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Abstract

In a grasping task, Johansson et al. (2001) found that subjects look at the position to which the finger tips are guided. However, in their experiment, only the contact position of the thumb was visible. We investigated what happens if the contact positions of both finger and thumb are visible. We recorded eye and finger movements. In a first experiment, subjects always grasped with the index finger at the top and the thumb at the bottom of a flat shape that was mounted on a horizontal bar. In order to see whether a salient feature of a shape would affect the fixation positions, we presented an (asymmetric) cross in 4 orientations (with the crossing of the bars representing the salient feature). In order to see whether gaze is attracted to the position where the finger has to be guided relatively precisely, we presented a triangle in two orientations that subjects had to contact at the base and at the pointed top (i.e., a higher required precision to contact the top than the base). We found that the crossing of the bars cross attracted the gaze whereas the top of a triangle did not. A prominent result was that subjects fixated above the center of the shape. In order to distinguish between subjects fixating the upper part of the shape versus being attracted by the index finger, we mounted a square and a triangle in two orientations on a vertical bar. We asked subjects to grasp first with one hand and then with the other so that the shape remained constant but the contact positions of the index finger and thumb were reversed. Subjects still looked above the center. In addition, the gaze was attracted to the index finger for the triangle but to the thumb for the square. We conclude that both features of the shape and the grasp affect gaze location. The exact location depends on the specific circumstances.