All the above studies were conducted in head-restrained conditions. Studies of saccades (e.g., Freedman & Sparks,
1997; Goossens & Van Opstal,
1997; Guitton, Munoz, & Galiana,
1990; Tomlinson & Bahra,
1986) and pursuit (e.g., Collins & Barnes,
1999; Daye, Blohm, & Lefèvre,
2012; Lanman, Bizzi, & Allum,
1978) with the head free to move did not account for saccade-pursuit interactions, except for Daye, Blohm, and Lefèvre (
2010) and Herter and Guitton (
1998). The present study sought to analyze saccade-pursuit interactions in head-unrestrained conditions, which introduces several challenges. First, because the eyes and head have different dynamics, the command timing to the eye and the neck muscles is crucial to ensure an accurate gaze trajectory. Second, with the head free to move, the CNS must account for the vestibulo-ocular reflex (VOR). To keep a clear vision, any head movement that generates a perturbation of the gaze goal must be negated. However, the VOR can be counterproductive in numerous situations (e.g., when gaze and head are moving in the same direction). Two mechanisms have been proposed to explain the modulation of the VOR. The first assumes that an opposite signal is added to the VOR to cancel the counter-rolling of the eye that would be caused by gaze movements (VOR cancellation, during pursuit, Lanman et al.,
1978). The second decreases the gain of the VOR when it would be counterproductive (VOR suppression, during gaze shifts, Cullen, Huterer, Braidwood, & Sylvestre,
2004; Lefèvre, Bottemanne, & Roucoux,
1992). Finally, although gaze and head movements usually have similar trajectories, they can also be controlled independently (Collins & Barnes,
1999). Therefore, the goals of the head and the gaze movements could partly be in conflict, e.g., when head movement is out of phase with the target movement.