Perplexity and its structural cerebral correlates: A promising marker for cognitive decline?

Abstract

AbstractBackgroundRecent studies identified linguistic changes as a promising marker of early Alzheimer´s disease (AD) and mild cognitive impairment (MCI). Perplexity – i.e. a measure of content complexity and therefore predictability of speech – is considered as a predictive marker of AD and MCI in otherwise healthy elderly (Frankenberg et al., 2019). A higher score indicates a reduced predictability, i.e. for persons with high variability in their speech, the perplexity feature will have a very high value. However, the underlying brain structures are still unknown.MethodPerplexity was automatically examined in voice recordings of extensive autobiographical interviews obtained in 38 participants in their mid‐70ties (N = 26 cognitively healthy, N = 12 with MCI) from the Interdisciplinary Longitudinal Study on Adult Development and Aging (ILSE). Perplexities of 1‐gram and 2‐gram language models are investigated: The 1‐gram language model considers one word at a time, the 2‐gram language model considers each word in the context of its predecessor. Whole‐brain correlations between gray matter and perplexity were controlled for total intracranial volume, sex and education.ResultsAnalyses yielded significant positive correlations between measures of perplexity and right parahippocampal gyrus, right middle occipital gyrus, left middle temporal gyrus and right superior temporal gyrus (1 gram) as well as left precuneus and left anterior cingulate (2 gram).ConclusionThe correlation of perplexity with parahippocampal gyrus is paralleled by associations between this structure and autobiographical memory (Frankenberg et al., submitted). The superior temporal gyrus points to planum temporale, while the precuneus is involved in motor coordination and is connected to the anterior cingulate, which is associated with functions such as error detection and attention. The middle temporal gyrus is also considered to be language‐relevant in particular for lexical‐semantic and conceptual semantic aspects and the middle occipital gyrus is discussed as the Occipital Word Form Sensitive Area.