# Prosody-Based Measures for Automatic Severity Assessment of Dysarthrie Speech
## 1 Introduction
An issue with this study is the use of the Neymours database, which contains a limited number of speakers and stimuli. Only speech from male speakers is collected, and only 1 healthy control is available. Furthermore, the collected speech consists of the same carrier sentence, \"The X is Y-ing the Z\", which can lead to unnatural prosody. A close study that uses prosody-based features for severity assessment is in Reference [38], which focuses on rhythm metrics without including a baseline feature set of non-prosodic features. Results indicate a 15% and 3.2% relative accuracy increase in Korean and English datasets, respectively, compared to only using pitch, speech rate, and voice quality features. Our study differs from the previously mentioned studies and contributes to the literature on dysarthric speech detection in several important aspects. First, our goal is to assess the severity of dysarthria with prosody-based measures; therefore, instead of a binary detection task where all dysarthric speakers are grouped into a single class, the speakers in our study are distinguished by four severity levels (healthy, mild, moderate and severe). In practical use, dysarthria assessment is most likely to be conducted on patients with early dysarthria, and therefore, it is important to include the speech of speakers with mild dysarthria. Second, we model prosody in a multidimensional manner by including measurements for pitch, speech rate, and rhythm. We also apply various feature selection algorithms to select measures that are more important for distinguishing different severity levels. Third, we conduct experiments with both English and Korean data, which allows for better generalization of using prosody with different languages. Furthermore, both databases contain a diverse set of speakers with multiple utterances from males, females, healthy, and dysarthric speakers. It is important to examine languages with different prosodic systems as the efficacy of using prosody for dysarthria assessment may vary depending on the language. English is a lexical stress language with a variety of cues to signal stress [39]. Stressed syllables in English are perceptually louder than unstressed ones and tend to be realized higher in pitch and longer in duration. Vowels in unstressed syllables are reduced, resulting in changes of F1 and F2 formant frequencies. However, standard Korean (or Seoul Korean) does not have either lexical stress or lexical pitch accent. Only phrase-level tones are used in Korean [40]. The pitch of a Korean syllable is affected by the intonation pattern at the sentence level, given that the pitch accent is used to signify prosodic phrases in Korean. There are also rhythmic differences in Korean and English. English is typically described as a stress-timed language where the duration of stressed syllables is relatively equal [41]. Korean is commonly known as a syllable-timed language where all syllables tend to be equal in duration [42; 43]. However, the status of rhythm in Korean is more contentious, and researchers have also proposed that Korean has stress-timed [44; 45] and mora-timed [46] patterns. The rest of the paper is organized as follows: Section 2 reviews the prosody-based measures, along with articulatory and phonatory features. Section 3 describes the experimental methods including the training process, classifiers, and feature selection methods. The Korean and English corpus is described in Section 4, along with an analysis of the prosodic measures. Results for all experiments are summarized in Section 5. Lastly, the study concludes in Section 6 with a discussion of the results and future directions.