Effects of Speaking Rate on Variability of Second Formant Frequency Transitions in Dysarthria.

INTRODUCTION: This study examined the utility of multiple second formant (F2) slope metrics to capture differences in speech production for individuals with dysarthria and healthy controls as a function of speaking rate. In addition, the utility of F2 slope metrics for predicting severity of intelligibility impairment in dysarthria was examined. METHODS: 23 speakers with Parkinson's disease and mild to moderate hypokinetic dysarthria (HD), 9 speakers with various neurological diseases and mild to severe ataxic dysarthria (AD), and 26 age-matched healthy control speakers (CON) participated in a sentence repetition task. Sentences were produced at habitual, fast, and slow speaking rate. A variety of metrics were derived from the rising second formant (F2) transition portion of the diphthong /ai/. To obtain measures of intelligibility for the two clinical speaker groups, 15 undergraduate SLP students participated in a transcription experiment. RESULTS: Significantly shallower slopes were found for the speakers with hypokinetic dysarthria compared to control speakers. Steeper F2 slopes were associated with increased speaking rate for all groups. Higher variability in F2 slope metrics was found for the speakers with ataxic dysarthria compared to the two other speaker groups. For both clinical speaker groups, there was a negative association between intelligibility and F2 slope variability metrics, indicating lower variability in speech production was associated with higher intelligibility. DISCUSSION: F2 slope metrics were sensitive to dysarthria presence, dysarthria type and speaking rate. The current study provided evidence that the use of F2 slope variability measures has additional value to F2 slope averaged measures for predicting severity of intelligibility impairment in dysarthria.

Impact of Cognitive Impairment and Dysarthria on Spoken Language in Multiple Sclerosis.

OBJECTIVE: To investigate the impact of cognitive impairment on spoken language produced by speakers with multiple sclerosis (MS) with and without dysarthria. METHOD: Sixty speakers comprised operationally defined groups. Speakers produced a spontaneous speech sample to obtain speech timing measures of speech rate, articulation rate, and silent pause frequency and duration. Twenty listeners judged the overall perceptual severity of the samples using a visual analog scale that ranged from no impairment to severe impairment (speech severity). A 2 × 2 factorial design examined main and interaction effects of dysarthria and cognitive impairment on speech timing measures and speech severity in individuals with MS. Each speaker group with MS was further compared to a healthy control group. Exploratory regression analyses examined relationships between cognitive and biopsychosocial variables and speech timing measures and perceptual judgments of speech severity, for speakers with MS. RESULTS: Speech timing was significantly slower for speakers with dysarthria compared to speakers with MS without dysarthria. Silent pause durations also significantly differed for speakers with both dysarthria and cognitive impairment compared to MS speakers without either impairment. Significant interactions between dysarthria and cognitive factors revealed comorbid dysarthria and cognitive impairment contributed to slowed speech rates in MS, whereas dysarthria alone impacted perceptual judgments of speech severity. Speech severity was strongly related to pause duration. CONCLUSIONS: The findings suggest the nature in which dysarthria and cognitive symptoms manifest in objective, acoustic measures of speech timing and perceptual judgments of severity is complex.

Relationship between acoustic measures and judgments of intelligibility in Parkinson's disease: a within-speaker approach.

This study investigated the acoustic basis of within-speaker, across-utterance variation in sentence intelligibility for 12 speakers with dysarthria secondary to Parkinson's disease (PD). Acoustic measures were also obtained for 12 healthy controls for comparison to speakers with PD. Speakers read sentences using their typical speech style. Acoustic measures of speech rate, articulatory rate, fundamental frequency, sound pressure level and F2 interquartile range (F2 IQR) were obtained. A group of listeners judged sentence intelligibility using a computerized visual-analog scale. Relationships between judgments of intelligibility and acoustic measures were determined for individual speakers with PD. Relationships among acoustic measures were also quantified. Although considerable variability was noted, articulatory rate, fundamental frequency and F2 IQR were most frequently associated with within-speaker variation in sentence intelligibility. Results suggest that diversity among speakers with PD should be considered when interpreting results from group analyses.

No differential effects of subthalamic nucleus vs. globus pallidus deep brain stimulation in Parkinson's disease: Speech acoustic and perceptual findings.

