FREQUENCY DEPENDENCE OF COMPLIANCE CONTRAST DETECTION

Neel DhruvGraduate Group in BioengineeringUniversity of California, BerkeleyBerkeley, California 94720Email: ndhruv@robotics.eecs.berkeley.eduFrank TendickDepartment of SurgeryUniversity of California, San FranciscoSan Francisco, California 94143Email: tendick@eecs.berkeley.eduABSTRACTPsychophysical experiments were performed to investigatethe haptic perception of objects with varying compliance overtheir surfaces. Perception of compliance contrast is importantin interaction with soft environments, as in teleoperative surgery.Simulated surfaces were presented using a haptic interface in vir-tual environments. An adaptive 2–down, 1–up procedure wasused to determine thresholds for compliance contrast detectionat a range of spatial frequencies. The maximum effective tempo-ral frequencies of compliance change, due to the subjects’ actionof scanning the surfaces, were calculated. Force contrast detec-tion thresholds were determined at a range of temporal frequen-cies to investigate the reliance of compliance contrast detectionon force cues. Compliance and force discrimination thresholdswere determinedin orderto comparewith low frequencycontrastthresholds. Compliance and force contrast detection thresholdswere found to be 2% and 1% above 1/2 cyc/cm and 20 Hz, re-spectively. Both were signiﬁcantly smaller than respective com-pliance and force discrimination thresholds and low frequencycontrast detection thresholds. It appears that the improvement incompliance contrast detection sensitivity is due to subjects be-ing more sensitive to force contrast at higher temporal frequen-cies. By changing environment exploration strategies or limbimpedance, subjects may be able to change their sensitivity toobject compliance contrast.INTRODUCTIONSurgeons depend on their ability to feel differences betweentissue types for a range of tasks. Signiﬁcant examples includefeeling pulsatile blood ﬂow through vasculature and embedded“anomalies” in soft tissues. Laparoscopic surgery is a minimallyinvasivesurgicalprocedureperformedwith tools that are insertedinto the patient through small incisions (Tendick and C¸avus¸o˘glu1997; C¸avus¸o˘glu et al. 1999). Laparoscopic surgery has beenaccepted as an alternative to open surgery because it results inreduced trauma and quicker patient recovery. However, somedrawbacks to the procedure exist. Traditional minimally inva-sive surgery (MIS) requires the surgeon to work in a very lim-ited workspace with greatly reduced haptic information(Tendicket al. 1993). In teleoperation (telesurgery), a surgeon uses amaster manipulator to control a robot located at the surgical site.Teleoperation solves some of the problems associated with MIS.Dexterityis enhancedby increasingdegrees of freedomand scal-ing down the surgeon’s motion (Charles 1996; C¸avus¸o˘glu et al.1999). However, a telesurgical procedure exacerbates the prob-lem of loss of tactile and kinesthetic information, as even theforces applied at the fulcrum along the shaft of the instrumentwould be masked from the surgeon. One way to solve this prob-lem is to provide haptic feedback to the master controller fromthe slave robot. This paper will focus on the nature of hapticfeedbacknecessaryto adequatelyrepresentthe complianceof theremote environment to the master or, equivalently, to representthe compliance in a virtual environment.There have been numerous psychophysical studies per-formed to determine thresholds for compliance discrimination.Jones and Hunter (1990) have determined Weber fractions forstiffness discrimination to be 0.23, while force and displacementdiscrimination Weber fractions were found to be 0.07 and 0.09,respectively. One drawbacktheauthorsnoteis that the sensitivityto stiffness is worse than what would be expected by combiningthe sensitivities for force and displacement, indicating complexneural processing in the discrimination for compliance. Theseexperiments were performed using a contralateral limb match-ing protocol, yet surgeons use their ﬁngers during active explo-ration to discriminate compliance. Therefore, the range of force1087 Copyright 2000 by ASME