Planum Parietale Volume in Antipsychotic-Naïve Schizophrenia

To the Editor: “The parietal lobe receives little attention in current neuropathological models of schizophrenia, and there has been little systematic investigation of this area,”¹ despite its recognized importance in processes that are likely disturbed in schizophrenia, such as language,² spatial working memory, and attention. The inferior parietal lobule (IPL), a part of the parietal lobe, also forms the part of the heteromodal association cortex that has been proposed as the site of the key abnormality in schizophrenia.³ It is subdivided into the supramarginal gyrus (area 39) and angular gyrus (area 40). These two structures are a part of the semantic-lexical network that supports “word meanings,” represented by a “grid of connectivity” that constitutes a “final pathway for the chunking of words into thought.”² Further, PET and fMRI studies have confirmed the role of the IPL, particularly the angular gyrus, in language comprehension.⁴ First-rank symptoms (FRS), a group of intriguing experiences characterized by a striking breach of “self versus non-self” boundaries, have had a critical influence on the diagnosis of schizophrenia.⁵ The IPL is also implicated in the pathogenesis of FRS in schizophrenia.³

Overall, IPL appears to be involved in the genesis of various symptoms of schizophrenia, as mentioned above. There are few imaging studies that have examined the IPL volume in schizophrenia with varied findings.⁶ Previously, a study by our team had reported significantly deficient right IPL volume, which correlated with FRS.⁷ With this background, we decided to study the volume of the planum parietale (PP), an unexplored area of the IPL. PP forms the cortex covering the posterior wall of the posterior ascending sylvian ramus, which is basically the continuation of the superior temporal gyrus. PP, although a subregion of the supramarginal gyrus, was recently discovered to function as an independent entity.⁸ It is thought to be involved in polymodal language functions in humans.⁹ Since, language dysfunction is one of the components of schneiderian FRS, the study aimed at studying the PP volume as well as its correlation with FRS in antipsychotic-naïve schizophrenia patients. We hypothesized that PP volume will be reduced in schizophrenia patients, compared with healthy comparison subjects, and will have a negative correlation with schneiderian FRS.

Methods

In this first-time study (to the best of our knowledge), we examined the volume of PP in antipsychotic-naïve schizophrenia patients (N=32; age: 29.3 [SD: 7.4] years; M:F=16:16) matched by age, sex, and handedness (as a group) with healthy comparison subjects (N=34; age: 29.4 [SD: 7.3] years; M:F=16:18), using a valid method with good interrater reliability. Intracranial volume was calculated using a method described earlier.¹⁰ Group comparisons were done using analysis of covariance statistics, controlling for the potential confounding effects of intracranial volume.

Results

Female schizophrenia patients showed significant volume reduction in the right PP as compared with female healthy controls (F=7.2; p=0.01). However, male patients did not (Table 1). There was a significant effect of schneiderian FRS in female patients (F=3.8; p=0.03); post-hoc analyses revealed that those women who had FRS had significantly smaller volume of right PP than healthy controls (p=0.01); whereas those female patients who were FRS-negative did not differ (p=0.12). Left PP volume did not differ between patients and controls.

Conclusions

The current study supports previous studies implicating the role of the parietal lobe in pathogenesis of FRS. The main function of PP is polymodal language function, which is independent of the planum temporale.⁸ Furthermore, we need systematic studies evaluating the language function associated with PP and the specific role of PP in FRS generation, and the possible implication of sex differences in schizophrenia.