  Faezeh Heydari Khabbaz* University of Toronto The Hospital for Sick Children Toronto Andrew Goldenberg University of Toronto James Drake The Hospital for Sick Children Toronto Force Discrimination Ability of Human Hand near Absolute Threshold for the Design of Force Feedback Systems in Teleoperations Abstract Force discrimination ability is an important surgical skill for micro or minimally invasive surgeries. This paper analyzes the force perception of the human hand for low- intensity stimuli. Psychophysical experiments were conducted to measure just noticea- ble differences (JNDs) at four reference forces below 1 N. Reference forces are cho- sen to be in the range of forces in minimally invasive surgery that do not follow Weber’s law. The force discrimination ability of two groups of subjects, surgeons and non-surgeons, is compared. Results demonstrate a superior ability of surgeons in dis- criminating small forces. The relationship between the JND and stimulus near absolute threshold is modeled. The application of the model in the design of force feedback sys- tems for surgical teleoperation is discussed. In a force augmenting teleoperation sys- tem, the force feedback signal is amplified based on the characteristics of the human perception system. Therefore, the degraded human sense of touch at low-intensity forces would be compensated. 1 Introduction Physicians rely heavily on their sense of touch during soft tissue examina- tions and surgery. The surgeon uses haptic perception to identify properties such as texture and stiffness and to discriminate between different organs and tissues. Information from the sense of touch can help physicians in many diag- nostic decisions such as detection of lumps, cysts, and tumors. Therefore, force feedback systems can play an important role in surgical teleoperations (Tavakoli, Patel, & Moallem, 2006; Wagner, Stylopoulos, Jackson, & Howe, 2007). With the latest technologies, force can be measured with very high accuracy. However, not all measured forces that are fed back to the human operator can be discriminated. This is particularly true for small forces where human force discrimination ability is degraded compared to that at larger forces (Hatzfeld & Werthschu ¨tzky, 2012). Force variations in soft tissue or microsurgery are smaller than the discrimination ability of the human hand, and yet carry impor- tant information for the surgeon. Consequently, precise force feedback would not be the best practice in soft tissue surgery because sometimes the discrimina- Presence, Vol. 25, No. 1, Winter 2016, 1–14 doi:10.1162/PRES_a_00245 ª 2016 by the Massachusetts Institute of Technology *Correspondence to faezeh.heydarikhabbaz@mail.utoronto.ca. Khabbaz et al. 1