TESTS OF A LINEAR-MODEL OF VISUAL-VESTIBULAR INTERACTION USING THE TECHNIQUE OF PARAMETER-ESTIMATION

The goal of this study was to test whether a superposition model of sm ooth-pursuit and vestibuloocular rifler (VOR) eye movements could acco unt for the stability of gaze that subjects show as they view a statio nary target, during head rotations at frequencies that correspond to n atural movements. Horizontal smooth-pursuit and the VOR were tested us ing sinusoidal stimuli with frequencies in the range 1.0-3.5 Hz. Durin g head rotation, subjects viewed a stationary target either directly o r through an optical device that required eye movements to be approxim ately twice the amplitude of head movements in order to maintain fovea l vision of the target. The gain of compensatory eye movements during viewing through the optical device was generally greater than during d irect viewing or during attempted fixation of the remembered target lo cation in darkness. This suggests that visual factors influence the re sponse, even at high frequencies of head rotation. During viewing thro ugh the optical device, the gain of compensatory eye movements decline d as a function of the frequency of head rotation (P < 0.001) but, at any particular frequency, there was no correlation with peak head velo city (P > 0.23), peak head acceleration (P > 0.22) or retinal slip spe ed (P > 0.22). The optimal values of parameters of smooth-pursuit and VOR components of a simple superposition model were estimated in the f requency domain, using the measured responses during head rotation, as each subject viewed the stationary target through the optical device. We then compared the model's prediction of smooth-pursuit gain and ph ase, at each frequency, with values obtained experimentally. Each subj ect's pursuit showed lower gain and greater phase lag than the model p redicted. Smooth-pursuit performance did not improve significantly if the moving tar-get was a 10 deg x 10 deg Amsler grid, or if sinusoidal oscillation of the target was superimposed on ramp motion. Further, s ubjects were still able to modulate the gain of compensatory eye movem ents during pseudo-random head perturbations, making improved predicto r performance during visual-vestibular interactions unlikely. We concl ude that the increase in gain of eye movements that compensate for hea d rotations when subjects view, rather than imagine, a stationary targ et cannot be adequately explained by superposition of VOR and smooth-p ursuit signals. Instead, vision may affect VOR performance by determin ing the context of the behavior.