Flocculus

The flocculus has a complex circuitry that is reflected in the structure of the zones and halves. These "zones" of the flocculus refer to five separate groupings of Purkinje cells that project to different areas of the brain. Depending upon where stimulus occurs in the flocculus, signals can be projected to very different parts of the brain. The first and third zones of the flocculus project to the superior vestibular nucleus, the second and fourth zones project to the medial vestibular nucleus, and the fifth zone projects to the interposed posterior nucleus, a part of the cerebellum.[2]

The anatomy of the flocculus shows that it is composed of two disjointed lobes or halves. The “halves” of the flocculus refer to the caudal half and the rostral half, and they indicate from where fiber projections are received and the path in which a signal travels.[3] The caudal half of the flocculus receives mossy fiber projections mainly from the vestibular system and tegmental pontine reticular nucleus, an area within the floor of the midbrain that affects the axonal projections or images received by the cerebellum. Vestibular inputs are also carried through climbing fibers that project into the flocculus, stimulating Purkinje cells. Leading research would suggest that climbing fibers play a specific role in motor learning.[4] The climbing fibers then send the image or projection to the part of the brain that receives electrical signals and generates movement. From the midbrain, corticopontine fibers carry information from the primary motor cortex.[1] From there, projections are sent to the ipsilateral pontine nucleus in the ventral pons, both of which are associated with projections to the cerebellum. Finally, pontocerebellar projections carry vestibulo-occular signals to the contralateral cerebellum via the middle cerebellar peduncle.[5] The rostral half of the flocculus also receives mossy fiber projections from the pontine nuclei; however, it receives very little projection from the vestibular system.

The idea that the flocculus is involved in motor learning gave rise to the “flocculus hypothesis.” This hypothesis argues that the flocculus plays a key role in the vestibulo-ocular system, most importantly the ability for the vestibular system to adapt to a shift in the visual field.[3] The learning of basic motor skills, including walking, balancing, and the ability to sit up, can be attributed to early patterns and pathways associated with the vestibulo-ocular reflex (VOR) and the pathways formed in the cerebellum that contribute to the learning of basic motor skills. The flocculus appears to be included in a VOR pathway that aids in the adaptation to a repeated shift in the visual field.[4] A shift in the visual field affects an individual's spatial recognition. The leading research would suggest that the flocculus aids in the synchronization of eye and motor functions after a visual shift occurs in order for the visual field and the motor skills to function together. If this shift is repeated the flocculus essentially trains the brain to fully readjust to these repeated stimuli.[6]

The conditions and systems associated with floccular loss are considered to be a subset of a vestibular disease. Some symptoms of common vestibular diseases include: head tilting, an inability to stand, ataxia, dizziness, vomiting and strabismus. Because of the flocculus’ role in the vestibular system, the inner ear, equilibrioception, and both peripheral and central vision is affected by any loss or damage to the flocculus. These systems are affected because damage to the flocculus prevents any changes from being stored in regards to visual and motor communication, meaning that although the VOR is still intact these systems are unable to store changes in gain or eye movement as the head is rotated back and forth.[10]