Background: Deep Brain Stimulation (DBS) in the Subthalamic Nucleus (STN) or the Globus Pallidus Interna (GPI) is well-established as a surgical technique for improving global motor function in patients with idiopathic Parkinson's Disease (PD). Previous research has indicated speech deterioration in more than 30% of patients after STN-DBS implantation, whilst speech outcomes following GPI-DBS have received far less attention. Research comparing speech outcomes for patients with PD receiving STN-DBS and GPI-DBS can inform pre-surgical counseling and assist with clinician and patient decision-making when considering the neural targets selected for DBS-implantation. The aims of this pilot study were (1) to compare perceptual and acoustic speech outcomes for a group of patients with PD receiving bilateral DBS in the STN or the GPI with DBS stimulation both ON and OFF, and (2) examine associations between acoustic and perceptual speech measures and clinical characteristics. Methods: Ten individuals with PD receiving STN-DBS and eight individuals receiving GPI-DBS were audio-recorded reading a passage. Three listeners blinded to neural target and stimulation condition provided perceptual judgments of intelligibility and overall speech severity. Speech acoustic measures were obtained from the recordings. Acoustic and perceptual measures and clinical characteristics were compared for the two neural targets and stimulation conditions. Results: Intelligibility and speech severity were not significantly different across neural target or stimulation conditions. Generally, acoustic measures were also not statistically different for the two neural targets or stimulation conditions. Acoustic measures reflecting more varied speech prosody were associated with improved intelligibility and lessened severity. Convergent correlations were found between UPDRS-III speech scores and perceptual measures of intelligibility and severity. Conclusion: This study reports a systematic comparison of perceptual and acoustic speech outcomes following STN-DBS and GPI-DBS. Statistically significant differences in acoustic measures for the two neural targets were small in magnitude and did not yield group differences in perceptual measures. The absence of robust differences in speech outcomes for the two neural targets has implications for pre-surgical counseling. Results provide preliminary support for reliance on considerations other than speech when selecting the target for DBS in patients with PD.

Influence of cognitive function on speech and articulation rate in multiple sclerosis.

We examined cognitive predictors of speech and articulation rate in 50 individuals with multiple sclerosis (MS) and 23 healthy controls. We measured speech and articulation rate from audio-recordings of participants reading aloud and talking extemporaneously on a topic of their choice (i.e., self-generated speech). Articulation rate was calculated for each speech sample by removing lexically irrelevant vocalizations and pauses of >200 ms. Speech rate was similarly calculated including pauses. Concurrently, the Minimal Assessment of Cognitive Function in Multiple Sclerosis (MACFIMS) battery, as well as standardized tests of sentence intelligibility and syllable repetition were administered. Analysis of variance showed that MS patients were slower on three of the four rate measures. Greater variance in rate measures was accounted for by cognitive variables for the MS group than controls. An information processing speed composite, as measured by the Symbol Digit Modalities Test (SDMT) and the Paced Auditory Serial Addition Test (PASAT), was the strongest predictor among cognitive tests. A composite of memory tests related to self-generated speech, above and beyond information processing speed, but not to oral reading. Self-generated speech, in this study, was not found to relate more strongly to cognitive tests than simple reading. Implications for further research are discussed.

Acoustic-perceptual relationships in variants of clear speech.

OBJECTIVE: The acoustic basis of intelligibility associated with varied clear speech instructions was studied. METHODS: Twelve healthy speakers read 18 sentences in 'habitual', 'clear', 'hearing impaired' and 'overenunciate' conditions. The latter 3 conditions are varieties of clear speech. Acoustic measures included tense and lax vowel space area, a measure of vowel spectral change, articulation rate and sentence-level vocal intensity. Sentences were mixed with multitalker babble to prevent ceiling effects and were orthographically transcribed by 40 listeners. Percent-correct scores were obtained for each speaker and condition. Regression analyses were used to quantify relationships between acoustic measures and intelligibility. RESULTS: Univariate regressions indicated that greater magnitudes of acoustic change in nonhabitual conditions were associated with greater increases in intelligibility. Multivariate regression analysis further indicated that lax vowel space, articulation rate and vocal intensity were significant predictors of intelligibility. CONCLUSIONS: Acoustic variables associated with intelligibility differed depending on whether relationships were examined using univariate or multivariate statistics. Multivariate statistics indicated that articulation rate was the strongest predictor of improvements in intelligibility above and beyond all other variables studied. The findings have implications for optimizing therapeutic use of clear speech for clinical populations.

Acoustic and perceptual consequences of clear and loud speech.

