Tracking in 3-D space under natural viewing condition

To track a small visual target in 3-D space, the two eyes have to move in different directions and/or at different velocities. This tracking might be accomplished by a disjunctive pursuit system, which uses separate motion processing of each individual eye but no disparity signal (hypothesis 1), or by the conjugate pursuit and the vergence system (hypothesis 2). To test the validity of the two hypotheses we recorded eye movements in five healthy human subjects with the scleral search-coil method. A small dim laser stimulus was presented on an earth horizontal platform. A position-ramp stimulus was presented in eight different directions: rightward or leftward, convergence or divergence, or a combination of them. We compared a fusible with an un-fused and a monocular viewing condition to assess whether a disparity signal is needed for 3-D tracking. Fusion was prevented by a vertical prism. We compared the monocular with the prism viewing condition to examine the effect of retinal motion signals of either one or both eyes on the tracking performance in the absence of disparity signals. Results revealed severe impairment of tracking in depth, while tracking in pure horizontal directions remained unaffected during the prism and monocular as compared to the binocular viewing condition. These data support hypothesis 2.