Vocabulary variation and lemma probabilities in speech analysis for diagnosing dementia and depression

Neurodegenerative and mental disorders significantly affect the manner of speaking, syntax, semantics and specific habits of word choice. The aim of our research was to develop linguistic speech analysis methods to provide screening tests in neurology and psychiatry as subjective techniques supporting medical diagnostics. Using features of speech samples recorded by the subjects and the control group, we created classifiers which distinguish one group of recordings from the other. We used two methods to diagnose dementia. The first method is based on the observation that people with dementia have lower vocabulary variations. The second diagnostic method is based on probabilities of lemmas. This method was also used in depression screening tests. For neurodegenerative and mental disorders, linguistic analysis appeared to be satisfactorily effective. Linguistic changes were easily detectable in dementia and less noticeable in depression. We achieved a 95% precision of diagnosis in the control group for dementia and 100% for the three stages of depression. This is higher than in groups of patients (dementia 93% and average 70% for the three stages of depression).