The purpose of this study was to determine perceived articulation rates in a subset of boys with FXS and CA controls previously investigated by Zajac et al. (2006)
. Two specific research questions were proposed: 1) Do listeners perceive boys with FXS as speaking faster than CA controls even when utterances are matched on overall acoustically-determined articulation rate? 2) If so, what factors within the utterances account for perceived articulation rate?
Relative to the first question, we found that when overall articulation rate in syllables per second was matched among groups, boys with FXS-ASD were judged to sound faster than CA boys. Although boys with FXS-O were also judged to sound faster than CA boys, the difference was not significant. These findings are somewhat at odds with previous research which suggests that FXS speakers are judged to exhibit a fast and fluctuating rate. While acoustic analysis of boys with FXS-ASD in our study revealed rate fluctuations, boys with FXS-O did not exhibit rate fluctuations compared to CA boys. We must emphasize, however, that previous studies have not reported the autism status of FXS speakers. Given that ASD may co-occur with FXS in up to 50% of cases (Hall et al., 2008
), it is possible that previous reports may have focused on speakers who had both diagnoses.
Relative to the second question, we found that both boys with FXS-ASD and FXS-O exhibited higher F0 levels and greater final F0 drops than CA boys. In addition, boys with FXS-ASD exhibited faster articulation rates minus the final word of the sentence (rate_mfw) than the other groups of boys. Multiple regression analysis, however, indicated that only rate_mfw and final F0 drop predicted approximately 75% and 16% of the variance in perceived articulation rate, respectively. While the large effect of rate_mfw was not unexpected, it highlights the perceptual influence of rate fluctuations within an utterance on perceived rate. The lack of diagnostic group as a significant predictor in the regression model was somewhat unexpected given that group was a significant effect in the ANOVA. The loss of group suggests that this variable was highly collinear with rate_mfw and final F0 drop. Also, we must note that power was reduced for the multiple regression analysis compared to the ANOVA given that 6 boys were omitted due to missing F0 data.
The above findings relative to F0
are generally consistent with previous research which has shown that overall pitch level and changes in pitch influence perception of articulation rate (e.g., Ainsworth & Lindsay, 1986
; Kohler, 1986
; Rietveld & Gussenhoven, 1987
). Kohler (1986)
, for example, conducted experiments using German speakers and listeners to show that the perception of speech rate is “multidimensional” and depends upon “various aspects of duration and F0
” (p. 135). Specifically, Kohler reported that listeners perceived faster speech when overall F0
levels were high and when utterance-final F0
was rising; listeners perceived slower speech when utterance-final F0
was falling. Kohler further noted that a) “sentence-internal F0
structure” involving pitch movement was dominant over global pitch level, and b) falling F0
was a “stronger” perceptual cue than rising F0
Given the perceptual importance of falling F0
suggested by Kohler (1986)
, one might question the relatively fast articulation rate estimated by listeners in the present study for boys with FXS-ASD who showed the largest final F0
drop. This apparent discrepancy may be explained by a combination of a) faster articulation rates at the beginning of the sentence, and b) the use of an extreme rise-fall F0
pattern at the end of the sentence. Relative to the latter, it may be that an extreme F0
rise is perceptually more salient than a F0
fall when both occur within a sentence. Regardless of the actual perceptual mechanisms, the present findings provide additional evidence of the role of sentence-final intonation contours on the perception of rate.
In the present study, boys with FXS-ASD used a fast articulation rate during the initial part of a sentence and a perceptually distinctive rise/fall F0
pattern at the end of a sentence. Atypical stress patterns have been reported to be characteristic of speakers with ASD. Indeed, Shriberg et al. (2001)
state, “… the prosody characteristics of a person with autism constitute one of the most significant obstacles to his or her social integration and vocational acceptance” (p. 1099). We believe that targeting rate and/or atypical prosodic patterns of boys with FXS-ASD in speech intervention could possibly result in reduced social stigma associated with perceptually “unusual” or “peculiar” speech. Obviously, research is needed to explore the effectiveness of such intervention in boys with FXS-ASD.
Several limitations of this study exist. First, some listeners find DME procedures difficult to perform even following adequate instructions and practice. As reported above, one listener in the second DME task showed relatively low intra-listener reliability. Although inter-listener reliability in the main DME task was high, we did not obtain repeated judgments for intra-listener reliability in this task. Second, because we separated boys with FXS into groups based upon autism status, the numbers in each group were small. Power was additionally reduced for the multiple regression analysis given the omission of some boys due to missing F0 data. This may have contributed to the loss of diagnostic group as a significant predictor. Last, because we used a sentence imitation task, the present findings may not be characteristic of boys with FXS during conversational speech.
A fast and/or fluctuating rate of speaking is a perceptual characteristic often reported for boys with FXS. Zajac et al. (2006)
reported no significant difference between boys with FXS and CA controls during conversation when overall articulation rate was calculated in syllables per second across utterances. In that study, however, boys with FXS used shorter utterances and a tendency to pause less often. In the present study, we used a sentence imitation task to control for utterance duration and pausing while we determined perceived articulation rates. The findings confirm previous descriptions of a perceptually fast and fluctuating articulation rate only among boys with FXS and a co-diagnosis of ASD. The findings also suggest that atypical sentence-final intonation may contribute to perceived articulation rate in boys with FXS-ASD.