THE HYPOTHESIS OF THE UNIQUENESS OF THE OCULOMOTOR NEURAL INTEGRATOR - DIRECT EXPERIMENTAL-EVIDENCE IN THE CAT

1. As far as horizontal eye movements are concerned, the well-known hy pothesis, not yet experimentally proved, of the common neural integrat or states that the eye-position signal is generated by a common networ k, regardless of the type of versional movement. The aim of this study was to evaluate the validity of this hypothesis by checking whether t he sensitivity to eye position of the neurones of the nucleus preposit us hypoglossi (NPH) (the main component of the system integrating the different incoming velocity signals) would be the same regardless of t he type of versional movement. 2. The discharge of sixty-five NPH neur ones was recorded in the alert cat during spontaneous eye movements ma de in the light and in response to sinusoidal rotations of the head in complete darkness. 3. For each NPH neurone, the sensitivity to eye po sition was determined from measurements carried out during intersaccad ic fixation. The discharge rate of the studied neurone was plotted aga inst eye position. The slope of the resulting regression line gave the sensitivity (measured during intersaccadic fixation in the light) of the. neurone to eye position, which was termed K-f. 4. A new method wa s developed to measure the sensitivity to eye position (K-v) of neuron es during vestibular slow phases. The difficulty came from the fact th at, during slow phases, eye velocity and eye position changed simultan eously and that each of those two variables could influence neuronal a ctivity For each neurone, the instantaneous firing rate was measured e ach time the eye passed through a given position during any slow phase generated during any vestibule-ocular reflex. At a given position, th e discharge rate of the neurone under study was plotted against the ey e velocity. From the resulting linear regression line, two interesting values were obtained: its slope, corresponding to the sensitivity of the neuron to eye velocity, R(v), (at that given eye position) and its 'y'-intercept, F(0), the interpolated firing rate when the eye veloci ty was zero. This procedure was repeated for different eye positions. The values of F(0) were then plotted against the eye positions. The sl ope of the resulting regression line gave the sensitivity (measured du ring vestibular stimulation) of the neurone to eye position, which was termed K-v. 5. The errors on the individual values of K-f and K-v wer e assessed in order to allow a statistical comparison at the single un it level. 6. We found that, for each of sixty-five neurones, the sensi tivity to eye position measured during intersaccadic fixation in the l ight was equal to the sensitivity to eye position measured during the vestibulo-ocular reflex (VOR) elicited in complete darkness. We conclu de that our results favour the hypothesis of a unique horizontal oculo motor integrator for all versional movements.