The relation between different fingers pressing elastic objects and the characteristics of hardness discriminability

Abstract

Identification of the hardness of an object in a human hand is different according to the hardness of the object. For example, chiropractors identify stiffness in the muscle by using their thumb, which can press more forcefully than the forefinger. However, it has not been shown experimentally if the characteristics of hardness discriminability are different according to fingers. In this study, to clarify the relation between different fingers pressing elastic objects and the characteristics of hardness discriminability, we investigated the differential threshold of hardness using the forefinger and thumb. Additionally, the influence of press force and press displacement on the characteristics of hardness discriminability was clarified. We used 7 elastic test pieces with different Young’s moduli as the presented stimuli. We also used the test pieces with hardness sensation judged as ‘hard’ in our previous research. We conducted the experiment using the constant stimuli method to calculate differential threshold of hardness as one evaluation index of hardness discriminability when using the forefinger and thumb. The results of this experiment showed that the hardness discriminability of the thumb was better than that of the forefinger. In addition, these results suggested that press displacement but not press force should be used to estimate the hardness discriminability and that the thumb is better than the forefinger to discriminate the displacement. These findings clarify the mechanism underlying hardness discriminability in the human hand.

Keywords

• hardness discriminability
• thumb
• forefinger
• constant stimuli method
• differential threshold
• press force
• press displacement