Haptic identification of stiffness and force magnitude

As haptics becomes an integral component of scientific data visualization systems, there is a growing need to study "haptic glyphs" (building blocks for displaying information through the sense of touch) and quantify their information transmission capability. The present study investigated the channel capacity for transmitting information through stiffness or force magnitude. Specifically, we measured the number of stiffness or force-magnitude levels that can be reliably identified in an absolute identification paradigm. The range of stiffness and force magnitude used in the present study, 0.2-3.0 N/min and 0.1-5.0 N, respectively, was typical of the parameter values encountered in most virtual reality or data visualization applications. Ten individuals participated in a stiffness identification experiment, each completing 250 trials. Subsequently, four of these individuals and six additional participants completed 250 trials in a force-magnitude identification experiment. A custom-designed 3 degrees-of-freedom force-feedback device, the ministick, was used for stimulus delivery. The results showed an average information transfer of 1.46 bits for stiffness identification, or equivalently, 2.8 correctly-identifiable stiffness levels. The average information transfer for force magnitude was 1.54 bits, or equivalently, 2.9 correctly-identifiable force magnitudes. Therefore, on average, the participants could only reliably identify 2-3 stiffness levels in the range of 0.2-3.0 N/mm, and 2-3 force-magnitude levels in the range of 0.1-5.0 N. Individual performance varied from I to 4 correctly-identifiable stiffness levels and 2 to 4 correctly-identifiable force-magnitude levels. Our results are consistent with reported information transfers for haptic stimuli. Based on the present study, it is recommended that 2 stiffness or force-magnitude levels (i.e., high and low) be used with haptic glyphs in a data visualization system, with an additional third level (medium) for more experienced users.