Discriminating differences in stiffness when performing a haptic task and when palpating real-world objects
Aims: To compare an individual's ability to discriminate stiffness using a haptic device and using real-world objects when cutaneous cues are either included or excluded. Background: Stiffness discrimination in tasks such as soft tissue palpation is informed by cutaneous receptors as well as kinaesthetic receptors in the muscles, joints and connective tissue. The relative contribution of cutaneous and kinaesthetic components in informing clinical judgements is not known but the ability to simply discriminate stiffness is decreased when either component is removed . Haptic devices are used to train stiffness discrimination  and incorporate stiffness discrimination into complex tasks including soft tissue palpation [3-4]. Haptic devices simulate kinaesthetic feedback for procedural tasks, but are generally unable to simulate cutaneous input that occurs during soft tissue palpation. It is not known whether there is a relationship between an individual's kinaesthetic ability to discriminate stiffness with haptic devices and with real-world objects or whether that relationship remains when cutaneous cues are also available in real-world tasks. The usefulness of haptic devices for teaching psychomotor skills may depend on the existence of a relationship between haptic and real-world skills. Methods: Sixteen penultimate year physiotherapy students (aged 20 to 29 years) participated. Stiffness discrimination in a haptic environment was assessed using the CoreSkills trainer , a haptic game-based program for developing generic psychomotor skills. In the 'firmness game' the user selects the softest of four virtual surfaces. Points are scored for correct answers and lives lost for errors. Students played the game once until they lost all of their lives. Stiffness discrimination with real-world objects was evaluated by the number of correct answers when ranking the relative stiffness of sets of five silicone discs. With four sets, students palpated the silicone discs directly (allowing both cutaneous and kinaesthetic input) and for the other four cutaneous information was excluded by capping the discs with a rigid plastic cover. Kendal's tau_b was calculated to assess the agreement between rankings. Local Ethics Review Committee approval was obtained. Results: A significant relationship was found between the ability to discriminate stiffness with the haptic device and with the capped silicone discs (kinaesthetic information only) (Kentals tau_b 0.475, p= .016). No relationship was found between the haptic device and the uncovered discs nor was there a relationship for the capped and uncovered discs. There were few errors with the uncovered discs which may have made it difficult to detect relationships between this and other tasks. Conclusions: Ability to discriminate stiffness using a haptic device seems related to stiffness discrimination in a real-world task; at least when that task only includes kinaesthetic information. It is unclear whether such a relationship also exists when cutaneous information is included. It would seem that a haptic environment can be used to evaluate an individual's relative kinesthetic ability to discriminate stiffness. Future research is necessary to establish whether improvement by an individual in stiffness discrimination in the real-world can be assessed haptically and whether training in a haptic environment will improve real-world skills in stiffness discrimination.