OBJECTIVE: Several issues concerning F2 slope in dysarthria were addressed by obtaining speech acoustic measures and judgments of intelligibility for sentences produced in Habitual, Clear and Loud conditions by speakers with Parkinson's disease (PD) and healthy controls. PATIENTS AND METHODS: Acoustic measures of average and maximum F2 slope for diphthongs, duration and intensity were obtained. Listeners judged intelligibility using a visual analog scale. Differences in measures among groups and conditions as well as relationships among measures were examined. RESULTS: Average and maximum F2 slope metrics were strongly correlated, but only average F2 slope consistently differed among groups and conditions, with shallower slopes for the PD group and steeper slopes for Clear speech versus Habitual and Loud. Clear and Loud speech were also characterized by lengthened durations, increased intensity and improved intelligibility versus Habitual. F2 slope and intensity were unrelated, and F2 slope was a significant predictor of intelligibility. CONCLUSION: Average diphthong F2 slope was more sensitive than maximum F2 slope to articulatory mechanism involvement in mild dysarthria in PD. F2 slope holds promise as an objective measure of treatment-related changes in the articulatory mechanism for therapeutic techniques that focus on articulation.

Speech and pause characteristics in multiple sclerosis: a preliminary study of speakers with high and low neuropsychological test performance.

This preliminary study investigated how cognitive-linguistic status in multiple sclerosis (MS) is reflected in two speech tasks (i.e. oral reading, narrative) that differ in cognitive-linguistic demand. Twenty individuals with MS were selected to comprise High and Low performance groups based on clinical tests of executive function and information processing speed and efficiency. Ten healthy controls were included for comparison. Speech samples were audio-recorded and measures of global speech timing were obtained. Results indicated predicted differences in global speech timing (i.e. speech rate and pause characteristics) for speech tasks differing in cognitive-linguistic demand, but the magnitude of these task-related differences was similar for all speaker groups. Findings suggest that assumptions concerning the cognitive-linguistic demands of reading aloud as compared to spontaneous speech may need to be re-considered for individuals with cognitive impairment. Qualitative trends suggest that additional studies investigating the association between cognitive-linguistic and speech motor variables in MS are warranted.

Lexical Characteristics of the Speech Intelligibility Test: Effects on Transcription Intelligibility for Speakers With Multiple Sclerosis and Parkinson's Disease.

PURPOSE: Lexical characteristics of speech stimuli can significantly impact intelligibility. However, lexical characteristics of the widely used Speech Intelligibility Test (SIT) are unknown. We aimed to (a) define variation in neighborhood density, word frequency, grammatical word class, and type-token ratio across a large corpus of SIT sentences and tests and (b) determine the relationship of lexical characteristics to speech intelligibility in speakers with multiple sclerosis (MS), Parkinson's disease (PD), and neurologically healthy controls. METHOD: Using an extant database of 92 speakers (32 controls, 30 speakers with MS, and 30 speakers with PD), percent correct intelligibility scores were obtained for the SIT. Neighborhood density, word frequency, word class, and type-token ratio were calculated and summed for each of the 11 sentences of each SIT test. The distribution of each characteristic across SIT sentences and tests was examined. Linear mixed-effects models were performed to assess the relationship between intelligibility and the lexical characteristics. RESULTS: There was large variability in the distribution of lexical characteristics across this large corpus of SIT sentences and tests. Modeling revealed a relationship between intelligibility and the lexical characteristics, with word frequency and word class significantly contributing to the model. CONCLUSIONS: Three primary findings emerged: (a) There was considerable variability in lexical characteristics both within and across the large corpus of SIT tests; (b) there was not a robust association between intelligibility and the lexical characteristics; and (c) findings from a study demonstrating an effect of neighborhood density and word frequency on intelligibility were replicated. Clinical and research implications of the findings are discussed, and three exemplar SIT tests systematically controlling for neighborhood density and word frequency are provided.

Premotor cortex is hypoactive during sustained vowel production in individuals with Parkinson's disease and hypophonia.

Introduction: Hypophonia is a common feature of Parkinson's disease (PD); however, the contribution of motor cortical activity to reduced phonatory scaling in PD is still not clear. Methods: In this study, we employed a sustained vowel production task during functional magnetic resonance imaging to compare brain activity between individuals with PD and hypophonia and an older healthy control (OHC) group. Results: When comparing vowel production versus rest, the PD group showed fewer regions with significant BOLD activity compared to OHCs. Within the motor cortices, both OHC and PD groups showed bilateral activation of the laryngeal/phonatory area (LPA) of the primary motor cortex as well as activation of the supplementary motor area. The OHC group also recruited additional activity in the bilateral trunk motor area and right dorsal premotor cortex (PMd). A voxel-wise comparison of PD and HC groups showed that activity in right PMd was significantly lower in the PD group compared to OHC (p < 0.001, uncorrected). Right PMd activity was positively correlated with maximum phonation time in the PD group and negatively correlated with perceptual severity ratings of loudness and pitch. Discussion: Our findings suggest that hypoactivation of PMd may be associated with abnormal phonatory control in PD.

