Recognition of tools relates closely to sensorimotor experiences of tool use. Here, we make a hypothesis that humans recognize graspable tools by making good use of an internal model of own hand that was trained through such sensorimotor experiences. By an internal simulation in the human brain, judging whether we can grasp an object is also useful to recognize tools such as a drinking cup.
In this study, in the first step to evaluate the plausibility, we built an experimental paradigm to relearn an internal model of the hand. Hand and finger movements were measured by CyberGlove; a geometrically transformed hand shape was displayed on a monitor; they moved the hand repeatedly while looking at the screen. As a result, they had relearned an internal model corresponding to the displayed hand shape. By using this paradigm, we investigated a relationship between cognitive judgment for size of graspable tools and the internal model. Ten right-handed subjects participated in the experiment. Before the experiment, six different tools were classified into two categories by a questionnaire about tool use. Before and after the learning, by presenting images one by one in different sizes of a tool, we measured the size regarded as the tool. For tools to be mainly used in the right hand (e.g., a nipper and a tweezers), after the internal model of the right hand was relearned, the size became almost twice: they became to recognize as the tool when an image of twice the size of the actual size was displayed. This is because the geometrically transformed hand can grasp an object of almost twice the size. After the model of the left hand was relearned, however, its size did not change. While, for tools to be used in either the right or the left hand (e.g., a drinking cup and a door knob), its size became almost twice in both of the learning. This is because all participants had been conscious of the hand of the side of the learning during the measurement. In addition, we ascertained that the effect of the learning did not transfer to the contralateral hand. The above results indicate that the internal model of the hand affects cognitive judgment of size of graspable tools.