It is difficult to say fully conclusively, but I would speculate there is not an effect.
The absence of the massa intermedia when it is associated with a noticable disorder is usually part of a larger midline deformation which impacts a wide array of extremely important structures, so attributing causation to the absence of the massa intermedia is difficult.
So absence of the massa intermedia may be more of a symptom of other disorders rather than a cause of any particular mental effect.
Anyway, horse’s mouth:
Massa intermedia (MI), also known as interthalamic adhesion is an inconsistent bridge connecting the two thalami. Recent studies suggest MI contains functional neuronal tissue and is responsible for interhemispheric communication. Absence of MI has been linked to cognitive differences and psychiatric disorders. However, MI is naturally absent in up to 35 percent of cases but its true prevalence during life in humans is unknown. High resolution anatomical MR studies of 1410 subjects aged 2 months to 93 years were reviewed and those with MI were identified. Prevalence and characteristics of MI were identified and grouped by gender and decade of life. MI was present in 87.3% of the studied subjects. Absence of MI was noted in as early as first decade of life as well as all decades of life, but its absence increased with age, suggesting additional factors during life as mediators. Females had 2.75 times higher likelihood of MI presence than males. Size of MI decreased with increasing age up to age 70. Size of MI was best predicted by third ventricular width and age mediating a larger MI with smaller third ventricular size and younger age. MI is present in 87.3% of the population as determined in this MRI study. Absence of MI is identifiable in very early years of life, suggesting a congenital cause. MI’s prevalence, however, was also found to decrease with advancing age, suggesting a dynamic process of MI disappearance during life.
Borghei A, Piracha A, Sani S. Prevalence and anatomical characteristics of the human massa intermedia. Brain Struct Funct. 2021 Mar;226(2):471-480.
That’s a remarkable thing. Has there been any research to see if its absence/presence has any effect on mental functions?
It is difficult to say fully conclusively, but I would speculate there is not an effect.
The absence of the massa intermedia when it is associated with a noticable disorder is usually part of a larger midline deformation which impacts a wide array of extremely important structures, so attributing causation to the absence of the massa intermedia is difficult.
So absence of the massa intermedia may be more of a symptom of other disorders rather than a cause of any particular mental effect.
Anyway, horse’s mouth:
Borghei A, Piracha A, Sani S. Prevalence and anatomical characteristics of the human massa intermedia. Brain Struct Funct. 2021 Mar;226(2):471-480.