The ineffectiveness of efference copy to eye gaze control can be clearly demonstrated by measuring pursuit of a single object that appears with regular, predictable timing and velocity, or undergoes a transient occlusion after steady-state pursuit has been achieved.
In the latter case, smooth pursuit decays rapidly following object occlusion and is then maintained at a reduced gain if the object is expected to reappear. We have shown in multiple experiments that when the reappearance characteristics are known in advance, both anticipatory and predictive smooth pursuit is observed during occlusion.
Notably, there is also a saccadic response that works in combination with smooth pursuit in order to locate the eyes close to the unseen position of the occluded object. To explain anticipatory and predictive gaze control, we proposed a model involving a direct and indirect pathway.
The model simulated well the ocular response during short-duration transient occlusion by incorporating short-term (i.e., within-trial) and long-term (between-trial) predictive influences in the form of a direct (e.g., efference copy) loop that operates during random-order trials, and an indirect (i.e., internal memory structure) loop that provides more persistent input during blocked-order trials.
The neurophysiological basis of the model was subsequently extended by other researchers to account for pursuit tasks that place an even greater demand on cognitive processes, and how these change as a function of normal ageing and neurodegenerative disease.
Smooth pursuit eye movements allow us to track moving objects, but when the object is briefly occluded, the brain must rely on prediction rather than visual input.
Earlier work showed that when participants generate the movement themselves (self‑generated motion), additional sensorimotor signals help maintain pursuit during occlusion.
Our project examined whether moving the arm in sync with an externally generated moving target (i.e., the arm does not cause the motion) could similarly support predictive pursuit.
Key findings were that concurrent upper‑limb movement improved smooth pursuit during occlusion, both for constant‑velocity and accelerating targets, but could not overcome the loss of drive of retinal input.
These results suggest that the brain integrates extra-retinal signals from the arm to enhance smooth pursuit during occlusion, supporting the idea that more complex predictive processes are required to account for target extrapolation in the absence of retinal input.
