Gaze strategies during linear motion in head-free humans.

1. Eye-head coordination strategies during horizontal displacements along the y (interaural) axis were investigated in human subjects seated on a sled (linear accelerator device) and tested in head-free conditions. They were instructed to stabilize their gaze, while in motion, on an earth-fixed memorized target and then, after cart immobilization, to look again at the real target. The last part of the test required a corrective saccade, which enabled us to evaluate the error of the subject's displacement estimation. Eye and head compensatory reflexes were tested within the 0.001-0.2 g acceleration range with a sinusoidal motion amplitude of 0.8 m peak to peak. 2. Fixation stabilization on a memorized target was achieved by different eye-head coordination strategies. According to the relative contribution of eye and head motion, a continuum among individual strategies was observed, covering a range of head contributions varying from 0 to almost 100%. All these strategies were well adapted because they contributed to the counteraction of the displacement and led to an optimal gaze accuracy. 3. The use of various gaze strategies during linear motion to achieve the same movement differed according to the subject, but also depended upon motion kinematics. As a rule, head contribution increased as the magnitude of linear acceleration was enhanced. 4. Different eye-head coordination strategies implicated either a linear vestibulo-ocular reflex (LVOR) or ocular responses composed of a combination of antagonistic angular and linear vestibulo-ocular reflexes (AVOR-LVOR). The slow phase direction of these two oculomotor responses for fixation stabilization on the target were compensatory and anticompensatory, respectively. 5. One of the major points of this study was the contribution of the saccadic system to gaze strategies, even in our experimental conditions where the head was free to move. We concluded that vestibular-saccadic cooperation appears to be a common feature in the elaboration of adequate fixation stabilization in daily life situations. 6. The functional coupling of these various subsystems involved in fixation stabilization depended on the range of motion: while the acceleration increased, the saccadic eye movements were replaced by vestibulo-ocular responses whose slow phase direction was opposite that of head motion and, therefore, directed away from the target. 7. Fast components of the nystagmic pattern of eye movements were able to improve gaze position accuracy, bringing the eyes toward the memorized target.(ABSTRACT TRUNCATED AT 400 WORDS)