A multi-digit tactile motion stimulator

Background: One of the hallmarks of haptic exploration is that it typically involves movement between skin and object. Explored objects may contact multiple digits simultaneously so information about motion must be integrated across digits, a process about which little is known. New method: To fill this gap, we have developed a stimulator that allows for the simultaneous and independent delivery of motion stimuli to multiple digits. The stimulator consists of individual units that deliver motion with three degrees of freedom: rotation (to produce motion), vertical excursion (to control depth of indentation into the skin) and arm orientation (to control the direction of motion). Each degree of freedom is controlled by a single motor. The compact design of the simulator allows for the side-by-side arrangement of the stimulator units such that they impinge upon adjacent fingers. Results: To demonstrate the functionality of the stimulator, we performed a series of psychophysical experiments that investigate the perception of motion on multiple fingers. We find that, while the sensitivity to changes in motion direction is equivalent whether stimuli are presented to the same or to different fingers, the perceived direction of motion depends on the relative configuration of the digits. Comparison with existing methods: We replicated the results of previous experiments investigating motion discrimination with a single digit and were able to extend these findings by investigating motion perception across multiple digits. Conclusion: The novel motion stimulator will be an invaluable tool to investigate how motion information is integrated across multiple digits.