Nerve fiber mapping of the human visual cortex using Polarized Light Imaging (PLI)

A neuroanatomic method to visualize and extract the main nerve fiber orientation in histological sections of the human brain is presented. The so-called Polarized Light Imaging (PLI) enables high resolution and large-scale studies of the human white matter architecture utilizing its birefringence properties. The birefringence of white matter can basically be attributed to the regular arrangement of lipids in the myelin sheaths that enclose nerve fibers. Consequently, linearly polarized light becomes elliptically polarized when traveling through histological brain sections. The final state of polarization depends on the fiber course and allows to quantify the in-plane orientation of the fibers in every section (direction) as well as the fiber out-of-section orientation (inclination). A dedicated apparatus was developed for large-area PLI of gross histological brain sections. The presented pilot study shows the potential of PLI with serial histological sections of the visual cortex of the human brain. Intensity maps and fiber orientation maps for every brain section were generated and finally 3D-reconstructed. The reconstructed volume includes valuable information about the spatial extent of nerve fiber tracts. Specific fiber tracking will deliver important insights into large-distance connectivity of functional brain areas. The fiber orientation maps obtained with PLI are suited to be compared with DT-MRI maps. Thus PLI represents a complementary method to DT-MRI and might be useful for DT-MRI validation.