In two experiments we investigated the control of fixation durations in naturalistic scene viewing. Empirical evidence from the scene onset delay paradigm and numerical simulations of such data with the CRISP model [Psychological Review 117 (2010) 382–405] have suggested that processing related difficulties may lead to prolonged fixation durations. Here, we ask whether processing related facilitation may lead to comparable decreases to fixation durations. Research in visual search and reading have reported only uni-directional shifts. To address the question of unidirectional (slow down) as opposed to bidirectional (slow down and speed up) adjustment of fixation durations in the context of scene viewing, we used a saccade-contingent display change method to either reduce or increase the luminance of the scene during prespecified critical fixations. Degrading the stimulus by shifting luminance down resulted in an immediate increase to fixation durations. However, clarifying the stimulus by shifting luminance upwards did not result in a comparable decrease to fixation durations. These results suggest that the control of fixation durations in scene viewing is asymmetric, as has been reported for visual search and reading.