## 6 Discussion
Thus, our study supports the previously announced concept that the differences in search accuracy among radiologists are domain-specific and do not apply to visual search tasks in general. In addition, our study emphasized the influence of stimulus physical properties on search accuracy. Accuracy decreased with the increase of target-distractor similarity but was unaffected by the amount of visual information on the volumetric display. This finding is in agreement with the perceptual study wherein the target discriminability was manipulated, and the response time was not limited [52]. Participants from all groups sometimes missed the slight differences between the target and distractors. Taking into account that performance is dependent on the visual capabilities of users and the quality of display, we emphasize that further improvements are required in the display resolution in order to enhance the usability of novel three-dimensional visualization systems. On a daily basis, radiologists must carefully inspect a vast number of images. For this reason, it is important to assess the time it takes to accomplish each search task. Previously, the domain-general temporal aspects of radiologists' performance were examined using eye-tracking [33; 35], and by recording the reaction time [29; 30] and trial completion time [34]. However, no differences were reported for gaze fixation characteristics [35], and radiologists demonstrated considerably faster reactions than other participants when detecting slight changes in non-radiological images [29] and finding low contrast targets [28; 30], which allowed us to assume that the physical properties of stimuli might play a critical role in the temporal evaluation of performance depending on radiologist experience. In our study, we saw a tendency for students to consume more time compared to radiologists and residents when target-distractor similarity was high, however, no significance was reached. Despite the relatively low contrast of visual stimuli on the volumetric display, we did not prove the superior performance of radiologists, presumably due to the complexity of the visual task. The search items were distributed in the physical space of the optical element, therefore, participants could require a longer inspection time than when viewing flat images, and this could potentially mask the faster reaction presented by radiologists. With the continuous emergence of new visualization methods, the way in which radiologists read and manipulate images changes substantially. The human-computer interaction was increasingly studied, investigating its relation to cognitive processes [8; 44; 45] and the visual search outcome [2; 33] during image interpretation in radiology. The major part of relevant research has been conducted using medical images (e.g., [2; 43; 45]), and few studies with non-medical search items have also included image manipulations in the experimental design [39; 42]. Similar to our findings, the number of manipulations was reported [39] to increase for radiologists and non-radiologists when they viewed high-complexity images on a flat-panel display. In our study, the complexity of the image depended on the amount of visual information and target conspicuity. The stimulus physics affected the search strategy and image manipulations of radiologists, residents, and students in our study in different ways. It is interesting that, in contrast to radiologists, students and residents made more direction changes due to changes in stimulus physics on the volumetric display. This finding is consistent with the results of earlier work [43], in which radiologists demonstrated a more organized search in the z-direction compared to non-radiologists when inspectingmulti-slice images on a flat-panel display. This result is compatible with the assumption that bottom-up processing plays a major role in the perception of early-career radiologists, as was emphasized in previous experimental studies [45; 53; 54].