Speech rate increase in primary progressive apraxia of speech and its cost on articulatory accuracy

Impaired articulation (e.g., articulatory accuracy) and prosody (e.g., slow speech rate) are considered primary diagnostic criterions for apraxia of speech both in neurodegenerative and post-stroke contexts. The primary aim of this study was to investigate the ability of participants with primary progressive apraxia of speech (PPAOS), a neurodegenerative disease characterised by initially isolated progressive apraxia of speech, to increase speech rate and the interaction between articulatory accuracy and speech rate. The secondary aim was to investigate the effect of syllable frequency and structure on this interaction. Four speakers with PPAOS, and four sex- and age-matched healthy speakers (HS) read eight two-syllable words embedded two times in a ten-syllable carrier phrase. Syllable frequency and structure were manipulated for the first syllable of the target words and controlled for the second syllable. All sentences were produced at three different target speech rates (conditions): habitual, regular (five syllables/second), and fast (seven syllables/second). Prosodic measures for target words and sentences were computed based on acoustic analysis of speech rate. Articulatory measures for words and sentences were rated based on a perceptual assessment of articulatory accuracy. Results show slower speech rate and reduced articulatory accuracy in speakers with PPAOS compared to HS. Results suggest that speakers with PPAOS also have limited ability to increase their speech rate. Finally, results suggest that articulatory complexity influences speech rate but that the cost of speech rate increase on articulatory accuracy varies greatly across speakers with PPAOS and is not necessarily related to the extent of the increase when measured in a highly structured sentence production task. Theoretical and clinical implications of these findings are discussed.