Gaze-contingent luminance shifts show evidence for asymmetrical control of fixation durations in scene viewing

In two experiments we investigated the control of fixation durations in naturalistic scene viewing. Evidence from the scene onset delay paradigm and numerical simulations of such data with the CRISP model (Nuthmann, Smith, Engbert, & Henderson, 2010) has suggested that processing related difficulties may result in prolonged fixation durations. Here, we ask whether processing related facilitation may lead to comparable decreases to fixation durations. Research in reading (Kennison & Clifton, 1995) and visual search (Hooge, Vlaskamp, & Over, 2007) has reported only uni-directional shifts. To address the question of uni-directional (slow down) as opposed to bi-directional (slow down and speed up) adjustment of fixation durations in the context of scene viewing we compared the effect of gaze contingent luminance shifts during saccadic eye-movements on fixation durations. Results demonstrate that degrading the stimulus by shifting luminance down resulted in an immediate increase to fixation durations. Furthermore, clarifying the stimulus by shifting luminance upwards also resulted in an immediate increase to fixation durations. The magnitude of this increase was less in the UP condition than in the DOWN condition. These results suggest that the control of fixation durations in scene viewing is asymmetric, as has been reported for reading and visual search.