conclusion that developmental stuttering may be associated with a deficit in rhythm processing.

Exploring the role of exposure frequency in recognizing pronunciation variants.

Words can be pronounced in multiple ways in casual speech. Corpus analyses of the frequency with which these pronunciation variants occur (e.g., Patterson & Connine, 2001) show that typically, one pronunciation variant tends to predominate; this raises the question of whether variant recognition is aligned with exposure frequency. We explored this issue in words containing one of four phonological contexts, each of which favors one of four surface realizations of word-medial /t/: [t], [ʔ], [ɾ], or a deleted variant. The frequencies of the four realizations in all four contexts were estimated for a set of words in a production experiment. Recognition of all pronunciation variants was then measured in a lexical decision experiment. Overall, the data suggest that listeners are sensitive to variant frequency: Word classification rates closely paralleled production frequency. The exceptions to this were [t] realizations (i.e., canonical pronunciations of the words), a finding which confirms other results in the literature and indicates that factors other than exposure frequency affect word recognition.

Long-Term Temporal Tracking of Speech Rate Affects Spoken-Word Recognition

Humans unconsciously track a wide array of distributional characteristics in their sensory environment. Recent research in spoken-language processing has demonstrated that the speech rate surrounding a target region within an utterance influences which words, and how many words, listeners hear later in that utterance. On the basis of hypotheses that listeners track timing information in speech over long timescales, we investigated the possibility that the perception of words is sensitive to speech rate over such a timescale (e.g., an extended conversation). Results demonstrated that listeners tracked variation in the overall pace of speech over an extended duration (analogous to that of a conversation that listeners might have outside the lab) and that this global speech rate influenced which words listeners reported hearing. The effects of speech rate became stronger over time. Our findings are consistent with the hypothesis that neural entrainment by speech occurs on multiple timescales, some lasting more than an hour.

When cues combine: How distal and proximal acoustic cues are integrated in word segmentation

Spoken language contains few reliable acoustic cues to word boundaries, yet listeners readily perceive words as separated in continuous speech. Dilley and Pitt (2010) showed that the rate of nonlocal (i.e., distal) context speech influences word segmentation, but present theories of word segmentation cannot account for whether and how this cue interacts with other acoustic cues proximal to (i.e., in the vicinity of) the word boundary. Four experiments examined the interaction of distal speech rate with four proximal acoustic cues that have been shown to influence segmentation: intensity (Experiment 1), fundamental frequency (Experiment 2), word duration (Experiment 3), and high frequency noise resembling a consonantal onset (Experiment 4). Participants listened to sentence fragments and indicated which of two lexical interpretations they heard, where one interpretation contained more words than the other. Across all four experiments, both distal speech rate and proximal acoustic manipulations affected the reported lexical interpretation, but the two types of cues did not consistently interact. Overall, the results of the set of experiments are inconsistent with a strictly-ranked hierarchy of cues to word boundaries, and instead highlight the necessity of word segmentation and lexical access theories to allow for flexible rankings of cues to word boundary placement.

Phonetic variation in consonants in infant-directed and adult-directed speech: the case of regressive place assimilation in word-final alveolar stops*

Pronunciation variation is under-studied in infant-directed speech, particularly for consonants. Regressive place assimilation involves a word-final alveolar stop taking the place of articulation of a following word-initial consonant. We investigated pronunciation variation in word-final alveolar stop consonants in storybooks read by forty-eight mothers in adult-directed or infant-directed style to infants aged approximately 0;3, 0;9, 1;1, or 1;8. We focused on phonological environments where regressive place assimilation could occur, i.e., when the stop preceded a word-initial labial or velar consonant. Spectrogram, waveform, and perceptual evidence was used to classify tokens into four pronunciation categories: canonical, assimilated, glottalized, or deleted. Results showed a reliable tendency for canonical variants to occur in infant-directed speech more often than in adult-directed speech. However, the otherwise very similar distributions of variants across addressee and age group suggested that infants largely experience statistical distributions of non-canonical consonantal pronunciation variants that mirror those experienced by adults.

Distal rhythm influences whether or not listeners hear a word in continuous speech: Support for a perceptual grouping hypothesis

Due to extensive variability in the phonetic realizations of words, there may be few or no proximal spectro-temporal cues that identify a words onset or even its presence. Dilley and Pitt (2010) showed that the rate of context speech, distal from a to-be-recognized word, can have a sizeable effect on whether or not a word is perceived. This investigation considered whether there is a distinct role for distal rhythm in the disappearing word effect. Listeners heard sentences that had a grammatical interpretation with or without a critical function word (FW) and transcribed what they heard (e.g., are in Jill got quite mad when she heard there are birds can be removed and Jill got quite mad when she heard their birds is still grammatical). Consistent with a perceptual grouping hypothesis, participants were more likely to report critical FWs when distal rhythm (repeating ternary or binary pitch patterns) matched the rhythm in the FW-containing region than when it did not. Notably, effects of distal rhythm and distal rate were additive. Results demonstrate a novel effect of distal rhythm on the amount of lexical material listeners hear, highlighting the importance of distal timing information and providing new constraints for models of spoken word recognition.

Rate dependent speech processing can be speech specific: Evidence from the perceptual disappearance of words under changes in context speech rate.

The perception of reduced syllables, including function words, produced in casual speech can be made to disappear by slowing the rate at which surrounding words are spoken (Dilley & Pitt, Psychological Science, 21(11), 1664–1670. doi: 10.1177/0956797610384743, 2010). The current study explored the domain generality of this speech-rate effect, asking whether it is induced by temporal information found only in speech. Stimuli were short word sequences (e.g., minor or child) appended to precursors that were clear speech, degraded speech (low-pass filtered or sinewave), or tone sequences, presented at a spoken rate and a slowed rate. Across three experiments, only precursors heard as intelligible speech generated a speech-rate effect (fewer reports of function words with a slowed context), suggesting that rate-dependent speech processing can be domain specific.

Auditory evoked potentials reveal early perceptual effects of distal prosody on speech segmentation

Prosodic context several syllables prior (i.e., distal) to an ambiguous word boundary influences speech segmentation. To assess whether distal prosody influences early perceptual processing or later lexical competition, EEG was recorded while subjects listened to eight-syllable sequences with ambiguous word boundaries for the last four syllables (e.g., tie murder bee vs. timer derby). Pitch and duration of the first five syllables were manipulated to induce sequence segmentation with either a monosyllabic or disyllabic final word. Behavioural results confirmed a successful manipulation. Moreover, penultimate syllables (e.g., der) elicited a larger anterior positivity 200–500 ms after the onset for prosodic contexts predicted to induce word-initial perception of these syllables. Final syllables (e.g. bee) elicited a similar anterior positivity in the context predicted to induce word-initial perception of these syllables. Additionally, these final syllables elicited a larger positive-to-negative deflection (P1-N1) 60–120 ms after onset, and a larger N400. The finding that prosodic characteristics of speech several syllables prior to ambiguous word boundaries modulate both early and late event-related potentials (ERPs) elicited by subsequent syllable onsets provides evidence that distal prosody influences early perceptual processing and later lexical competition.