Interaction of Eye and Head Movements during Smooth Pursuit in Macular Degeneration

Purpose: There is limited evidence that suggests head movements can improve smooth pursuit performance in participants with healthy vision (Ackerley & Barnes 2011). We have previously shown that smooth pursuit is impaired in individuals with macular degeneration (MD), and the degree of impairment depends on the direction of target motion (Shanidze et al. 2016). In this study, we developed a novel eye and head movement tracking approach to investigate whether participants with MD use head movements in pursuit and how these movements relate to scotoma and pursuit performance. Methods: Four MD individuals with differing degrees of central field loss (75-95 yo) and two controls (75-76 yo) participated. With their heads unrestrained, observers were seated in front of a screen and asked to pursue targets moving in a step ramp (Rashbass 1961). They first fixated a central target (1°), which then made a 6° step, and moved at 10°/s in one of 6 possible directions (horizontally, vertically or on a diagonal), selected at random (Fig 2A, black line). Participants were asked to follow the target with their eyes. Eye movements were recorded using a head-mounted eyetracker (PupilLabs, Germany). Head movements were recorded synchronously with eye movements, using a 9-axis inertial measurement unit (IMU, LP-Research, Japan, Fig 1). Results: Both MD and control participants demonstrated a range of eye-head coordination pursuit strategies. In both groups, there were individuals whose overall pursuit strategy was more reliant on eye movements and others who used a combination of eye and head. Average head velocities of participants ranged between 0.9 and 5.4 °/s for MDs and 0.3 and 3.4 °/s for controls (Fig 2B). In controls, increased head velocities clearly corresponded to a decrease in velocities of the eyes relative to the head. This trend was evident in some, but not all of the MD participants (P1&P3). For participants who did not exhibit this coordination, pursuit gains tended to be greater than 1, suggesting an increased retinal slip (Fig 2C). Extent of binocular scotoma did not predict eye-head movement strategy across observers. Conclusions: Head movement strategies vary across individuals during smooth pursuit. Eye-head coordination strategy does not appear to depend on scotoma extent across observers, although improper coordination of eye and head movements may be related to decrement in smooth pursuit performance.