Objective: Phonemic verbal fluency reflects executive function and involves both within-category lexical retrieval (cluster size) and switching between categories. We compared semantic search behavior in patients with basal ganglia disorders and healthy controls (HC), incorporating natural language processing (NLP)-based semantic space analysis.

Background: Verbal fluency performance depends on cluster size and switching frequency (Troyer et al., 1997). While reduced fluency has been reported in Parkinsonian disorders, quantitative evaluation of search dynamics within a computationally defined semantic space remains limited, particularly in progressive supranuclear palsy (PSP).

Method: Participants included 96 patients with Parkinson’s disease (PD), 23 with PSP, 16 with multiple system atrophy (MSA), and 34 age-matched HCs. Participants generated words beginning with the Japanese syllable “ka” for 60 seconds. Mean cluster size and switching frequency normalized by total word count were calculated. In addition, traveled semantic distance defined using NLP-based word2vec embeddings was quantified and compared across groups. Linear regression analysis was performed between total semantic movement distance and total word count in each group. The study was approved by institutional ethics committees at participating centers.

Results: Mean word counts were 7.8±3.8 (PD), 5.8±4.0 (PSP), 8.0±2.9 (MSA), and 9.8±3.7 (HC). PSP produced significantly fewer words than HC and PD (p<0.01). Mean cluster size was also significantly smaller in PSP compared with HC and PD (p<0.05).

Analysis of NLP-defined semantic space revealed that movement distance was significantly smaller in PSP than in PD (p<0.01). Furthermore, the slope of the linear regression between total semantic movement distance and total word count was significantly smaller in PSP compared with other groups (p<0.01), indicating reduced expansion of semantic search with increasing word production.

Conclusion: Compared with HC and PD, PSP demonstrated significantly fewer generated words and smaller cluster size. Moreover, PSP showed reduced movement within NLP-defined semantic space and a diminished increase in semantic exploration relative to word output. These findings suggest impaired ability to explore distant semantic regions in PSP, likely reflecting executive dysfunction.

References: Troyer AK, Moscovitch M, Winocur G. Neuropsychology 1997:11 (1):138-46.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/evaluation-of-semantic-space-search-metrics-in-phonemic-fluency-comparison-among-basal-ganglia-disorders-and-healthy-controls/