Answer: Also called longitudinal callosal fascicles, Probst bundles are aberrant anterior-posterior white matter tracts that form in agenesis of the corpus callosum.
The corpus callosum is the major white matter tract that connects and right and left hemisphere. It allows for cross communication between the two hemispheres.
In certain rare developmental disorders, a person may be born with agenesis of the corpus callosum (ACC). In this condition, the white matter tracts do not develop in a neurotypical fashion. There is often a spectrum of symptom severity, ranging from poor muscle tone and coordination to developmental delays and difficulty with cognitive tasks.
Probst bundles are a diagnostic sign that indicates ACC. Instead of a white matter tract that spans the medial-lateral axis, Probst bundles appear to run parallel to the interhemispheric fissure, the midline where the two hemispheres separate. They run along the anterior-posterior axis, from the front of the brain to the back of the brain. Most of the axons that form the Probst bundles are from layer 2 / 3 of the cortex, the axons that normally reach out and span the two hemispheres in neurotypical development.
Probst bundles can be seen using magnetic resonance imaging (MRI) or diffusion tensor imaging (DTI). In scans, agenesis of the corpus callosum produces a “racing car” shape.
They were first described by Austrian neuroanatomist Moriz Probst in 1901.