MODULATION OF THE HUMAN VESTIBULOOCULAR REFLEX DURING SACCADES - PROBING BY HIGH-FREQUENCY OSCILLATION AND TORQUE PULSES OF THE HEAD

1. We probed the gain and phase of the vestibuloocular reflex (VOR) du ring the execution of voluntary gaze saccades, with continuous oscilla tion or acceleration pulses, applied through a torque helmet. 2. Small -amplitude (<1 degrees), high-frequency (10-14 Hz) head oscillations i n the horizontal or vertical plane were superimposed on ongoing horizo ntal gaze saccades (40-100 degrees). Torque pulses to the head (''with '' or ''against'' gaze) were superimposed on 40 degrees horizontal sac cades. Eye and head movements were precisely measured with sensor coil s in magnetic fields. 3. Techniques were developed to separate the osc illatory (horizontal or vertical) component from the gaze shift and ob tain VOR gain and phase with Fourier techniques from the relation betw een eye-in-head and head oscillations. These involved either subtracti on of exactly matching saccades with and without oscillation (drawback : low yield) or time shifting of successive trials to synchronize the oscillations (drawback: slight time blurring of saccades). 4. The resu lts of these matching and synchronization methods were essentially ide ntical and consistent. Presaccadic gain values of the horizontal VOR ( typically about unity) were reduced by, on average, similar to 20 and 50% during horizontal saccades of 40 and 100 degrees, respectively. Th ese percentages may be truncated because of methodological limitations , but even after taking these into account (on the basis of simulation experiments with 2 different, theoretical profiles of suppression) ou r results do not support a complete saccadic VOR suppression for any s ubstantial fraction of saccadic duration. Qualitatively similar change s were found when the vertical VOR was probed during 100 degrees horiz ontal saccades. 5. Concomitantly with the reductions in gain, VOR phas e was advanced by similar to 20 degrees during the saccade. 6. In the wake of gaze saccades, VOR gain was consistently elevated (to similar to 1.0) above the presaccadic level (similar to 0.9). We submit that t his mechanism ensures stable fixation of the newly acquired target at a time when the head is still moving substantially. 7. Although the re sponses to head torque pulses showed idiosyncratic asymmetries, analys is of the differences in eye and head movements for pulses with and ag ainst consistently showed a sharp fall of VOR gain at saccadic onset, following an approximately exponential course with a time constant of similar to 50 ms. This decay may be assumed to reflect VOR gain for a period of similar to 50 ms, after which secondary gaze control mechani sms become dominant. 8. The time course of the gain decay and phase sh ift of the VOR suggest that suppression of the ''integrative (position ) loop'' of the VOR circuit was more complete than suppression of the direct, ''velocity'' pathway.