This study evaluated the possible influence of linguistic demands on speech motor control by measuring articulatory movement stability during conditions of increasing grammatical complexity. There were 60 participants in three age groups: 20-30 years, 40-50 years, and 60-70 years, with equal numbers of men and women in each group. These speakers produced 10 repetitions of five different sentence or phrase conditions. These five conditions included two baseline measurements and three sentences of varying complexity. Each complexity condition had an MLU count of 23, word length of 17, syllable length of 25, and contained the phrase open boxes of pompoms. Complexity was measured by node-count and grammatical structure. Lower lip movements during production of the target phrase were used to compute the spatiotemporal index (STI), a measure of lip movement stability over 10 repetitions. It was predicted that STI would be lower (indicating greater stability) in the baseline and low complexity conditions. Comparison of complexity conditions against the baseline-counting condition demonstrated significant differences in the upper lip's STI, displacement, and velocity, as well as in vocal intensity. Speech motor differences between the grammatical complexity levels were minimal and could be attributed to several factors, such as speaking rate or semantic differences. An unexpected finding of this study was the influence of age on speech production. Participants from the 60 year-old group had significantly longer utterance duration, while those from the 20 year-old group had the highest lower lip and jaw STI values. These findings suggest that speech motor control matures even beyond young adulthood and that linguistic complexity does not appear to have a consistent effect on speech movement variables.



College and Department

David O. McKay School of Education; Communication Disorders



Date Submitted


Document Type





divided attention, syntactic complexity, speech, language