Rare but precious: Microsaccades are highly informative about attentional allocation

Abstract

To clarify the relation between attention and microsaccades, we monitored microsaccades while observers performed tasks with different attentional demand. In four high-demand conditions, observers shifted attention covertly to a peripheral location, or focused attention at fixation. Three corresponding low-demand conditions on physically identical displays provided a basis for comparison. Our results revealed two distinct effects of attentional load: higher loads were associated consistently with lower microsaccade rates, but also with increased directional selectivity (up to 98% congruent). In short, when microsaccades were most rare, the direction of microsaccades proved most informative about the location of the attention focus. The detailed time-courses of the two effects differed, however, suggesting that they may reflect independent processes.

On the Dissociation between Microsaccade Rate and Direction after Peripheral Cues: Microsaccadic Inhibition Revisited

“…This was justified because, when microsaccade directions were biased by peripheral stimuli, these effects were strongly clustered around the true location of the stimuli or vectoraverage (see Results). As an additional sanity check, we also repeated our direction time courses but now using the same time binning as that used for the rate time courses described above (20 ms bin widths stepped in 20 ms jumps), and we confirmed that the dissociation between rate and direction that we show in Results was not the result of different time binning strategies (see also Pastukhov and Braun, 2010, in which the same dissociation was also described).…”

“…For example, if at time t Ϯ t 0 ms, there were 100 microsaccades across trials, and 75 of them were directed toward the peripheral stimulus, then the fraction of movements directed toward the stimulus during this time window was 0.75. Using this approach, we isolated microsaccade direction biases independent of rate, similar to previous studies (Pastukhov and Braun, 2010;Pastukhov et al, 2012).…”

“…Our model was aimed at parsimoniously explaining the apparent dissociation between time and space in the microsaccades of the tasks above (and also observed previously in both humans and monkeys) (Laubrock et al, 2005;Pastukhov and Braun, 2010;Hafed et al, 2011).…”

“…This phenomenon is described as microsaccadic inhibition or, more generally, as a microsaccadic rate signature. In parallel, microsaccade directions are also biased (e.g., Hafed and Clark, 2002;Engbert and Kliegl, 2003;Pastukhov and Braun, 2010;Pastukhov et al, 2012). Early models of these effects have attributed to the superior colliculus (SC) a role in microsaccade generation (Rolfs et al, 2008;Hafed et al, 2009;, consistent with neurophysiology (Hafed et al, 2009Hafed and Krauzlis, 2012).…”

“…However, these models diverge in some of their predictions, as we discuss in this paper, and recent results also call for possible additions to them. For example, Pastukhov and Braun (2010) found that the strongest directional biases of microsaccades toward an attended location surprisingly occurred at the time of lowest microsaccade rate. Based on these results, these authors suggested that there is dissociation between microsaccade rate and direction.…”