## 3 Ergonomic Evaluation of Visualization Methods  
The ergonomic evaluation of new visualization methods requires the consideration of different factors that may influence user performance, such as physical properties of visual stimuli [25] and expertise in particular visual tasks [26]. Radiologists read approximately several hundred medical images per day [1,5], and the majority of their professional tasks can be defined as visual search tasks [27]. Radiologists search for anatomical abnormalities with different physical properties, such as shape, size, contrast and wall thickness. The visual search of radiologists is regularly challenged due to the physical similarity of lesions compared to nearby normal structures. For instance, in lung cancer screening, spherical lung nodules look similar to tubular vessels in a cross-section [1], and may be occluded by the ribs [5]. Therefore, true three-dimensional visualization of multi-slice images is expected to facilitate visual search by providing radiologists with the opportunity to see the entire view of the anatomical structure and separate different image elements in physical space. Extensive training leads to the increased tuning of visual perception [28] and changes in search strategy which radiologists implement during image reading [29]. Most previous studies have demonstrated the strong top-down influence of expertise when comparing the performance of radiologists and non-radiologists viewing medical images [29; 30; 31; 32; 33]. In addition, the tracking of brain activity revealed significant differences among radiologists and non-radiologists, not only for radiology-related images, but also for control images [34]. As a result, researchers hypothesized that radiology expertise could change visual processing in general. However, the investigation of perceptual processes for edited medical images [35; 36] and non-medical search items [37; 38; 39; 40; 28; 32; 37; 36; 38; 39; 40] clarified that the superior skills of radiologists were shown only in the presence of specific physical properties of visual stimulus. Namely, considerable differences were revealed in the temporal aspects of task outcome when low contrast stimuli were employed [28; 35], suggesting that contrast sensitivity might be enhanced in radiologists. Therefore, one can expect differences in the performance of radiologists when not only viewing domain-specific images, but also domain-general images. The ability of experienced radiologists to engage in the global processing of the entire image is another piece of evidence for their tuned visual perception. Namely, experts can process large regions of the image in parallel using their parafoveal and/or peripheral vision [41]. The psychophysical assessment of perceptual expertise has revealed that experienced radiologists can quickly extract the relevant information from the complete visual scene, guiding their selective visual attention efficiently, and further conducting a more organized visual search when compared to their less-experienced colleagues. However, in reality, the opportunity for global processing has been lost when displaying multi-slice images on flat-panel displays. Three-dimensional visualization methods may permit radiologists to recapture the opportunity to develop a comprehensive view before examining in detail. In addition to visual performance, another important aspect of ergonomic evaluation is the interaction technique that users implement when viewing images on a display. The interaction technique of radiologists was broadly assessed in the context of testing new radiology workstations [39; 42], studying effective search strategies for multi-slice computer tomography viewing [2; 33; 43], and studyingthe cognitive processes of radiologists during medical image reading [44; 45]. With the advent of multi-slice images, researchers mostly investigated how radiologists manipulated images in the z-direction, the feature which sharply distinguishes the new type of images from two-dimensional ones.