Figure 8.

A, B, Premicrosaccadic ocular drifts using different eye tracking techniques. We repeated the same box counting procedure of Engbert and Mergenthaler (2006) on both monkeys, but now using the video-based eye tracker (black curves). In monkey N (A), we also measured the same eye simultaneously using scleral search coils (gray curve; note that the box counts for the coils are lower than those for the video-based eye tracker because coils are much less noisy). In both monkeys, the video-based data showed an apparently artifactual reduction in ocular drift ∼200 ms before microsaccade onset, replicating Engbert and Mergenthaler (2006). This reduction was not present in simultaneously measured search coil data (A, gray) and in all our other analyses with coils (Figs. 1–5). This discrepancy likely reflects the limitation of video-based eye tracking for measuring slow ocular drifts (Kimmel et al., 2012). All conventions are similar to Figure 4A and C.