Role of the cerebellar floccular region in generation of three dimensional pursuit eye movements - Comparison with frontal eye field signals -

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Frontal-eyed primates use both smooth-pursuit in frontal planes (frontal-pursuit) and pursuit-in-depth (vergence-pursuit) to track objects of interest moving slowly in three-dimensional (3D) space. The cerebellar floccular region is well known to play a crucial role in initiation and maintenance of frontal-pursuit and control of the vestibulo-ocular reflex (VOR). By assigning monkeys to pursue a spot moving in 3D space, I examined whether pursuit Purkinje (P) cells in the floccular region have vergence sensitivity and how they are involved in vergence-pursuit. Of 112 pursuit P cells examined for both frontal and vergence-pursuit, the majority (63%) discharged for both, 24% discharged only for vergence-pursuit and 13% discharged only for frontal-pursuit. The majority of (58%) of vergence-related P cells had velocity sensitivity, thus they carry 3D-pursuit signals. 72% also had vergence-position sensitivity. In contrast, pursuit P cells that discharged specifically for frontal-pursuit ar … More e in the minority in the floccular region. 80% of 3D-pursuit P cells discharged after the onset of vergence-pursuit with the average lag of 56 ms. I also examined activity of floccular P cells during fore/aft translation to test whether they carry gaze-velocity signals. The majority of P cells (17/22=77%) did not respond to fore/aft translation when monkeys were required to fixate a target that moved with them in order to maintain the vergence angle during translation. However, most (13/22=59%) responded if the monkeys were required compensatory vergence eye movements. Actual responses during the latter condition were well predicted by their eye velocity sensitivity during vergence-pursuit. These results suggest the involvement of the floccular region in 3D-pursuit rather than gaze (eye-in-space) control. Pursuit signals in the floccular region could provide internal feedback signals for pursuit gain control. Vergence eye position-related P cell activity could also provide the viewing distance information for appropriate control of the VOR. Less