May the force be with you: exploring force discrimination in chimpanzees using the force-feedback device.

While force-feedback devices have been developed in areas such as virtual reality, there have been very few comparative cognitive studies in nonhuman animals using these devices. In addition, although cross-modal perception between vision and touch has been actively studied in nonhuman primates for several decades, there have been no studies of their active haptic perception. In this study, we attempted to train force discrimination in chimpanzees using a force-feedback device modified from a trackball. Chimpanzees were given different levels of force feedback (8.0 vs. 0.5 N) when moving the on-screen cursor to the target area by manipulating the trackball and were required to select one of two choice stimuli based on the force cue. The experiment was conducted using a trial-block procedure in which the same force stimulus was presented for a fixed number of trials, and the force stimulus was changed between blocks. The block size was progressively reduced from ten trials. Four chimpanzees were trained, but none reached the learning criterion (80% or more correct responses under the condition that the force stimuli were presented randomly). However, a detailed analysis of the chimpanzees' performance before and after the trial-block switching revealed that their choice behavior could not be explained by a simple win-stay/lose-shift strategy, suggesting that the switching of the force stimuli affected the chimpanzees' choice behavior. It was also found that the chimpanzees performed better when switching from small to large force stimuli than when switching from large to small force stimuli. Although none of the chimpanzees in this study acquired force discrimination, future studies using such force-feedback devices will provide new insights for understanding haptic cognition in nonhuman primates from a comparative cognitive perspective.