Although eyes and upper limbs work together over the whole course of arm movement, vision usually dominates in visual-haptic perception. The dependence of the brain on visual information is demonstrated by the wide variety of visual illusions, such as “shade and shape,” that have been developed. In order to quantitatively measure upper-limb motion, a haptic device can be used to obtain numerical data. Although, as the name suggests, such a device is capable of transmitting force, human perception mechanisms tend not to measure force accurately owing to interference. In this paper, we analyze the role played by visual information in line drawing. In experiments where participants were asked to estimate the length of a line using a haptic device operated by an experimenter and then to draw a line on their own, there was a tendency to draw a longer line when their eyes were open. These results reinforce the importance of visual information to length recognition in drawing.
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