Human subthalamic nucleus neurons differentially encode speech and limb movement.

Deep brain stimulation (DBS) of the subthalamic nucleus (STN), which consistently improves limb motor functions, shows mixed effects on speech functions in Parkinson's disease (PD). One possible explanation for this discrepancy is that STN neurons may differentially encode speech and limb movement. However, this hypothesis has not yet been tested. We examined how STN is modulated by limb movement and speech by recording 69 single- and multi-unit neuronal clusters in 12 intraoperative PD patients. Our findings indicated: (1) diverse patterns of modulation in neuronal firing rates in STN for speech and limb movement; (2) a higher number of STN neurons were modulated by speech vs. limb movement; (3) an overall increase in neuronal firing rates for speech vs. limb movement; and (4) participants with longer disease duration had higher firing rates. These data provide new insights into the role of STN neurons in speech and limb movement.

Minimally Detectable Change of Speech Intelligibility in Speakers With Multiple Sclerosis and Parkinson's Disease.

PURPOSE: This study sought to determine the minimally detectable change (MDC) of sentence intelligibility for speakers with multiple sclerosis (MS) and Parkinson's disease (PD). METHOD: Speakers included 78 participants consisting of 32 neurologically healthy control speakers, 30 speakers with MS, and 16 speakers with PD. All speakers were recorded reading 11 sentences comprising the Speech Intelligibility Test (SIT), which were subsequently transcribed by inexperienced listeners. Percent correct scores were calculated for each sentence. An average percent correct score was also calculated for each speaker. The MDC at the 95% confidence interval was calculated using the following formula: MDC95 = 1.96 × âˆš2 × standard error of measurement. Speakers were divided into operationally defined categories of severity, and MDCs were calculated for each category to permit comparison to MDCs reported for speakers with amyotrophic lateral sclerosis (ALS). Kruskal-Wallis tests were conducted to compare MDCs between groups and severity categories. RESULTS: The average MDC95 for control speakers was 5.53% (range = 3.21%-7.47%) and was statistically smaller than MDCs for speakers with MS (average = 10.08%, range = 5.30%-15.62%) and those for speakers with PD (average = 10.98%, range = 8.60%-13.98%). Statistical analyses further revealed significant differences between MDCs across severity categories. CONCLUSIONS: In agreement with previous work in ALS conducted under similar conditions (i.e., orthographic transcription of SIT sentences in a quiet listening environment), the MDC95 of speech intelligibility ranged from 3% to 10% for speakers with MS and PD who have mildly impaired speech. These estimates are a step toward the development of a universal language with which to evaluate speech changes in a variety of patient populations.

Intelligibility Across a Reading Passage: The Effect of Dysarthria and Cued Speaking Styles.

OBJECTIVE: Reading a passage out loud is a commonly used task in the perceptual assessment of dysarthria. The extent to which perceptual characteristics remain unchanged or stable over the time course of a passage is largely unknown. This study investigated crowdsourced visual analogue scale (VAS) judgments of intelligibility across a reading passage as a function of cued speaking styles commonly used in treatment to maximize intelligibility. PATIENTS AND METHOD: The Hunter passage was read aloud in habitual, slow, loud, and clear speaking styles by 16 speakers with Parkinson's disease (PD), 30 speakers with multiple sclerosis (MS), and 32 control speakers. VAS judgments of intelligibility from three fragments representing the beginning, middle, and end of the reading passage were obtained from 540 crowdsourced online listeners. RESULTS: Overall passage intelligibility was reduced for the two clinical groups relative to the control group. All speaker groups exhibited intelligibility variation across the reading passage, with trends of increased intelligibility toward the end of the reading passage. For control speakers and speakers with PD, patterns of intelligibility variation across passage reading did not differ with speaking style. For the MS group, intelligibility variation across the passage was dependent on speaking style. CONCLUSIONS: The presence of intelligibility variation within a reading passage warrants careful selection of speech materials in research and clinical practice. Results further indicate that the crowdsourced VAS rating paradigm is useful to document intelligibility in a reading passage for different cued speaking styles commonly used in treatment for dysarthria.

