Introduction: Massa intermedia (MI) a midline bridge between the two thalami is reportedly absent in 2.3%-22.3% of human brains[1,2]. Prior studies demonstrated higher prevalence and bigger size of MI in female subjects[2-4]. Recently, behavioral studies have revealed MI playing a role in age-related attention[3].
Our group has recently demonstrated passage of decussating stria medullaris fibers through MI using probabilistic tractography, which further supports MI involvement in limbic and behavioral networks[5].
In this study, we investigated differences in fractionated anisotropy(FA) in healthy brains between those with and without MI.
Methods: 3T MRI studies were obtained from the Human Connectome Project(HCP)[6]. Subjects used were 100 healthy unrelated volunteers(age 25-35, 54% female). All subjects included preprocessed T1-weighted structural (TR:2400,TE:2.14,flip-angle:8°,FOV:224*224,isotopic:0.7mm) and diffusion weighted (TR:5520,TE:89.5,flip-angle:78°,FOV:210*180,partial-Fourier:6/8,b-values:1000,2000,3000,isotropic:1.25mm,slice:111) sequences[7].
T1 sequences were manually inspected to identify MI by three investigators and inter-rater agreement was calculated using Fleiss-kappa (kappa:061,p-value:0.00). Ten brains were excluded due to inability to confirm MI presence/absence. Diffusion tensor was reconstructed using FSL dtifit. FA data were then used to perform TBSS pre-statistical stages[8]. Subjects were grouped into those with and without MI. Two-sample unmatched t-test was performed using randomise commandline[9]. P-value was set at 0.05. Results were projected onto MNI152 standard-space.
Results: Out of 90 brain studied (53 female and 37 male), there were 11 individuals (5 female and 6 male) with no MI. FA-values were significantly higher in subjects with MI over anatomical location of MI. Results further showed significant difference in FA across the internal capsule particularly on the left, uncinate fasciculus (left>right) and temporal stem (left~right).
Conclusions: This is the first study using computational neuroimaging which demonstrated MI as a true conduit of fiber tracts in the human brain. MI presence did result in an increase of FA values of specific WM tracts with apparent lateralization. Further studies can shed light on functional significance of this midline commissure.
Patient Care: MI is a large commissural conduit that is absent in 4-20% of humans. It appears to bear limbic network fibers. Understanding its structure and function with further shine light on related diseases such as schizophrenia and depression.
Learning Objectives: Role of massa intermedia in structural connectivity of the human brain.
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