A Systematic Comparison of Perceptual Performance in Softness Discrimination with Different Fingers

In studies investigating haptic  softness perception, participants are typically instructed to explore soft objects by indenting them with their index finger. In contrast, performance with other fingers has rarely been investigated. We wondered which fingers are used in spontaneous exploration and if performance differences between fingers can explain spontaneous usage. In Experiment 1 participants discriminated the softness of two rubber stimuli with hardly any constraints on their finger movements. Results indicate that humans perform multiple phases during an exploration, in which they successively use different fingers and finger combinations, preferring index, middle and (to a lesser extent) ring finger. In Experiment 2 we compared discrimination thresholds between conditions, with participants using any of the four fingers of the dominant hand. Participants compared the softness of rubber stimuli in a 2IFC discrimination task. Performance with index and middle finger was better as compared to ring and little finger. Performance with the little finger was worst. In Experiment 3 we again compared discrimination thresholds, but prescribed participants to use constant peak force. Performance with the little finger was worst compared to performance with the other fingers, whereas the other fingers did not differ. We conclude that in spontaneous exploration behavior the preference of combinations of index, middle, and partly ring finger seems to be well chosen, as indicated by improved performance with the spontaneously used fingers. Better performance seems to be based on both different motor abilities to produce force, mainly linked to using the index and middle finger, and different sensory sensitivities, mainly linked to avoiding the little finger.