Speech dysfunction, cognition, and Parkinson's disease.

Communication difficulties are a ubiquitous symptom of Parkinson's disease and include changes to both motor speech and language systems. Communication challenges are a significant driver of lower quality of life. They are associated with decreased communication participation, social withdrawal, and increased risks for social isolation and stigmatization in persons with Parkinson's disease. Recent theoretical advances and experimental evidence underscore the intersection of cognition and motor processes in speech production and their impact on spoken language. This chapter overviews a growing evidence base demonstrating that cognitive impairments interact with motor changes in Parkinson's disease to negatively affect communication abilities in myriad ways, at all stages of the disease, both in the absence and presence of dementia. The chapter highlights common PD interventions (pharmacological, surgical, and non-pharmacological) and how cognitive influences on speech production outcomes are considered in each.

Clear Speech Variants: An Investigation of Intelligibility and Speaker Effort in Speakers With Parkinson's Disease.

PURPOSE: This study investigated the effects of three clear speech variants on sentence intelligibility and speaking effort for speakers with Parkinson's disease (PD) and age- and sex-matched neurologically healthy controls. METHOD: Fourteen speakers with PD and 14 neurologically healthy speakers participated. Each speaker was recorded reading 18 sentences from the Speech Intelligibility Test in their habitual speaking style and for three clear speech variants: clear (SC; given instructions to speak clearly), hearing impaired (HI; given instructions to speak with someone with a hearing impairment), and overenunciate (OE; given instructions to overenunciate each word). Speakers rated the amount of physical and mental effort exerted during each speaking condition using visual analog scales (averaged to yield a metric of overall speaking effort). Sentence productions were orthographically transcribed by 50 naive listeners. Linear mixed-effects models were used to compare intelligibility and speaking effort across the clear speech variants. RESULTS: Intelligibility was reduced for the PD group in comparison to the control group only in the habitual condition. All clear speech variants significantly improved intelligibility above habitual levels for the PD group, with OE maximizing intelligibility, followed by the SC and HI conditions. Both groups rated speaking effort to be significantly higher for both the OE and HI conditions versus the SC and habitual conditions. DISCUSSION: For speakers with PD, all clear speech variants increased intelligibility to a level comparable to that of healthy controls. All clear speech variants were also associated with higher levels of speaking effort than habitual speech for the speakers with PD. Clinically, findings suggest that clear speech training programs consider using the instruction "overenunciate" for maximizing intelligibility. Future research is needed to identify if high levels of speaking effort elicited by the clear speech variants affect long-term sustainability of the intelligibility benefit.

Dysarthria Subgroups in Talkers with Huntington's Disease: Comparison of Two Data-Driven Classification Approaches.

Although researchers have recognized the need to better account for the heterogeneous perceptual speech characteristics among talkers with the same disease, guidance on how to best establish such dysarthria subgroups is currently lacking. Therefore, we compared subgroup decisions of two data-driven approaches based on a cohort of talkers with Huntington's disease (HD): (1) a statistical clustering approach (STATCLUSTER) based on perceptual speech characteristic profiles and (2) an auditory free classification approach (FREECLASS) based on listeners' similarity judgments. We determined the amount of overlap across the two subgrouping decisions and the perceptual speech characteristics driving the subgrouping decisions of each approach. The same speech samples produced by 48 talkers with HD were used for both grouping approaches. The STATCLUSTER approach had been conducted previously. The FREECLASS approach was conducted in the present study. Both approaches yielded four dysarthria subgroups, which overlapped between 50% to 78%. In both grouping approaches, overall bizarreness and speech rate characteristics accounted for the grouping decisions. In addition, voice abnormalities contributed to the grouping decisions in the FREECLASS approach. These findings suggest that apart from overall bizarreness ratings, indexing dysarthria severity, speech rate and voice characteristics may be important features to establish dysarthria subgroups in HD.

Voice handicap Index in Parkinson's patients: Subthalamic versus globus pallidus deep brain stimulation.

