Relative Position of the Fingers Affects Length Perception While Grasping Objects

Background: Representation of objects can be obtained through tactual perception alone. The previous studies showed that there were no significant differences between the index finger and middle finger when the participants perceived the length of the presented objects, however, the length perception characteristics between the thumb and each of the four other fingers remains unclear. Objective: To investigate the length perception characteristics between the thumb and each of the four other fingers, the current study using a four-degree-of-freedom (4DOF) length display device with an adjustable distance conducted length perception experiments. Methods: We performed two experiments: experiment I, perception of different lengths (natural position), and experiment II, perception of the same length (same length position). In experiment I, the length presented to each pair of fingers was different. In experiment II, the length presented to finger pairs of the participants was the same. Results: The results showed that in both experiments, the perceived length was relatively shorter than the presented length, and when the presented length was longer than 70 mm, the error becomes smaller. The comparison of results indicated that for the perception of the index finger, no significant differences existed between the two experiments under any condition; for the middle and ring fingers, significant differences were found only when the presented length was approximately 100 mm; for the little finger, significant differences were found for lengths ranging from 45 mm to 90 mm. Conclusion: These results indicated that the perceptual accuracy at the natural position of the fingers is worse than that when the finger is in a bent position, and when the finger is spread beyond the neutral position, the relative positon affects the length perception.