Evidence from Retractor Bulbi EMG for Linearised Motor Control of Conditioned Nictitating Membrane Responses

Classical conditioning of nictitating membrane (NM) responses in rabbits is a robust model learning system, and experimental evidence indicates that conditioned responses (CRs) are controlled by the cerebellum. It is unknown whether cerebellar control signals deal directly with the complex non-linearities of the plant (blink-related muscles and peripheral tissues), or whether the plant is linearised to ensure a simple relation between cerebellar neuronal firing and CR profile. To investigate this question, the retractor bulbi muscle EMG was recorded with implanted electrodes during NM conditioning. Pooled activity in accessory abducens motoneurons was estimated from spike trains extracted from the EMG traces, and its temporal profile found to have an approximately Gaussian shape with peak amplitude linearly related to CR amplitude. The relation between motoneuron activity and CR profiles was then accurately fitted by a first order, linear filter, with each spike input producing an exponentially decaying impulse response with time constant of order 0.1 s. Application of this first-order plant model to CR data from other laboratories suggested that in these cases also motoneuron activity had a Gaussian profile, with time-of-peak close to unconditioned stimulus (US) onset and standard deviation proportional to the interval between conditioned stimulus and US onsets. These results suggest that for conditioned NM responses the cerebellum is presented with a simplified 'virtual' plant that is a linearised version of the underlying non-linear biological system. Analysis of a detailed plant model suggests that one method for linearising the plant would be appropriate recruitment of motor units.