Microsaccades and Attention: Does a Weak Correlation Make an Index?

Abstract

In the recent literature on microsaccades and attention, two questions have been conflated. There is a broad question of whether microsaccades are related to attention, and there is a more specific question about whether microsaccades serve as an index of attention. We are happy to agree that microsaccades are related to attention. However, the claim that ''microsaccades are an index of covert attention'' (in the title of Laubrock, Engbert, Rolfs, & Kliegl, 2007, this issue) depends on a strong correlation. Were this claim true, a researcher might be able to conduct a study relying entirely upon microsaccade direction as a measure of attentional deployment. Although we would be delighted to be able to conduct such an experiment, both our data and those of Laubrock et al. suggest that microsaccades cannot be used as a reliable marker of covert attention.Laubrock et al. make two arguments. First, reviewing our experiment (Horowitz, Fine, Fencsik, Yurgenson, & Wolfe, 2007, this issue), they criticize our selection of trials on which the directions of the cue and microsaccade disagreed, arguing that this selection would inevitably lead to a negative microsaccade-congruency (MC) effect. Second, they report new data demonstrating a correlation between MC and reaction time (RT). We deal with these points in reverse order.Laubrock et al. have demonstrated a statistically reliable relationship between MC and RT. An incongruent microsaccade was associated with a 6-ms slowing of RT. This is a weak effect, an order of magnitude smaller than the 81-ms effect associated with an invalid cue. This effect may be statistically significant, but it suggests that microsaccade direction provides very little useful information about the spatial distribution of attention.Also, Laubrock et al. argue, under a seemingly reasonable set of assumptions, that in our study, trials on which the microsaccade direction diverged from the cue direction were dominated by trials on which the microsaccade did not follow attention, even if microsaccades usually did follow attention. The argument is as follows. Assume that observers direct attention toward the cue with probability w, and that the microsaccade reflects the direction of attention with probability x. Let v denote cue validity. There are two kinds of trials on which the cue is valid but the cue direction and microsaccade direction disagree: (a) valid trials (v) on which attention does not follow the cue (1 À w) and the microsaccade follows attention (x) and (b) valid trials (v) on which attention follows the cue (w) but the microsaccade does not reflect attention (1 À x). The proportion of trials of the first type is given by p 1 5 v(1 À w)x, and the proportion of trials of the second type is given by p 2 5 vw(1 À x). Laubrock et al. note that if w 5 v and x 5 .75 (i.e., the microsaccade is almost as good an index of attention as the cue), the predictions would be qualitatively consistent with our results.However, this scenario is not quantitatively consistent with our results. Although w a...

Capacity and precision in an animal model of visual short-term memory

“…Although there is some debate concerning the suitability of microsaccades as an index of attention (Horowitz, Fencsik, Fine, Yurgenson, & Wolfe, 2007; Horowitz, Fine, Fencsik, Yurgenson, & Wolfe, 2007; Laubrock, Engbert, Rolfs, & Kliegl, 2007), we found that making a microsaccade significantly affected the subject’s behavioral performance, suggesting that in this case, microsaccades can indeed reveal the locus of covert spatial attention. Similar to the findings in humans, we found that focusing attention on an item significantly affects the representational precision of memories.…”

Sustained spatial attention accounts for the direction bias of human microsaccades

“…The cause of this microsaccade direction effect has been controversial. While there have been multiple suggestions that microsaccades can serve as an overt indicator of covert spatial attention [6][7][8]13,14,[18][19][20][21] , others propose a dependency on other factors, such as complicated oculomotor response to visual stimuli [22][23][24][25][26][27][28] .…”

Event-related functional MRI of cortical activity evoked by microsaccades, small visually-guided saccades, and eyeblinks in human visual cortex

“…Barlow (1952) proposed that microsaccades occur upon a shift of visual attention, and it has since been shown that microsaccade rate is indeed modulated by shifts of attention (Engbert and Kliegl, 2003; Hafed and Clark, 2002; Horwitz and Albright, 2003; Rolfs et al, 2004; Tse et al, 2002, 2004; Turatto et al, 2007; Valsecchi and Turatto, 2009) and perceptual state (Hsieh and Tse, 2009; Martinez-Conde et al, 2006), although microsaccade occurrence is not limited to the time of attentional shifts or perceptual shifts (Tse, Caplovitz and Hsieh, 2009). However, there has been a debate about whether microsaccade directionality is also influenced by the directionality of attentional shifts, with some arguing for an influence of attention (Engbert, 2006; Engbert and Kliegl, 2003; Hafed and Clark, 2002; Laubrock et al, 2007; Rolfs et al, 2004; Turatto et al, 2007; Valsecchi and Turatto, 2009), and others finding that attention plays no significant role in the distribution of microsaccade directions (Horwitz and Albright, 2003; Horowitz et al, 2007a, b, Tse et al, 2002, 2004). Whether microsaccade directionality is correlated with attentional directionality or not, the fact that microsaccade rate is modulated by attentional shifts suggests that eye-shifting circuitry is not independent of attention-shifting circuitry.…”