TESTING THE COMMON NEURAL INTEGRATOR HYPOTHESIS AT THE LEVEL OF THE INDIVIDUAL ABDUCENS MOTONEURONS IN THE ALERT CAT

1. As far as horizontal eye movements are concerned, the well-known hy pothesis of a common neural integrator states that the eye-position si gnal is generated by a common network, regardless of the type of versi onal movement. The aim of this study was to evaluate the validity of t his 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 th e sensitivity to eye position would be the same regardless of the type of versional movement. If there were multiple integrators, the sensit ivity to eye position in one type of versional movement might be diffe rent from the sensitivity to eye position in another type of versional movement, at least for occasional motoneurones. 2. The discharge of t hirty-one antidromically identified abducens motoneurones was recorded in the alert cat during spontaneous eye movements made in the light a nd in response to sinusoidal rotations of the head in complete darknes s. 3. All of the abducens motoneurones exhibited a burst of action pot entials for lateral saccades. During fixation between saccades, they m aintained a steady firing rate that increased as the cat fixated incre asingly lateral eye positions. 4. For each abducens motoneurone, the s ensitivity to eye position (K(f)) was determined from measurements car ried out during intersaceadic fixations. K(f) was calculated from the slope of the firing rate-eye position linear regression line. 5. The d ischarge rate of the identified motoneurones was observed during four sinusoidal vestibular stimulations (+/-10 deg, 0.10 Hz; +/-20 deg, 0.1 0 Hz; +/-30 deg, 0.10 Hz; +/-40 deg, 0.10 Hz). The motoneurones exhibi ted a burst of activity during fast Phases in the lateral direction an d paused during fast phases in the opposite direction, During slow pha ses, motoneurones modulated their activity as a function of the vestib ularly induced eye movements except for slow phases that occurred in P osition ranges below their recruitment threshold. In these cases their activity was cut off. 6. A new method was developed to measure, the s ensitivity to eye position of neurones during vestibular slow phases. The difficulty came from the fact that, during slow phases, eye veloci ty and eye position changed simultaneously and that each of those two variables could influence neuronal activity. 7. For each motoneurone, the instantaneous firing rate was measured each time the eye passed th rough a given position during any slow phase generated during any vest ibulo-ocular reflex. At a given position, the discharge rate of the mo toneurone under study was plotted against the eye velocity. From the r esulting linear regression line, two interesting values were obtained: its slope corresponding to the sensitivity of the motoneurone to eye velocity, R(v), (at that given eye position) and its y-intercept, F(0) , the interpolated firing rate when the eye velocity was zero. This pr ocedure was repeated for different eye positions. The values of F(0) w ere then plotted against the eye positions. The slope of this regressi on line gave the sensitivity of the motoneurone to eye position measur ed during vestibular stimulation. It was termed K(v). 8. We found that , for each of our thirty-one abducens motoneurones, the sensitivity to eye position measured during intersaccadic fixation in the light (K(f )) was equal to the sensitivity to eye position measured during the ve stibulo-ocular reflex elicited in complete darkness (K(v)). 9. This re sult is compatible with the hypothesis of the common oculomotor neural integrator. It does not prove it but it is worth emphasizing that a c onverse result would have ruled out the hypothesis of the common oculo motor neural integrator.