Performing accurate actions such as goal-directed arm movements requires taking into account visual and body orientation cues to localize the target in space and produce appropriate reaching motor commands. We experimentally tilted the body and/or the visual scene to investigate how visual and body orientation cues are combined for the control of unseen arm movements. Subjects were asked to point toward a visual target using an upward movement during slow body and/or visual scene tilts. When the scene was tilted, final pointing errors varied as a function of the direction of the scene tilt (forward or backward). Actual forward body tilt resulted in systematic target undershoots, suggesting that the brain may have overcompensated for the biomechanical movement facilitation arising from body tilt. Combined body and visual scene tilts also affected final pointing errors according to the orientation of the visual scene. The data were further analysed using either a body-centered or a gravity-centered reference frame to encode visual scene orientation with simple additive models (i.e., ‘combined’ tilts equal to the sum of ‘single’ tilts). We found that the body-centered model could account only for some of the data regarding kinematic parameters and final errors. In contrast, the gravity-centered modeling in which the body and visual scene orientations were referred to vertical could explain all of these data. Therefore, our findings suggest that the brain uses gravity, thanks to its invariant properties, as a reference for the combination of visual and non-visual cues.

The distinct tactile feedback provided by mechanical keyboards notifies users that their actions have been successfully recorded. The presence of these subtle yet informative tactile cues is one of the reasons why mechanical keyboards are still preferred to their virtual counterparts. An artificial sensation of pressing a mechanical switch can be produced by varying the coefficient of friction as the user is pressing down on a glass surface using ultrasonic vibration. We examined the factors involved in producing a vivid sensation of a stimulus by measuring the mechanical impedance, the frictional behavior of the fingertip and the perceptual thresholds. Subjects who experienced weaker sensations also showed a weaker sensitivity to friction modulation, which may in turn be attributable to the presence of a larger or a smaller than average impedance. In the second experiment, the user's finger impedance was measured during the click, and it was observed that the successful detection of the stimulus was correlated with the presence of considerable discontinuity in the mechanical impedance added to the plate by the finger. This discontinuity in the evolution of the impedance supports the idea that the skin is being reconfigured towards a new equilibrium state after the change in friction.

Ultrasonic friction reduction is one potential technology for bringing tangibility to flat touchscreens. We previously established that this approach can be used to create an artificial sensation of pressing a mechanical switch by varying the coefficient of friction, which depends on the force applied by the user. This sensation proves effective majority of, but a non-negligible fraction reported feeling only weak sensations or none at all. In the present study, we examined the factors possibly involved in producing a vivid perception of a stimulus by measuring the mechanical impedance of the fingertip as an index to the frictional behavior, and performing psychophysical experiments. Subjects who experienced weaker sensations were found to have a weaker susceptibility to friction modulation, which may in turn be attributable to either a larger or a smaller than average/normal impedance; whereas those with a mechanical impedance of around 55 N.s/m clearly perceived the ultrasonic switch. Measuring and factoring the users impedance in real time could therefore provide a useful means of improving the rendering of ultrasonic surface haptic devices.