## 4 Materials and Methods
### Visual Stimuli  
Medical images contain a high variability of physical properties and image artifacts, which altogether lead to distracted visual attention [47]. Moreover, the semantic context of medical images may mask the effect of the physical properties of the display and visual stimulus [25]. Taking into account this consideration, we chose to evaluate the influence of stimulus physical properties and the three-dimensional representation of visual information without the semantic context of medical images. Thus, we could ensure that the search items were identifiable by experienced and early-career radiologists, as well as non-radiologists, under controlled viewing conditions, as was previously conducted on flat-panel displays [26; 39].  
The detection of small spherical lung nodules is considered to be one of the most challenging and important tasks frequently performed by radiologists [14]. Inspired by the appearance of lung nodules, which are frequently missed due to their physical similarity when compared with pulmonary blood vessels in a cross-section, we designed a visual search array consisting of bright circles of a constant angular size on a dark background. The outer diameter was \(0.5^{\circ}\), and the inner diameter was \(0.4^{\circ}\). The luminance of stimuli was 21 cd/m\({}^{2}\) (measured with the Photo Research spectroradiometer PR-655). The background luminance was 0.6 cd/m\({}^{2}\). In the beginning of the visual search task, the circles were distributed among ten consequent display planes. The target differed from other search items (distractors) with its line width, which was larger in the direction of the circle's center. Based on previous studies investigating the role of peripheral vision in search performance, differences in this target feature were expected to affect visual processing [48] and search time [49; 50]. Visual search tasks without semantic context are considered to be more sensitive to visual degradation effects compared to the ones that simulate usual activities [25]. Thus, we had an opportunity to test whether the slight changes of the target feature were visible to medical professionals and students on the volumetric display.  
The central part of each display plane (\(4.75^{\circ}\times 4.75^{\circ}\)) was divided into an invisible grid of \(5\times 5\) equal cells. The search items were randomly assigned to these cells according to the set size, with no items sharing the same points. In order to avoid misconceptions, the target item could not intersect the vertical or horizontal meridian of the display planes (where the borders of the invisible grid were formed), but the distractors were allowed to be demonstrated there. In the z-direction, the target was located in any of the 6 th to 10 th slices of the three-dimensional image. The slices on the closest planes of the volumetric display did not contain the target item, in order to avoid the pop-out effect due to the depth component [51].  
In order to study how the physical properties of visual information affect visual search outcomes and image manipulation, we varied the target conspicuity and amount of visual information in the search tasks by employing the target-distractor similarity and set size as controlled variables. In the test session, the set size could be two or four search items per each display plane, which resulted in 20 or 40 items per trial. The target-distractor similarity could be high or low, with an increased line thickness of 15% or 25% towards the center of the circle, respectively.