The effect of speaking rate on serial order sound-level errors in non-brain damaged participants and persons with aphasia

While many speech errors can be generated at either a linguistic or motoric level of production, phonetically well formed sound-level serial order errors are generally assumed to result from disruption of phonologic encoding (PE) processes. An influential model of PE (Dell, 1986; Dell, Burger & Svec, 1997) predicts that speaking rate should affect the relative proportion of these serial order sound errors (anticipations, perseverations, exchanges). These predictions have been extended to, and have special relevance for persons with aphasia (PWA) because of the increased frequency with which speech errors occur and because their localization within the functional linguistic architecture may help in treatment management. Supporting evidence regarding the effect of speaking rate on phonological encoding has been provided by studies using young normal language (NL) speakers and computer simulations. Limited data exist at present for older NL users and no group data exist for PWA. The purpose of this study was to examine the effects of speech rate on the ratio of phonological sequencing errors (anticipation/exchange (AE), anticipation/perseveration) (AP)) and other error types (vocal reaction time and distortion) in non-brain-damaged individuals and in persons with aphasia who were without concomitant motor speech disorders. Sixteen NL users and 16 PWA performed a phonologically challenging (tongue twister) speech production task at their typical and two faster speaking rates. A significant effect of rate was obtained for the AP ratio but not for the other comparisons. Contrary to the predictions of the model, the AP ratio increased with faster speaking rates. There was also a significant effect of rate and group for the VRT measure. The results for the serial order error ratios did not provide support for the model derived predictions regarding the direction of change for error type proportions. However, the significant effect of rate for the AP ratio provided support that changes in speaking rate did affect phonological encoding. Additionally, the results suggest that the relationships among slow post-selection inhibition and normal residual activation functions postulated to create an increase in perseverations relative to anticipation serial order errors, needs to be reconsidered within the Dell, et al. (1997) model