The posterior commissure is a bundle of neural fibers that connects midbrain and diencephalon structures. This bundle is just in front of the superior colliculi and below the pineal gland.
This commissure is usually very difficult to see in all but the most detailed structural scans, and is generally invisible on functional scans (like positron emission tomography). The posterior commissure is used as a landmark when brain structures are located using Talairach coordinates.