|
Article Excerpt
This investigation was designed to examine articulatory variability over time in an individual with apraxia of speech (AOS) and aphasia. Speech recordings were made on three occasions within an 8-day period. The second and third recordings were conducted 1 day and 1 week following the initial session, respectively. Initial stop consonant productions were examined via perceptual, acoustic, and kinematic analyses. Perceptual analyses revealed that voicing errors were the predominant type of error across sampling times. Sound errors were not predictable for specific words within a sampling session, but were relatively invariable by sound. The AOS speaker produced more variable voice onset times (VOTs) for [TEXT NOT REPRODUCIBLE IN ASCII] and longer total word durations (TWD) across all sounds in comparison to a control speaker. Lower lip plus jaw movement data revealed increased displacement and velocity for alveolar stops and variable displacement and reduced velocity for bilabial stops for the AOS speaker when compared to the control speaker. For bilabial stops, spatiotemporal index and labial coordination were more variable for the AOS speaker. Extra velocity peaks were found across all sounds produced by the speaker with AOS. Acoustic and kinematic analyses revealed patterns across sounds; however; some distinct patterns were observed for individual sounds.
**********
Acquired apraxia of speech (AOS) is a neurogenic sensorimotor speech disorder associated with cortical and/or subcortical damage (McNeil, Robin, & Schmidt, 1997; Square, Martin, & Bose, 2001). Its primary identifying features are slow speech rate (evidenced by lengthened sound segments and intersegment durations), phoneme distortions, distorted sound substitutions, and prosodic abnormalities (McNeil et al., 1997). Other clinical characteristics that frequently occur with AOS, but that are nondiscriminatory in terms of differential diagnosis, include articulatory groping, perseverative errors, increasing errors with increasing word length, and speech initiation difficulties (McNeil et al., 1997).
The characterization of AOS has evolved since its initial description, and controversy remains with respect to several speech behaviors. In particular, the variability, or invariability, of sound errors in AOS has not been definitively established (Croot, 2002). A high degree of variability in errors was long considered a hallmark of AOS (Deal & Darley, 1972; Johns & Darley, 1970; Wertz, LaPointe, & Rosenbek, 1984). Speakers with AOS were thought to demonstrate errors that were highly unpredictable in terms of both the location of the error (Johns & Darley, 1970; LaPointe & Johns, 1975) and the nature of the error (i.e., variability of error type; Johns & Darley, 1970; LaPointe & Johns, 1975) across repeated productions of the same target. Although early investigations reported a high degree of variability for groups of participants with AOS (Johns & Darley, 1970; LaPointe & Horner, 1976), later investigations suggested that AOS errors may not be extremely variable (McNeil, Odell, Miller, & Hunter, 1995; Mlcoch, Darley, & Noll, 1982; Skenes & Trullinger, 1988). In terms of speech production, variability can be defined as a lack of uniformity in a particular task (i.e., producing a word differently) over repeated trials, whereas consistency is the ability to repeat a particular task (i.e., produce a word similarly) over a number of trials with minimal variation.
It is not surprising that the notion of high variability of errors has been called into question. The oft-cited investigation of variability by Johns and Darley (1970) provided data that indicated that as a group, 10 participants with AOS varied in overall articulatory accuracy across type of word (nonsense words elicited more errors than real words) and mode of stimulus presentation (auditory only mode evoked more errors than auditory plus visual mode). Johns and Darley provided a summarization of sound errors made by the AOS group in terms of percentages of overall errors and types of errors per sound. Although the types of errors generally varied widely across and within sounds, some predictability of errors was evident for the group for certain sounds. For example, errors in production of [TEXT NOT REPRODUCIBLE IN ASCII] took the form of an intrusive schwa 77% of the time. Johns and Darley did not provide data for the AOS group, or AOS individuals, relative to consistency of error location or error type across repeated productions. Only anecdotal evidence was offered regarding variability of performance within individuals. Their claims that "inconsistency and unpredictability--best describe the efforts of the apraxic group" (p. 580), appear to be derived from the fact that sound errors were not homogenous within the group and that stimulus conditions differentially impacted the articulatory accuracy of the group.
The effect of stimulus conditions on articulatory accuracy in AOS was further elucidated by Deal and Darley (1972). These investigators found that for a group of 12 AOS participants, "errors were not scattered randomly ... but tended to occur in a somewhat predictable fashion" (p. 644). Specifically, error occurrence was associated with the weighting of words in terms of grammatical class and length. Furthermore, they examined the types of errors made by the group across experimental conditions and found a high degree of consistency in the rank ordering of the types of errors. Deal and Darley's findings illustrate the importance of attempting to understand and control variability rather than necessarily ascribing variability to the disorder.
