Task 1.4: Principles and Evaluation of Human-Machine Systems is in its second year. It concentrates efforts, previously distributed across Thrust 1, in human factors and ergonomic studies of surgical assistance techniques and systems. An important effort in this area is the evaluation of different types of virtual fixture control laws and their effectiveness for different system architectures. At JHU and Harvard, we have studied the role of haptic feedback in surgery, considering appropriate levels of force feedback, interaction between force and tactile feedback, and the effect of limited degrees-of-freedom of haptic feedback. Work in multi-model interaction and sensory substitution has shown that visual displays of force information can provide significant performance enhancement without the complexity of direct force feedback. Models of task performance developed in Task 1.3 are being applied to the analysis of surgical learning curves for minimally invasive procedures. This Task also includes human factors infrastructure, meaning that we provide information and contacts to center researchers that facilitate appropriate studies involving human subjects.