Purpose
This study was designed to test two hypotheses about apraxia of speech (AOS) derived from the DIVA model (Guenther et al., 2006): (1) the Feedforward System Deficit Hypothesis and (2) the Feedback System Deficit Hypothesis.
Method
We used noise masking to minimize auditory feedback during speech. Six speakers with AOS and aphasia, four with aphasia without AOS, and two unimpaired speaker groups (younger and older adults) participated. Acoustic measures of vowel contrast, variability, and duration were analyzed.
Results
Younger, but not older unimpaired speakers showed significantly reduced vowel contrast with noise masking. Relative to older controls, the AOS group showed longer vowel durations, and a greater reduction in vowel contrast under masking conditions. There were no significant differences in variability. Three of the six speakers with AOS demonstrated the group pattern. Speakers with aphasia without AOS did not differ from controls in contrast, duration, or variability.
Conclusion
The greater reduction in vowel contrast with masking noise for the AOS group is consistent with the Feedforward System Deficit Hypothesis but not with the Feedback System Deficit Hypothesis; however, effects were small and not present in all individual speakers with AOS. Theoretical implications and alternative interpretations of these findings are discussed.