Dynamics of abducens nucleus neuron discharges during disjunctive saccades

In this report, we provide the first characterization of abducens nucleus neuron (ABN) discharge dynamics during horizontal disjunctive saccades. These movements function to rapidly transfer the visual axes between targets located at different eccentricities and depths. Our primary objective was to determine whether the signals carried by ABNs during these movements are appropriate to drive the motion of the eye to which they project. We also asked whether ABNs encode eye movements similarly during disjunctive saccades and disjunctive fixation. To address the first objective we 1) assessed whether we could predict the discharge dynamics of individual neurons during disjunctive saccades based on their discharge properties during conjugate saccades and 2) directly estimated the sensitivity of individual neurons to either the ipsilateral/contralateral eye or the conjugate/vergence position and velocity using bootstrap statistics. Our main finding was that during disjunctive saccades in the direction ipsilateral to the recording site (ON-direction), the majority of ABNs preferentially encoded the velocity and the position of the ipsilateral eye. The remaining neurons predominantly encoded the conjugate motion of the eyes (i.e., were equally sensitive to the motion of both eyes). Generally, ipsilateral/contralateral eye based models better described neuronal discharges than conjugate/vergence based models, yet both model structures yielded similar conclusions. Moreover, the preferred eye of individual neurons based on their position and velocity sensitivities were generally well matched. We also found that for saccades in the OFF-direction, the pausing behavior of ABNs was similar during conjugate and disjunctive saccades, with the exception that for movements of small amplitudes, more ABNs paused during conjugate saccades. Finally, we found that putative motoneurons and internuclear neurons encoded ON- and OFF-direction disjunctive saccades in a similar manner. To address our second objective, we compared the discharge properties of individual ABNs during disjunctive saccades and disjunctive fixation. Good coherence was observed between the preferred eye of individual ABNs during the two behaviors. Taken together, our results indicate that although individual ABNs can encode the motion of both eyes to various degrees, the population drive of ABNs accounts for most of the movement of the ipsilateral eye during disjunctive saccades and disjunctive fixation.