Can sex influence the neurocognition of language? Evidence from Parkinson's disease

Abstract

Parkinson's disease (PD), which involves basal ganglia degeneration, affects language as well as motor function. However, which aspects of language are impaired in PD and under what circumstances remains unclear. We examined whether lexical and grammatical aspects of language are differentially affected in PD, and whether this dissociation is moderated by sex as well as the degree of basal ganglia degeneration. Our predictions were based on the declarative/procedural model of language. The model posits that grammatical composition, including in regular inflection, depends importantly on left basal ganglia procedural memory circuits, whereas irregular and other lexicalized forms are memorized in declarative memory. Since females tend to show declarative memory advantages as compared to males, the model further posits that females should tend to rely on this system for regulars, which can be stored as lexicalized chunks. We tested non-demented male and female PD patients and healthy control participants on the intensively studied paradigm of English regular and irregular past-tense production. Mixed-effects regression revealed PD deficits only at regular inflection, only in male patients. The degree of left basal ganglia degeneration, as reflected by right-side hypokinesia, predicted only regular inflection, and only in male patients. Left-side hypokinesia did not show this pattern. Past-tense frequency effects suggested that the female patients retrieved regular as well as irregular past-tense forms from declarative memory, whereas the males retrieved only irregulars. Sensitivity analyses showed that the pattern of findings was robust. The results, which are consistent with the declarative/procedural model, suggest a grammatical deficit in PD due to left basal ganglia degeneration, with a relative sparing of lexical retrieval. Female patients appear to compensate for this deficit by relying on chunks stored in declarative memory. More generally, the study elucidates the neurocognition of inflectional morphology and provides evidence that sex can influence how language is computed in the mind and brain.

Keywords: Basal ganglia; Compensation; Hypokinesia; Parkinson's disease; Regular and irregular inflectional morphology; Sex differences.

Figure 1:

Logit-transformed adjusted means and standard errors from the logistic regression performed for the group comparison. The means can be back-transformed into probabilities of correct responses with the equation y = 1/(1+e^−x), where x is the logit-transformed adjusted mean; standard errors cannot be back-transformed. PD: Parkinson’s disease; NC: normal control.

Figure 2:

Logit-transformed adjusted accuracy at the production of regular (solid lines) and irregular (dashed lines) past-tense forms, as a function of log-transformed right-side hypokinesia, separately for males (A) and females (B). Regression lines are shown for the range of right-side hypokinesia in each sex. Shaded bands represent standard errors (95% confidence intervals are approximately twice the width of standard error bands). See Figure S1 in Supplementary Material for scatterplots showing the untransformed by-subject data.