Aim: 

Ocular and manual pursuit depends on sensory and motor representation of the target in space. In the present research pursuit system has been analysed to evidence how sensory and motor coordinate systems control manual and ocular tracking.

Methods: 

Subjects were asked to remember a position of a target presented before the trials, and to track it as imaginary target in darkness, while they were asymmetrical oscillated in the horizontal plane (40° amplitude, 20% asymmetry ratio and 0.15 Hz frequency). Eye tracking was recorded by EOG and hand tracking by measuring the position of a pointer. Trials were carried out in two different conditions 1) by changing horizontal head position re-trunk and 2) by unilaterally vibrating neck muscles.

Results: 

The asymmetric vestibular stimulation induced a progressive illusory deviation of the imaginary target re-body in the opposite direction of fast stimulation side. Subjects were able to pursuit the imaginary target according to the illusory self-motion perception. However, while hand tracking did not present any discontinuity, eye tracking changed abruptly from a pursuit movement to a typical VOR at ~50° of target displacement. The limit of ocular pursuit increased to ~70° with the head re-trunk deviated toward the faster stimulation side and decreased to ~30° with the head in the opposite direction. Unilateral neck muscle vibration had effects similar to head deviation on the same side.

Conclusion: 

The limit of ocular pursuit and its modulation by head-trunk angle suggests that the eye movement pursuit is under a control of a coordinate system defined by the trunk in space. In contrast with the eye, manual pursuit does not present limit and this may indicate a different central sensory representation.