Oculomotor anatomy and the motor-error problem: the role of the paramedian tract nuclei

Abstract: Anatomical evidence indicates that copies of premotor commands to ocular motoneurons are sent to the cell groups of the paramedian tract, whose projections constitute a major mossy-fibre input to the floccular region of the cerebellum. Damage to this relay impairs gaze-holding, so producing retinal slip signals that are also relayed to the flocculus, in this case as climbing-fibre input. These observations suggest that the relation between efference copy and sensory error is important, and might be used by the cerebellum to learn accurate movements. By modelling the flocculus as an adaptive filter using a covariance learning rule, we show that in simulation the cerebellar cortex can in fact learn to decorrelate efference copy from motor command, and thereby compensate for changes to the oculomotor plant. This mechanism appears to be very robust with respect to plant characteristics and noise, and can cope with error-signal delay provided there is a brainstem site of plasticity. Its general significance is that it removes the need for motor-error signals, which are typically unavailable and in any case not conveyed by climbing fibres. This appears to be an example where anatomical findings have helped address a long-standing problem in adaptive control.