A dissociation between syntactic and lexical processing in Parkinson's disease

Abstract

Parkinson's disease (PD), which involves the degeneration of dopaminergic neurons in the basal ganglia, has long been associated with motor deficits. Increasing evidence suggests that language can also be impaired, including aspects of syntactic and lexical processing. However, the exact pattern of these impairments remains somewhat unclear, for several reasons. Few studies have examined and compared syntactic and lexical processing within subjects, so their relative deficits remain to be elucidated. Studies have focused on earlier stages of PD, so syntactic and lexical processing in later stages are less well understood. Research has largely probed English and a handful of other European languages, and it is unclear whether findings generalize more broadly. Finally, few studies have examined links between syntactic/lexical impairments and their neurocognitive substrates, such as measures of basal ganglia degeneration or dopaminergic processes. We addressed these gaps by investigating multiple aspects of Farsi syntactic and lexical processing in 40 Farsi native-speaking moderate-to-severe non-demented PD patients, and 40 healthy controls. Analyses revealed equivalent impairments of syntactic comprehension and syntactic judgment, across different syntactic structures. Lexical processing was impaired only for motor function-related objects (e.g., naming 'hammer', but not 'mountain'), in line with findings of PD deficits at naming action verbs as compared to objects, without the verb/noun confound. In direct comparisons between lexical and syntactic tasks, patients were better at naming words like 'mountain' (but not words like 'hammer') than at syntactic comprehension and syntactic judgment. Performance at syntactic comprehension correlated with the last levodopa equivalent dose. No other correlations were found between syntactic/lexical processing measures and either levodopa equivalent dose or hypokinesia, which reflects degeneration of basal ganglia motor-related circuits. All critical significant main effects, interactions, and correlations yielded large effect sizes. The findings elucidate the nature of syntactic and lexical processing impairments in PD.

Keywords: Parkinson’s disease; dopamine; hypokinesia; language; lexicon; syntax.

Figure 1.

Example item from the syntactic comprehension task (Paradis et al., 1987). The item was shown during auditory presentation of the sentence In dokhtar ast ke be anha ab mipashad (‘It is the girl that sprays them’).

Figure 2.

NC (normal control) and PD (Parkinson’s disease) performance at the Farsi syntactic comprehension task. A) Comparisons between NC and PD participants for the three sentence types, showing log-odds transformed accuracy means and standard errors (error bars), with p values from independent measures t-tests. B) Mean untransformed accuracy scores (and standard errors) for each group, for each sentence type.

Figure 3.

NC (normal control) and PD (Parkinson’s disease) performance at the Farsi syntactic judgment task. A) Comparisons between NC and PD participants, showing d-prime means and standard errors (error bars), with p value from an independent measures t-test. B) Accuracy results as a percentage (over both correct and incorrect sentences), with mean accuracy scores (and standard errors) shown for each group.

Figure 4.

NC (normal control) and PD (Parkinson’s disease) performance at the Farsi object naming task. A) Comparisons between NC and PD participants for the two object types, showing log-odds transformed accuracy means and standard errors (error bars), with p values from independent measures t-tests. B) Mean untransformed accuracy scores (and standard errors) for each group, for each object type.