Dysfluencies on function words in the speech of people who stutter mainly occur when function words precede, rather than follow, content words (Au-Yeung, Howell, & Pilgrim, 1998). It is hypothesized that such function word dysfluencies occur when the plan for the subsequent content word is not ready for execution. Repetition and hesitation on the function words buys time to complete the plan for the content word. Stuttering arises when speakers abandon the use of this delaying strategy and carry on, attempting production of the subsequent, partly prepared content word. To test these hypotheses, the relationship between dysfluency on function and content words was investigated in the spontaneous speech of 51 people who stutter and 68 people who do not stutter. These participants were subdivided into the following age groups: 2-6-year-olds, 7-9-year-olds, 10-12-year-olds, teenagers (13-18 years), and adults (20-40 years). Very few dysfluencies occurred for either fluency group on function words that occupied a position after a content word. For both fluency groups, dysfluency within each phonological word occurred predominantly on either the function word preceding the content word or on the content word itself, but not both. Fluent speakers had a higher percentage of dysfluency on initial function words than content words. Whether dysfluency occurred on initial function words or content words changed over age groups for speakers who stutter. For the 2-6-year-old speakers that stutter, there was a higher percentage of dysfluencies on initial function words than content words. In subsequent age groups, dysfluency decreased on function words and increased on content words. These data are interpreted as suggesting that fluent speakers use repetition of function words to delay production of the subsequent content words, whereas people who stutter carry on and attempt a content word on the basis of an incomplete plan.

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A theory is outlined that explains the disruption that occurs when auditory feedback is altered. The key part of the theory is that the number of, and relationship between, inputs to a timekeeper, operative during speech control, affects speech performance. The effects of alteration to auditory feedback depend on the extra input provided to the timekeeper. Different disruption is predicted for auditory feedback that is out of synchrony with other speech activity (e.g., delayed auditory feedback, DAF) compared with synchronous forms of altered feedback (e.g., frequency shifted feedback, FSF). Stimulus manipulations that can be made synchronously with speech are predicted to cause equivalent disruption to the synchronous form of altered feedback. Three experiments are reported. In all of them, subjects repeated a syllable at a fixed rate (Wing & Kristofferson, 1973). Overall timing variance was decomposed into the variance of a timekeeper (Cv) and the variance of a motor process (Mv). Experiment 1 validated Wing and Kristofferson's method for estimating Cv in a speech task by showing that only this variance component increased when subjects repeated syllables at different rates. Experiment 2 showed DAF increased Cv compared with when no altered sound occurred (Experiment 1) and compared with FSF. In Experiment 3, sections of the subject's output sequence were increased in amplitude. Subjects just heard this sound in one condition and made a duration decision about it in a second condition. When no response was made, results were like those with FSF. When a response was made, Cv increased at longer repetition periods. The findings that the principal effect of DAF, a duration decision and repetition period is on Cv whereas synchronous alterations that do not require a decision (amplitude increased sections where no response was made and FSF) do not affect Cv, support the hypothesis that the timekeeping process is affected by synchronized and asynchronized inputs in different ways.

The voiceless affricate/fricative contrast has played an important role in developing auditory theories of speech perception. This type of theory draws some of its support from experimental data on animals. However, nothing is known about differential responding of affricate/fricative continua by animals. In the current study, the ability of hooded rats to "label" an affricate/fricative continuum was tested. Transfer (without retraining) to analogous nonspeech continua was also tested. The nonspeech continua were chosen so that if transfer occurred, it would indicate whether the animals had learned to use rise time or duration cues to differentiate affricates from fricatives. The data from 9 of 10 rats indicated that rats can discriminate between these cues and do so in a similar manner to human subjects. The data from 9 of 10 rats also demonstrated that the rise time of the stimulus was the basis of the discrimination; the remaining rat appeared to use duration.

This program of work is intended to develop automatic recognition procedures to locate and assess stuttered dysfluencies. This and the preceding article focus on developing and testing recognizers for repetitions and prolongations in stuttered speech. The automatic recognizers classify the speech in two stages: In the first the speech is segmented and in the second the segments are categorized. The units segmented are words. The current article describes results for an automatic recognizer intended to classify words as fluent or containing a repetition or prolongation in a text read by children who stutter that contained the three types of words alone. Word segmentations are supplied and the classifier is an artificial neural network (ANN). Classification performance was assessed on material that was not used for training. Correct performance occurred when the ANN placed a word into the same category as the human judge whose material was used to train the ANNs. The best ANN correctly classified 95% of fluent, and 78% of dysfluent words in the test material.