Role of massa intermedia in human neurocognitive processing.

Massa intermedia (MI) is an inconsistent midline structure in the human brain that is absent in approximately 30% of the population. Absence of MI is seen more frequently in schizophrenia spectrum disorder and bipolar disorder. However, very little is known about the normal role of MI in the human brain. The purpose of this study was to investigate the role of human MI in cortical and subcortical cognitive processing as determined by differences between subjects with and without MI. Using the Human Connectome Project database, a cohort of randomly selected participants were selected to (1) identify presence, absence, and size of MI, and (2) explore possible cognitive process mediated by the presence of MI. Four hundred and two brains were included (216 females) in the final analysis. Four independent blind raters identified 360 brains with MI (202 females) and 42 without MI using anatomical T1-weighted MR scans. Presence of MI was significantly more prevalent in female participant (p = 0.005) and they had significantly larger size of MI (p = 0.001 and 0.000 for anteroposterior and craniocaudal dimensions, respectively). There were no statistically significant differences in the presence of MI with regards to age, race and ethnicity. Further analysis revealed gender, flanker test, and loneliness as predictor of the presence of MI in a Firth logistic regression model (p = 0.0004). This is the largest study of human MI to date. MI may contribute to interhemispheric cortical and subcortical connectivity with resulting subtle neuropsychological differences among individuals with a present versus absent MI.