Control of rabbit nictitating membrane movements. II. Analysis of the relation of motoneuron activity to behavior.

The objective of this study is to understand more precisely the relationship of motoneuron activity to movements of the rabbit nictitating membrane (NM). We use a model of the oculomotor plant to investigate what NM movements are generated by a given pattern of neural input and what inputs are required to generate particular NM movements. Simulated peak NM extensions can occur well over 50 ms after the end of motoneuron activity. The neural input required for the model to generate full amplitude NM extension responses is more consistent with single accessory abducens unit recordings from awake rabbits than recordings from anesthetized rabbits. An initial high frequency burst of neural activity followed by a rapid decay is required for simulations of conditioned responses (CRs) trained at a 125 ms interstimulus interval (ISI). For CRs trained with a 250 ms ISI, a more slowly rising and decaying neural activity is required. Model simulations show that the linear correlation between the shape of histogrammed motoneuron activity and the shape of NM movements can be high for long duration responses (> 400 ms) but is low for short duration responses (< 200 ms). Simulations are also consistent with the hypothesis that NM retraction is generally passive.