Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disorder that affects language. We applied a computerized information-theoretic technique to assess the type and severity of language-related FTLD symptoms. Audio-recorded samples of 48 FTLD patients from three participating medical centers were elicited using the Cookie-Theft picture stimulus. The audio was transcribed and analyzed by calculating two measures: a perplexity index and an out-of-vocabulary (OOV) rate. The perplexity index represents the degree of deviation in word patterns used by FTLD patients compared to patterns of healthy adults. The OOV rate represents the proportion of words used by FTLD patients that were not used by the healthy speakers to describe the stimulus. In this clinically well-characterized cohort, the perplexity index and the OOV rate were sensitive to spontaneous language manifestations of semantic dementia and the distinction between semantic dementia and progressive logopenic aphasia variants of FTLD. Our study not only supports a novel technique for the characterization of language-related symptoms of FTLD in clinical trial settings, it also validates the basis for the clinical diagnosis of semantic dementia as a distinct syndrome.
- Frontotemporal lobar degeneration
- Semantic dementia
- Statistical language modeling