For the quality estimation, the places fixated on were mostly the semantically important areas of an image, which were then examined in detail with short saccades indicating local processing. The same semantic ROIs were attended to in the difference task, but there were also areas fixated on that were both outside the ROIs and outside the area of low-level saliency. The semantic ROIs were crucial for both tasks, as is suggested by change blindness studies.51 Nevertheless, our results also show the influence of the task: in the difference task, ROIs alone were insufficient. The quality estimation may have depended solely on semantically important places, but for detecting differences, an evaluation of the whole image was needed and a larger area was scanned. Contrary to this, an earlier study comparing free viewing and quality estimation found that free viewing attracted more attention to the ROIs than quality estimation, where fixations were spread also outside this area.24 This could reflect the differing demands of free-viewing and quality estimation tasks: in the free-viewing task, no aspect of an image is important for the task’s sake, since there is no question to answer later, and in the quality estimation task, there is. However, the results could also be influenced by the fact that the fixations from the free-viewing task were used as the bases for calculating the ROIs. The differences between the tasks could have been related to differing cognitive requirements. The requirements have also been shown to change when the range of image quality changes. Poor image quality has been shown to be estimated using low-level attributes, such as sharpness and darkness, but high image quality is estimated using high-level attributes, such as naturalness.9 In the current experiment, the observers estimated differences in high-quality images, so it might be that higher-level quality concepts were needed in the quality estimation more than in the difference estimation. This, however, needs further research.