PURPOSE: Subthalamic nucleus (STN) and globus pallidus interna (GPI) are the two most common sites for deep brain stimulation (DBS) in people with Parkinson's disease (PWP). Voice impairments are a common symptom of Parkinson's disease and information about voice outcomes with DBS is limited. Most studies in speech-language pathology have focused on STN-DBS and few have examined the effects of GPI-DBS. This was an initial effort to examine the impact of DBS location on Vocal Handicap Index (VHI) scores, which assess the impact of a voice disorder on an individual. METHOD: Twenty-four gender-matched PWP (12 STN-DBS and 12 GPI-DBS) completed the VHI post-DBS implantation. Two-tailed independent samples t-tests were used to compare each VHI scale score (physical, functional, emotional, total) and patient factors between the two groups. RESULTS: No significant differences in total or subscale VHI scores were identified between the two DBS groups. A trend toward greater impairment in PWP with GPI-DBS was noted. An association between higher VHI scores and DBS settings was found. CONCLUSIONS: Studies directly comparing speech outcomes for different DBS targets are lacking. The current findings provide new insights concerning voice outcomes following DBS by adding to the limited literature directly comparing speech outcomes in multiple DBS targets. Limitations and directions for future research are discussed.

The impact of rate reduction and increased loudness on fundamental frequency characteristics in dysarthria.

OBJECTIVE: This study examined the extent to which articulatory rate reduction and increased loudness were associated with adjustments in utterance-level measures of fundamental frequency (F(0)) variability for speakers with dysarthria and healthy controls that have been shown to impact on intelligibility in previously published studies. More generally, the current study sought to compare and contrast how a slower-than-normal rate and increased vocal loudness impact on a variety of utterance-level F(0) characteristics for speakers with dysarthria and healthy controls. PATIENTS AND METHODS: Eleven speakers with Parkinson's disease, 15 speakers with multiple sclerosis, and 14 healthy control speakers were audio recorded while reading a passage in habitual, loud, and slow conditions. Magnitude production was used to elicit variations in rate and loudness. Acoustic measures of duration, intensity and F(0) were obtained. RESULTS AND CONCLUSIONS: For all speaker groups, a slower-than-normal articulatory rate and increased vocal loudness had distinct effects on F(0) relative to the habitual condition, including a tendency for measures of F(0) variation to be greater in the loud condition and reduced in the slow condition. These results suggest implications for the treatment of dysarthria.

Effects of speaking task on intelligibility in Parkinson's disease.

Intelligibility tests for dysarthria typically provide an estimate of overall severity for speech materials elicited through imitation or read from a printed script. The extent to which these types of tasks and procedures reflect intelligibility for extemporaneous speech is not well understood. The purpose of this study was to compare intelligibility estimates obtained for a reading passage and an extemporaneous monologue produced by 12 speakers with Parkinson's disease (PD). The relationship between structural characteristics of utterances and scaled intelligibility was explored within speakers. Speakers were audio-recorded while reading a paragraph and producing a monologue. Speech samples were separated into individual utterances for presentation to 70 listeners who judged intelligibility using orthographic transcription and direct magnitude estimation (DME). Results suggest that scaled estimates of intelligibility for reading show potential for indexing intelligibility of an extemporaneous monologue. Within-speaker variation in scaled intelligibility also was related to the number of words per speech run for extemporaneous speech.

Investigating Acoustic Correlates of Intelligibility Gains and Losses During Slowed Speech: A Hybridization Approach.

Purpose This exploratory study sought to identify acoustic variables explaining rate-related variation in intelligibility for speakers with dysarthria secondary to multiple sclerosis. Method Seven speakers with dysarthria due to multiple sclerosis produced the same set of Harvard sentences at habitual and slow rates. Speakers were selected from a larger corpus on the basis of rate-related intelligibility characteristics. Four speakers demonstrated improved intelligibility and three speakers demonstrated reduced intelligibility when rate was slowed. A speech analysis resynthesis paradigm termed hybridization was used to create stimuli in which segmental (i.e., short-term spectral) and suprasegmental variables (i.e., sentence-level fundamental frequency, energy characteristics, and duration) of sentences produced at the slow rate were donated individually or in combination to habitually produced sentences. Online crowdsourced orthographic transcription was used to quantify intelligibility for six hybridized sentence types and the original habitual and slow productions. Results Sentence duration alone was not a contributing factor to improved intelligibility associated with slowed rate. Speakers whose intelligibility improved with slowed rate showed higher intelligibility scores for duration spectrum hybrids and energy hybrids compared to the original habitual rate sentences, suggesting these acoustic cues contributed to improved intelligibility for sentences produced with a slowed rate. Energy contour characteristics were also found to play a role in intelligibility losses for speakers with decreased intelligibility at slowed rate. The relative contribution of speech acoustic variables to intelligibility gains and losses varied considerably between speakers. Conclusions Hybridization can be used to identify acoustic correlates of intelligibility variation associated with slowed rate. This approach has further elucidated speaker-specific and individualized speech production adjustments when slowing rate.