Purpose : Saccades and smooth pursuit are inextricably linked, particularly in cases of low gain, where saccades can help bring the fovea back on target. Individuals with macular degeneration (MD) have compromised foveas due to central field loss, which impacts both fixation stability and saccades, as well as the interaction between the saccade and pursuit systems. To investigate how saccades associated with pursuit are affected, we conducted a quantitative analysis of binocular smooth pursuit eye movement data collected for a prior study (Shanidze et al., 2016) of smooth pursuit in MD. Here we extend that work by characterizing saccadic intrusions in MD participants during pursuit and pre-pursuit fixation.
Methods : We examined saccade frequency, magnitude, and direction across viewing conditions for MD (7, 4F) and control participants (4, 1F). Participants were asked to pursue a 1° annular target, moving in a step ramp (6° step, 12° ramp) in one of 6 directions (4 cardinal & 2 oblique). Saccades were detected offline when eye velocity exceeded 40°/s, or acceleration exceeded 150 (°/s2) and confirmed manually by an experimenter during the fixation and pursuit portions of each trial.
Results : Individuals with MD made significantly more saccades during fixation and pursuit than controls (Fig F). During pursuit, both control and MD participants made saccades aligned with the target direction. However, MD participants also made saccades in non-target directions (Fig A-D). To quantify this difference, we computed the anisotropy index (a comparison of saccades aligned with and orthogonal to the target, Fig E). We found controls had a significantly higher anisotropy index than MDs, indicating greater alignment with target direction.
Conclusions : Our analysis suggests that despite higher frequency, a large number of saccades during pursuit in MD participants are not in the target direction, and thus are not catch-up saccades that serve to keep the eye on the target. The saccades in non-target directions appear to be associated with the significant increase in saccades during fixation. Thus, MD participants do not effectively use saccades to compensate for the lower pursuit gains reported previously.