Animals move the same body parts in diverse ways. How the central nervous system executes one action over related ones is poorly understood. To investigate this, I assessed the behavioural manifestation and neural control of saccadic eye rotations made by larval zebrafish, since these movements are simple and easy to investigate at a circuit level.
I first classified the larva’s saccadic repertoire into 5 types, of which hunting specific convergent saccades and exploratory conjugate saccades were the main types used to orient vision. Convergent and conjugate saccades shared a nasal eye rotation, which had kinematic differences and similarities that suggested the rotation was made by overlapping but distinct populations of neurons between saccade types. I investigated this further, using two-photon Ca2+ imaging and selective circuit interventions to identify a circuit from rhombomere 5/6 to abducens internuclear neurons to motoneurons that was crucial to nasal eye rotations. Motoneurons had distinct activity patterns for convergent and conjugate saccades that were consistent with my behavioural observations and were explained largely by motoneuron kinematic tuning preferences. Surprisingly, some motoneurons also modulated activity according to saccade type independent of movement kinematics. In contrast, pre-synaptic internuclear neuron activity profiles were almost entirely explained by movement kinematics, but not neurons in rhombomere 5/6, which had mixed saccade type and kinematic encoding, like motoneurons. Regions exerting descending control on this circuit from the optic tectum and anterior pretectal nucleus had few neurons tuned to saccade kinematics compared to neurons selective for convergent saccades.
My results suggest a transformation from encoding action type to encoding movement kinematics at successive circuit levels. This transformation was not monotonic or complete, and suggests that control of even simple, highly comparable, movements cannot be entirely described by a shared kinematic encoding scheme at a motor or premotor level.