Objective and subjective measures of performance in virtual reality environments increase as more sensory cues are delivered and as simulation fidelity increases. Some cues (colour or sound) are easier to present than others (object weight, vestibular cues) so that substitute cues can be used to enhance informational content in a simulation at the expense of simulation fidelity. This study evaluates how substituting cues in one modality by alternative cues in another modality affects subjective and objective performance measures in a highly immersive virtual reality environment. Participants performed a wheel change in a virtual reality (VR) environment. Auditory, haptic and visual cues, signalling critical events in the simulation, were manipulated in a factorial design. Subjective ratings were recorded via questionnaires. The time taken to complete the task was used as an objective performance measure. The results show that participants performed best and felt an increased sense of immersion and involvement, collectively referred to as ‘presence’, when substitute multimodal sensory feedback was provided. Significant main effects of audio and tactile cues on task performance and on participants' subjective ratings were found. A significant negative relationship was found between the objective (overall completion times) and subjective (ratings of presence) performance measures. We conclude that increasing informational content, even if it disrupts fidelity, enhances performance and user’s overall experience. On this basis we advocate the use of substitute cues in VR environments as an efficient method to enhance performance and user experience.

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The ideal intracorporeal lithotriper would comminute all types of calculi into small readily excreted particles. It would be small and flexible with an energy source safe for the uroepithelium. It should not break, should be inexpensive, and should not retropulse the stone up the urinary tract. This investigation was designed to quantify the last quality for the holmium:YAG laser. The mechanism of action of the pulsed Ho:YAG laser (wavelength 2100 nm) is the generation of a gas plasma at the stone-fluid interface causing a shockwave. The holmium laser was employed for lithotripsy of model stones composed of silicate with a ferrous coating. Stones were selected with a mass of 2 mg +/- 0.1 mg. We sequentially investigated three variables: energy (0.6, 0.8, and 1.0 J), frequency (10, 16, and 20 Hz), and fiber diameter (200, 365, 550, and 1000 microm). Ten stone trials were performed with each of the 36 possible combinations of energy, pulse frequency, and fiber diameter. Our model ureter consisted of a clear rigid polymer tube filled with 0.9% saline. The system was closed and permitted intertrial flushing of generated air bubbles. The laser fiber was maintained at constant extension from the ureteroscope, with stones positioned at the fiber tip before each trial. Laser energy was applied for 2 seconds, with maximum and net retropulsion recorded in millimeters. Each measurement series was recorded in a database for paired Student t-tests. Net retropulsion was then compared by statistically holding each of the three variables constant (fiber size constant with power and frequency varying; frequency constant with power and fiber size varying; and power constant with fiber size and frequency varying). Most retropulsion occurred with the 365-microm and 550-microm fibers. Most comminution was also noted with these fiber sizes. There was no statistical correlation between observed retropulsion and efficiency of comminution. This self-contained model for laser lithotripsy allowed us to measure retropulsion accurately. Silicate stones are not chemically similar to human uroliths but are of uniform composition. The irregular surface characteristics are similar to human stones, making them ideal for retropulsion investigations.

An approximate theoretical analysis is presented for the mean crushing forces generated during the dynamic axial loading of top-hat and double-hat sections. The theoretical predictions compare favourably with experimental results reported in a companion article on spot-welded mild steel sections and with the empirical equations and theoretical predictions of other authors.