Testing the common neural integrator hypothesis at the level of the individual abducens motoneurones in the alert cat.

Abstract

1. As far as horizontal eye movements are concerned, the well-known hypothesis of a common neural integrator states that the eye-position signal is generated by a common network, regardless of the type of versional movement. The aim of this study was to evaluate the validity of this hypothesis by analysing the behaviour of the abducens motoneurones, the system into which the horizontal neural integrator(s) project(s). If there were a common neural integrator, the different motoneurones would receive the eye position signal through the same pathway and the sensitivity to eye position would be the same regardless of the type of versional movement. If there were multiple integrators, the sensitivity to eye position in one type of versional movement might be different from the sensitivity to eye position in another type of versional movement, at least for occasional motoneurones. 2. The discharge of thirty-one antidromically identified abducens motoneurones was recorded in the alert cat during spontaneous eye movements made in the light and in response to sinusoidal rotations of the head in complete darkness. 3. All of the abducens motoneurones exhibited a burst of action potentials for lateral saccades. During fixation between saccades, they maintained a steady firing rate that increased as the cat fixated increasingly lateral eye positions. 4. For each abducens motoneurone, the sensitivity to eye position (Kf) was determined from measurements carried out during intersaccadic fixations. Kf was calculated from the slope of the firing rate-eye position linear regression line. 5. The discharge rate of the identified motoneurones was observed during four sinusoidal vestibular stimulations (+/- 10 deg, 0.10 Hz; +/- 20 deg, 0.10 Hz; +/- 30 deg, 0.10 Hz; +/- 40 deg, 0.10 Hz). The motoneurones exhibited a burst of activity during fast phases in the lateral direction and paused during fast phases in the opposite direction. During slow phases, motoneurones modulated their activity as a function of the vestibularly induced eye movements except for slow phases that occurred in position ranges below their recruitment threshold. In these cases their activity was cut off. 6. A new method was developed to measure the sensitivity to eye position of neurones during vestibular slow phases. The difficulty came from the fact that, during slow phases, eye velocity and eye position changed simultaneously and that each of those two variables could influence neuronal activity.(ABSTRACT TRUNCATED AT 400 WORDS)