Several studies suggest that retinal image shifts (e.g., due to microsacccades) can cause flips in perceptual dominance (Sabrin & Kertesz, 1980, 1983; van Dam & van Ee, 2006). Several findings in the microsaccade literature make us wonder whether the onset biases reported in the paper are related to eye movement biases. First, microsaccade direction has a very strong idiosyncratic component to it; i.e., subjects preferrably generate microsaccades in certain directions (e.g., Nachmias, 1959). Second, it is well established that the direction of microsaccades is correlated with the direction of covert visual attention (e.g., Engbert & Kliegl, 2003; Hafed & Clark, 2002; Laubrock et al., 2005; Rolfs et al., 2005). Further, sudden stimulus onsets like in the onset-rivalry case will lead to an increased likelihood of microsaccades after an initial onset-related suppression.

We have noticed that many subjects show local biases at diagonal locations, corresponding to the orientations of the gratings used. If microsaccades account for perceptual flips in these data, these flips would have appeared most likely for the diagonal stimulus positions, since in these cases attention-aligned microsaccades (i.e., towards the test location) will shift the retinal image parallel to one eye's, but orthogonal to the other eye's grating. Thus, the retinal image will change only in one eye, which according to van Dam and van Ee's (2006) results would increase the chance of a perceptual flip towards the interpretation suggested by that eye's stimulus.

Based on this speculation, we predict, for instance, that subject S8 in Figure 3 had a red grating with a 45° orientation (////), while subject S4 in Figure 3 had a red grating with a 135° orientation (\\\\). Thus, we wonder whether there is any relation between the orientation of the gratings and the onset biases. In general, we would appreciate a discussion of the potential influences of fixational eye movements on these results.

References:
Engbert, R., & Kliegl, R. (2003). Microsaccades uncover the orientation of covert attention. Vision Research, 43, 1035-1045.
Hafed, Z. M., & Clark, J. J. (2002). Microsaccades as an overt measure of covert attention shifts. Vision Research, 42, 2533-2545.
Laubrock, J., Engbert, R., & Kliegl, R. (2005). Microsaccade dynamics during covert attention. Vision Research, 45, 721-730.
Nachmias, J. (1959). Two-dimensional motion of the retinal image during monocular fixation. Journal of the Optical Society of America, 49, 901-908.
Rolfs, M., Engbert, R., & Kliegl, R. (2005). Crossmodal coupling of oculomotor control and spatial attention in vision and audition. Experimental Brain Research, 166, 427-439.
Sabrin, H. W., & Kertesz, A. E. (1980). Microsaccadic eye movements and binocular rivalry. Perception & Psychophysics, 28, 150-154.
Sabrin, H. W., & Kertesz, A. E. (1983). The effect of imposed fixational eye movements on binocular rivalry. Perception & Psychophysics, 34, 155-157.
van Dam, L.C.J. & van Ee, R. (2006). Retinal image shifts, but not eye movements per se, cause alternations in awareness during binocular rivalry. Journal of Vision, 6, 1172-1179.