Academic journal article Attention, Perception and Psychophysics

When Do Microsaccades Follow Spatial Attention?

Academic journal article Attention, Perception and Psychophysics

When Do Microsaccades Follow Spatial Attention?

Article excerpt

Following up on an exchange about the relation between microsaccades and spatial attention (Horowitz, Fencsik, Fine, Yurgenson, & Wolfe, 2007; Horowitz, Fine, Fencsik, Yurgenson, & Wolfe, 2007; Laubrock, Engbert, Rolfs, & Kliegl, 2007), we examine the effects of selection criteria and response modality. We show that for Posner cuing with saccadic responses, microsaccades go with attention in at least 75% of cases (almost 90% if probability matching is assumed) when they are first (or only) microsaccades in the cue-target interval and when they occur between 200 and 400 msec after the cue. The relation between spatial attention and the direction of microsaccades drops to chance level for unselected microsaccades collected during manual-response conditions. Analyses of data from four cross-modal cuing experiments demonstrate an above-chance, intermediate link for visual cues, but no systematic relation for auditory cues. Thus, the link between spatial attention and direction of microsaccades depends on the experimental condition and time of occurrence, but it can be very strong.

Eye movements and visual attention are intimately related. The major function of saccadic eye movements is to move objects of interest into the fovea, the retinal region of highest acuity, for close inspection during the following fixation (Findlay & Gilchrist, 2003). Fixating an object means overtly attending to it. However, attention can also be covert-that is, dissociated from fixation position. When covert shifts of attention are induced with a centrally presented cue, responses to targets subsequently appearing at the cued peripheral location (valid-cue trials) are faster than responses to targets at the opposite location (invalid-cue trials; Posner, 1980; Posner, Snyder, & Davidson, 1980). With the phrase "covert shifts of attention," reference is made to the absence of large saccadic eye movements during the cue-target interval (CTI). Whereas covert attention shifts are by definition not accompanied by overt saccades, there is evidence that saccades are obligatorily preceded by covert shifts of attention (i.e., processing at the saccade target is enhanced before saccade execution; Deubel & Schneider, 1996; Hoffman & Subramaniam, 1995; Kowler, Anderson, Dosher, & Blaser, 1995).

A measure that can be used to track the deployment of covert attention may be useful in a number of contexts. Given the close relationship between saccades and visual attention, one might wonder whether traces of covert attention shifts can be detected in oculomotor activity during fixations. Since the absence of saccades during covert attention shifts does not imply the absence of fixational eye movements, microsaccades-small saccade-like movements with amplitudes <10 that occur during attempted ocular fixation (see Engbert, 2006, for a review)-have been proposed as a measure of covert attention. Given the result that microsaccades and saccades are probably regulated by the same physiological structures at the level of the superior colliculus (SC; Hafed, Goffart, & Krauzlis, 2009; Rolfs, Kliegl, & Engbert, 2008) and downstream (van Gisbergen, Robinson, & Gielen, 1981), a relationship between covert attention and microsaccades may not be particularly surprising. In fact, evidence in favor of such a relationship has been presented: A considerable amount of research has demonstrated effects of attentional cue presentation on rate and direction of microsaccades (Corneil, Munoz, Chapman, Admans, & Cushing, 2008; Engbert & Kliegl, 2003; Galfano, Betta, & Turatto, 2004; Gowen, Abadi, Poliakoff, Hansen, & Miall, 2007; Hafed & Clark, 2002; Kohama & Usui, 2002; Laubrock, Engbert, & Kliegl, 2008; Turatto, Valsecchi, Tamè, & Betta, 2007).

A number of physiological control loops at several levels converge on the SC. For example, there is the low-level, reflexive, retino-tectal loop bypassing even the lateral geniculate nucleus. …

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