Grouped results from Experiment 1. (A) Sample grasp-force trajectories from Experiment 1, from a single subject. In each plot the x-axis denotes time in seconds, and the y-axis the force in Newtons. The plots show four different experimental conditions: lifting a heavy object without (top left), and with vibrotactile feedback (top right); lifting a lightweight object without (bottom left), and with vibrotactile feedback (bottom right). For this subject, tactile feedback offers little utility in reducing grasp force, only in reducing variability. Object weight, on the other hand, has a clear effect on grasp forces. (B) Data from Experiment 1, grouped by factor, using three metrics to compare performance. Error bars denote standard error. N = 6. Comparison of within-subject factors of tactile feedback condition (green bars) and object weight (blue bars). Weight is split into lightweight ('L') and heavy ('H'). ANOVA results revealed a significant main effect of object weight, but not of tactile feedback condition, denoted by the stars. (C) Data from Experiment 1, grouped by feedback condition, using three metrics. Error bars denote standard error. N = 6. Comparison of subjects' ability to discriminate object weight as a function of feedback condition. Feedback conditions were with tactile feedback ('tactile') and without tactile feedback ('none'). The two bars per condition indicate performance with the lightweight object ('L') and heavy object ('R'). Successful discrimination is indicated by a positive slope. Subjects were able to discriminate equally well in either feedback condition.