| Under natural conditions, most shifts of the line of sight are to objects that he in different directions and different distances. Such gaze shifts require a combination of a saccade and vergence movement; electrophysiological studies have indicated that separate neuronal populations generate each type of movement. The research presented here addresses, in three parts, the issue of how the neural control systems that generate these gaze shifts interact. First, the dynamics of anticipatory vergence responses were studied, and it was shown that the vergence system possesses predictive properties similar to, but independent of, the saccadic system. Second, memory-guided saccade-vergence movements were studied to test two current models; the findings supported a Hering model that incorporated separate saccadic and vergence generators but were evidence against a Helmholtz model in which each eye was driven by monocular saccadic generators. Finally, although most saccadic-vergence movements are synchronized, it was found that positioning the near target above the far target caused temporal dissociation of saccadic and vergence components, thus providing an experimental strategy to investigate interaction of the two systems. Synchronized and temporally dissociated responses were used to formally test and extend a model of saccade-vergence interactions, which hypothesized that the saccadic-generating mechanism influences the activity of the vergence-generating mechanism, when the two movements are synchronized. This phenomenon of one class of movements being able to improve the performance of another can be extended to other motor control systems, and has implications for a variety of natural activities. |
