Speech perception in rats: use of duration and rise time cues in labeling of affricate/fricative sounds.

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.

Timing interference to speech in altered listening conditions.

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 and 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.

Speech Rate Modification and Its Effects on Fluency Reversal in Fluent Speakers and People Who Stutter.

A theory has been proposed recently that asserts that the problem that people who stutter have arises when these speakers attempt to execute speech at a faster rate than planning processes allow. This leads speakers to complete words before the following one is ready. Plan unavailability usually happens on the more complex content words rather than the relatively simple function words. There are two ways of dealing with this situation when it arises. Speakers can (1) delay production of a content word by repeating prior function words or (2) carry on and attempt to produce the following content word and gamble that the remainder of the plan arrives while it is being executed. The former strategy does not lead speakers to persist in their dysfluency, while the latter does. It is proposed that the pressure on speech rate that leads speakers to adopt the latter strategy is particularly acute around adolescence. In this article two experiments are reported which test the effects of rate on fluency. In Experiment 1, fluent speakers are induced to produce stuttering-like dysfluencies on content words using a commentary task. A prediction of the theory is that procedures known to induce fluency have to produce local slowing of speech so that planning and execution can get back in synchrony. This prediction is confirmed for frequency-shifted feedback and when speakers who stutter have to sing in Experiment 2. Results are discussed in terms of the model for the etiology of stuttering based on plan unavailability. The implications of the results are also discussed with respect to the diagnosis of the disorder and how it can be treated.

INTERNAL STRUCTURE OF CONTENT WORDS LEADING TO LIFESPAN DIFFERENCES IN PHONOLOGICAL DIFFICULTY IN STUTTERING.

This study investigated whether frequency of stuttering was affected by factors that specify the phonological difficulty of a sound and whether and how any influences vary across age groups. Analyses were performed separately on content words and function words. The phonological factors examined were: a) Whether the word contained a late emerging consonant (LEC); and b) Whether the word contained a consonant string (CS). Analyses showed that these factors occurred at different rates across the age groups used (children under 12, teenagers between 12 and 18, and adults). A more detailed breakdown was also reported of frequency of usage of LEC and CS over age groups depending on whether and where these factors occurred in the content words; all nine combinations of no LEC, word-initial LEC, non-initial LEC with no CS, word-initial CS, and non-initial CS were examined. Usage of certain of these nine categories varied over age groups. Friedman statistic on the ratio of stuttering (proportion of stuttered words in a particular word class divided by the proportion of words in that particular word class) showed that the frequency of stuttering remained high for adult speakers when CS and LEC both occurred in a word and when they appeared in word-initial position. These findings support a recently proposed theory that accounts for life-span changes in stuttering.

Differential effects of frequency shifted feedback between child and adult stutterers.

It has been reported previously that presentation of an altered form of the voice enhances the fluency of people who stutter. One of these forms of alteration is frequency shifted feedback. The effects of frequency shifted feedback was compared between two speaker groups that differed in age. The fluency enhancing effects of frequency shifted feedback was greater for adult speakers (mean age 21;3) than for children (mean age 9;11). The results are discussed in terms of their implications for theory and treatment.