Introduction: The incorporation of haptics, the sense of touch, into medical simulations increases their capabilities by enabling the users to "feel" the virtual environment. We are involved with haptics-augmented virtual reality training for palpatory diagnosis. We have developed a stiffness discrimination program to train and test users in finding subtle differences in human tissue stiffness for medical diagnoses. In this article, we studied the effect of surface stiffness on the stiffness discrimination task and analyzed the palpation force and speed during haptic exploration.
Methods: The ability to discriminate stiffness differences was studied by means of a psychophysical experiment with 13 second-year medical students (eight women and five men). Subjects were asked to identify the stiffer of two virtual computer-generated surfaces (top surfaces of two cylinders) by means of a PHANToM Omni (SensAble Inc.) haptic device with a modified stylus to accommodate their fingers. The modification of the stylus provided the mechanical advantage to simulate surface stiffness values that are beyond the original capability of the haptic device. An adaptive two-alternative forced-choice procedure was used on each trial. Palpation velocity and force vectors were recorded directly from the haptic device for further analyses. Weber fraction was determined by using an automated mastery algorithm.
Results: Four standard stiffness values (0.25, 0.50, 1.00, and 1.25 N/mm), typical of the stiffness range of human soft tissues, were used as references. The average experimental Weber fractions observed were 0.20, 0.27, 0.26, and 0.30, respectively, with higher Weber fractions corresponding to lower stiffness discrimination ability. At 1.00 and 1.25 N/mm standard stiffness, the correlation analysis for Weber fraction and the palpation speed revealed significant differences (P < 0.05). These differences suggested that the subjects with a higher palpation velocity tended to have a higher Weber fraction. There was no significant difference between male and female subjects. There was no significant difference between subjects new to the haptic device and those who had used it previously. The average amount of force that was applied by the subjects to the standard stiffness side and the comparison stiffness side within the sessions was not significantly different. However, the subjects increased the average force they applied with increasing standard stiffness value across the sessions (P < 0.05).
Conclusions: For the four standard stiffness values investigated, 0.25, 0.50, 1.00, and 1.25 N/mm, the resulting average stiffness-discrimination Weber fractions were 0.20, 0.27, 0.26, and 0.30, respectively. The average of the forces applied by the subjects was constant within a single session (with a single standard stiffness value). This average force monotonically increased as the standard stiffness value increased across the sessions. We also found positive correlation between the Weber fraction and the palpation speed in the sessions tested with 1.00 and 1.25 N/mm standard stiffness. This correlation suggested that higher speed is related to lower sensitivity in discrimination of stiffness differences for these two standard stiffness values. Our results are applicable to tasks involving stiffness discrimination between multiple objects.