A potential confound that may relate to the large ranges in reported variability across subjects and that pertains to many early investigations of AOS is the issue of possible inclusion of participants with phonemic paraphasias (McNeil et al., 1997). For example, LaPointe and Horner (1976) specifically selected participants who were considered to have "phonological selection-sequencing" impairments (p. 263). Therefore, it is likely that at least some sound errors reflected phonological level disruptions rather than sensorimotor disruptions (i.e., linguistic disturbances rather than AOS). Of course, most individuals with AOS will also have co-occurring aphasia (Duffy, 2005). Consequently, there are few investigations of AOS involving participants with relative "pure" AOS.
Although Deal and Darley's (1972) investigation provided important insights into the issue of variability and consistency in AOS, it did not address the issue of individual variability in performance. A thorough understanding of variability necessitates study of the individual (Barlow & Hersen, 1984; Johnston & Pennypacker, 1993). LaPointe and Horner (1976) investigated group variability, as well as individual variability, of repeated productions with seven individuals with AOS and varying degrees of aphasia. Five participants in the study suffered a left cerebral vascular accident (CVA) and two suffered a right CVA. The diagnosis of AOS was based on participants exhibiting predominantly substitution errors, variable error patterns on repeated trials of polysyllabic words, and/or difficulty initiating speech. Participants were asked to repeat stimulus words 10 times each following a model, and consistency and variability analyses were conducted; however it was unclear how data were analyzed (i.e., orthographic transcription or phonetic transcription). Stimuli was comprised of five monosyllable words (i.e., words included two or three initial consonant clusters) and five polysyllable words (three syllables or greater). For the group, the consistency analysis was conducted to compare accuracy of production of the first five repetitions to the last five repetitions. Findings revealed no significant difference in accuracy levels, although a trend toward deterioration was noted. For the individual participants, LaPointe and Horner examined the variability in production over the course of the 10 repetitions of target words. Each repeated production was examined relative to its similarity to the preceding production and a percentage of variability was calculated. Variability of productions ranged from 23.3% to 50.8%, with four of the seven participants demonstrating approximately 50% variability.
In a similar investigation, Skenes and Trullinger (1988) reported comparable findings from an investigation in which nine participants with AOS were asked to produce four consecutive repetitions of monosyllabic words comprised of initial and final stop consonants (i.e., [TEXT NOT REPRODUCIBLE IN ASCII]) in three vowel contexts (i.e., bab, bib, bub, dad, did, dud). Eight of the participants suffered a left CVA, and one suffered left hemisphere brain damage secondary to tumor excision. Participants were judged to have AOS if they exhibited effortful trial and error groping, dysprosody, inconsistent errors on repeated trials, and difficulty initiating speech during a motor speech examination conducted by one of the authors. Participant's productions were phonetically transcribed and judged to be correct or incorrect productions of the target. Authors also compared the first and last productions of target words and scored them as same or different. Findings revealed that 66% of the comparisons yielded the same productions for the group. However, individual agreement between the two trials for each participant ranged from 25% to 100%. The findings from LaPointe and Horner (1976) and Skenes and Trullinger (1988) highlight the need to study variability in AOS at the level of the individual.
McNeil et al. (1995) addressed both variability of error type and consistency of error location in an investigation of repeated productions with four speakers with pure AOS. Three of the participants suffered a CVA and the other participant suffered surgical sequela following tumor excision. The diagnosis of AOS was based on individuals exhibiting effortful trial and error groping upon initiation of speech gestures, frequent single featured sound substitutions, articulation and prosody similar on imitative and spontaneous speech tasks, and variable articulation and prosody on repeated trials of the same utterance. Aphasia was ruled out utilizing standardized testing. Narrow phonetic transcription was utilized to examine variability of error type across three productions of multisyllabic words. An average of only 13% of the errors varied across successive productions with a range of 0% to 16%. Error location was found to be highly consistent, with errors occurring in the same location 90% of the time (range 86%-94%).
Recent evidence suggests that variability of error type may differ across sounds within an individual with AOS and accompanying aphasia (Wambaugh, Nessler, Bennett, & Mauszycki, 2004). Wambaugh et al. (2004) examined repeated productions (not successive) of monosyllabic words beginning with the stop consonants [TEXT NOT REPRODUCIBLE IN ASCII] as produced by an individual with AOS and aphasia subsequent to a left CVA. The speaker's speech production was described as being consistent with AOS characteristics described by McNeil et al. (1997) (e.g., consistently reduced rate of speech production, distorted sound errors, segregated syllable production, disrupted prosody). Target words were elicited through randomized and blocked (by sound) presentation modes on three separate occasions. Productions were transcribed using broad phonetic transcription, and analyses were conducted to determine if the same type of error occurred across different exemplars for a sound within and across sampling times and conditions. Findings revealed that variability in error types across sounds ranged from 0% to 58%. That is, the same type of error (e.g., voicing error) occurred for all of the errors made on some sounds, whereas the type of error varied approximately half of the time for other sounds). Voiced sounds in the blocked condition were the...
|
