An important aspect of natural behavior is that observers have to coordinate body, head, and eye movements to sample the visual world. However, this is often not given in an experimental setting. For example, to understand oculomotor control, experiments often use a setup with a computer monitor and a chin-rest to isolate eye movement behavior. In contrast, in natural behavior, the head is rarely stable and the body moves freely (
Hayhoe & Lerch, 2022;
Land, 1992;
Pelz, Hayhoe, & Loeber, 2001). The addition of these degrees of freedom can have a substantial influence on how we move our eyes: even for the small saccade amplitudes between 5° and 10° of visual angle that are typically measured in the lab under head-restrained conditions if tested under unrestrained conditions, observers will naturally use accompanying head movements (
Stahl, 1999), which changes the dynamics of the oculomotor system (
Morasso, Bizzi, & Dichgans, 1973). For example, a very basic observation for saccadic eye movements is the so-called main sequence, a strong relationship between the amplitude and peak velocity of a saccade (
Bahill, Clark, & Stark, 1975), but even this fundamental relationship changes when the eyes move alone or together with the head (
Epelboim et al., 1997). In addition, while eye and head often work together to explore scenes (
Bischof, Anderson, & Kingstone, 2023), how we control the head can differ from how we control the eyes (
David, Beitner, & Võ, 2020;
Solman, Foulsham, & Kingstone, 2017). In particular, in unconstrained viewing, multiple saccades are often made during one continuous head movement, with intervening fixations on objects while the eyes counterrotate with respect to the head to keep the gaze stable (
Fang, Nakashima, Matsumiya, Kuriki, & Shioiri, 2015). This finding indicates that measurements of isolated eye movements, while being head fixed, might not be a good descriptor of gaze dynamics (which represents the combination of eye and head movements) under unconstrained, natural conditions.