Combined Eye Head Movements

Combined eye-head movements occur when the head is passively perturbed and the eyes compensate by the VOR. However, under natural circumstances saccadic gaze shifts and smooth pursuit consist of a combination of eye and head movements, especially when the target eccentricity is too large to be reached with the eye alone. Active combined eye-head movements raise several questions [104], for example whether the VOR is active during the gaze shift, or whether the local feedback loops in the saccadic system operate on gaze (eye plus head) or eye-in-head signals. While it is usually accepted that the VOR is shut off during the gaze shift, models on combined eye-head gaze shifts reached different conclusions concerning the feedback loops: while most models assume that gaze is the controlled variable [105-107], others propose that eye and head movements are controlled separately with the head controller influencing the saccadic burst generator for the eye [108]. The 3-D behavior of eye and head during gaze shifts has successfully been explained by an elegant model [109] which shows how the eye may anticipate the final head position. Finally, a recent neural network model showed how superior colliculus and cerebellum may interact for combined eye-head gaze shifts [50].

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