## 6 Discussion  
This is the first study to investigate the differences in visual search performance and strategy depending on radiologist experience for real three-dimensional images on a volumetric display. We designed the visual stimulus with known physical properties for the multi-plane display in the absence of a semantic context relating to medical images in order to control the bottom-up influence. This way, we could quantify the search accuracy, response time, and image manipulation strategy in the z-direction to estimate the usability of the novel display, as well as evaluating the viewing requirements of individuals with varying amount of specific search experience (i.e., reading medical images) when working with a three-dimensional visualization system. Based on our study results, the three-dimensional visualization of information provides experienced radiologists with a valuable opportunity to grasp the overview of the entire rendered image without image manipulations before switching to a more focused local search. In all three groups, individuals performed their best when the target-distractor similarity was low and the amount of visual information was relatively small on the display. In general, performance was significantly affected by target-distractor similarity, but no considerable differences were demonstrated between radiologists, residents and students in terms of their average search accuracy. The assessment of the temporal aspects revealed that radiologists and residents presented considerably faster search times compared to students. Moreover, the analysis of image manipulation strategy in the z-direction revealed differences in terms of the viewing preferences of radiologists, residents, and medical students. Specifically, radiologists preferred seeing the entire three-dimensional image for search purposes, whereas residents and students did not. Mostly, residents and radiologists tended to reduce the amount \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Task Parameters** & **Radiologists** & **Residents** & **Students** \\ \hline High target-distractor similarity (15\% difference) & Small set & 0.22 (0.27) & 1.07 (0.47) & 1.10 (0.37) \\ Large set & 0.30 (0.21) & 1.52 (0.50) & 1.93 (0.61) \\ \hline Low target-distractor similarity (25\% difference) & Small set & 0.04 (0.06) & 0.63 (0.36) & 0.58 (0.22) \\
0.36 (0.28) & Large set & 0.36 (0.28) & 0.89 (0.46) & 1.15 (0.35) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number of direction changes, averaged among all participants in three groups (SE). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Image Coverage** & **Radiologists** & **Residents** & **Students** \\ \hline No changes & 52.5\% & 38.0\% & 33.0\% \\ Half image & 33.0\% & 20.5\% & 25.0\% \\ Full image & 14.5\% & 41.5\% & 42.0\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Total percentage of types of image coverage within manipulations, presented for three experience-adjacent groups of participants. of information on the display and selectively inspect slices of three-dimensional images, similar to the multi-slice image-reading procedure commonly executed on flat-panel displays. With the increase of the amount of visual information on the display, students made more direction changes when compared with residents and radiologists. Of all participants, only radiologists persuasively showed the image manipulation pattern that was independent of changes in target-distractor similarity. The accuracy of visual search is of primary importance in radiology, because it influences further decision making and diagnosis establishment. For this reason, the majority of previous experimental studies focused on investigating the possible impact of various expertise and stimulus-related factors on the search outcome. Similar to our findings, search accuracy did not vary considerably in earlier studies, in which the search performance of radiologists and non-radiologists (medical students or lay people) was compared using non-radiological search items [38; 39; 40].