The Distribution of Microsaccade Directions Need Not Reveal the Location of Attention: Reply to Rolfs, Engbert, and Kliegl

Rolfs, Engbert, and Kliegl have demonstrated that microsaccades can reveal the direction of covert attentional shifts either toward (Engbert Kliegl, 2003) or away from (Rolfs, Engbert, Kliegl, this issue) a peripheral cue under certain circumstances. Our previous analysis (Tse, Sheinberg, Logothetis, 2002) of the fixational eye movements collected at the same time as the change-detection data upon which Rolfs et al. comment revealed no effect of cue location on mean fixational eye movements. Other researchers subsequently reported finding changes in the distribution of microsaccades after the occurrence of a peripheral cue (Engbert Kliegl, 2003; Hafed Clark, 2002), contradicting our null finding. Examining macaques, Horwitz and Albright (2003) found that the incidence of microsaccades around fixation increased after the onset of a peripheral cue, but, like us, they found no relationship between cue location and microsaccade direction. Because we limited our previous analysis to mean trajectories of thousands of fixational eye movement traces, it was possible that we missed changes that would be evident only in the spatial distribution of microsaccades, which could have contributed negligibly to averaged eye movement traces. We therefore reanalyzed our data using Engbert and Kliegl's (2003) code for microsaccade detection. We now report that there was indeed no consistent effect of the location of a peripheral cue on the distribution of microsaccade directions in these data. Thus, we find that the distribution of microsaccades need not correlate with the direction of attentional allocation