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Sebastian Pannasch, Johannes Schulz, Boris Velichkovsky; Explaining visual fixation durations in scene perception: Are there indeed two distinct groups of fixations?. Journal of Vision 2010;10(7):138. doi: 10.1167/10.7.138.
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Everyday oculomotor behavior reveals a substantial variation in the duration of visual fixations. This variance can be generally explained by direct and indirect control mechanisms. Recent experiments using a scene onset delay paradigm demonstrated a prolongation of a part of the fixations which lasted until the reapperance of the scene (Henderson & Pierce, 2008; Henderson & Smith, 2009). This has been considered as evidence for two distinct ‘subpopulations of fixations’, one under direct and another under indirect control. However, a similar prolongation can be obtained in as much as all fixations when distractors are presented during free visual exploration of a scene (e.g. Graupner, Velichkovsky, Pannasch, & Marx, 2007; Pannasch, Dornhoefer, Unema, & Velichkovsky, 2001; Reingold & Stampe, 2000). Since the behavior of fixations is similar in both paradigms, we argue that the findings by Henderson et al. (2008, 2009) can be explained in terms of the distractor effect from the scene onset together with artifacts produced by a variable delay in the scene changes. We provide new data from two experiments with a variable delay of distractor application that allow to numerically simulate the Henderson et al. (2008, 2009) results from the scene onset delay paradigm. Based on these findings, we question the particular version of the mixed control model. We also discuss the issue of control mechanisms for visual fixation duration. Instead of suggesting two distinct groups of fixations our results demonstrate that each fixation can be under shifting control of several brain mechanisms (see Pannasch & Velichkovsky, 2009). Most prominent among them seem to be mechanisms of ambient and focal mode of processing or, respectively, that of dorsal and ventral streams (pathways) within the visual system.
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