The present paper integrates the results of experimental studies in which cognitive differences between stuttering and nonstuttering adults were investigated. In a monitoring experiment it was found that persons who stutter encode semantic information more slowly than nonstuttering persons. In dual-task experiments the two groups were compared in overt word-repetition and sentence-production experiments. The results of the two word-repetition experiments indicate that the speech of stuttering persons is sensitive to interference from concurrent attention-demanding cognitive processing-particularly when phonological coding is involved. In two sentence-generation and -production experiments it was found that under dual-task conditions stuttering persons produced sentences containing a smaller number of content units whereas persons who do not stutter did not show a significant single- vs. dual-task contrast. These results suggest that sentence generation and production required greater sustained attentional processing in stuttering than in nonstuttering persons and that persons who stutter reduce the amount of "conceptual work" in order to keep their stuttering rates low. Data from an fMRI-study indicate that in persons who stutter the neural systems activated during sentence generation and production overlap to a greater extent than those of persons who do not stutter. It is suggested that in persons who stutter neural subsystems involved in speech planning are "modularized" to a lesser extent than in persons who do not stutter.
The aim of the present experiment was to investigate differences between persons who stutter and persons who do not stutter during the production of sentences in a single task versus two dual-task conditions. Participants were required to form a sentence containing 2 unrelated nouns. In dual-task conditions, rhyme and category decisions were used as secondary tasks. The results for 14 adults who stutter and 16 adults who do not stutter are reported. Dependent variables were the number of correct rhyme and category decisions, decision latencies, length, number of propositions, sentence latency, speech rate of sentences, disfluencies, and stuttering rates. The results indicated that both groups reduced the average number of correct rhyme and category decisions when this task was performed concurrently with sentence generation and production. Similarly, the 2 groups of participants did not differ with respect to the correctness and latency of their decisions. Under single-task conditions the sentences of both groups had a comparable number of propositions. But under dual- as compared with single-task conditions persons who stutter significantly reduced the number of propositions whereas persons who do not stutter did not show a significant dual- versus single-task contrast. Experimental conditions did not significantly influence stuttering rates. These results suggest that persons who stutter require more processing capacity for sentence generation and articulation than persons who do not stutter and that both groups keep stuttering rates at a constant level by adjusting the number of propositional units of their linguistic productions. The results support the view that the organization of the speech-production system of persons who stutter makes it more vulnerable to interference from concurrent attention-demanding semantic tasks.
Longer rehearsal times presumably reduce the efficiency of rehearsal and, hence, of short-term recall. The present experiment examined the question as to whether the slower subvocalization rate of people who stutter is correlated with inferior short-term serial recall and recognition performance. Rate of overt articulation was taken as a measure of rehearsal time. Lists of four nonlexical CVC syllables were presented for short-term serial recall and for short-term recognition. Nineteen adults who stutter and 30 nonstutterers participated in the experiment. In the serial reproduction task the subjects who stuttered reproduced significantly fewer items correctly than did nonstutterers. Recognition performance was measured by nonparametric measures of sensitivity and bias as defined in signal detection theory. The stuttering subjects had a significantly lower sensitivity resulting primarily from a higher false alarm rate. Rate of overt articulation was significantly related to one measure of short-term recall but not to the sensitivity of recognition. These results were interpreted as suggesting that people who stutter have slower phonological encoding and rehearsal times, that they make less use of nonphonological forms of coding than do nonstutterers, and that within their phonological system, activation more easily spills over to similar items.
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