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Zijiang J. He, Ji Hong, Teng Leng Ooi; On judging surface slant using haptic (palm-board) and verbal-report task. Journal of Vision 2007;7(9):282. doi: 10.1167/7.9.282.
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Recent studies (e.g., Proffitt et al, 1995) showed the judged geographical slant of the ground is exaggerated with the verbal-report task, and is more veridical with the palm-board task. Is there a correlation between the two tasks? Our observers viewed a 1.5×2.4m checkerboard texture-surface with varying slants (5, 10, 20, 30, 45deg) relative to the horizontal ground from either 1m or 4m. In task-1, the observer judged the slant of the texture-surface relative to the ground; by adjusting the slant of the palm-board to match the perceived slant of the ground, and separately, to the perceived slant of the texture-surface. The difference between these two measurements reveals the judged slant of the texture-surface. For each set of measurements, the angular position of the observer?s arm to the palm-board was fixed at 0, 30, or 60deg from the horizontal. We found that judged slants were significantly affected by the arm angle. A larger arm angle caused a smaller measured slant. In task-2, the observer verbally reported the slant of the texture-surface. Consistent with Proffitt et al, the verbal judgment was less accurate (overestimated) than with the palm-board task. To reveal the scaling relationship between tasks 1 and 2, we tested the same observers in task-3, where the observers blindly rotated the palm-board to a verbally instructed slant angle (0, 5, 10, 20, 30, or 45deg). We found that the adjusted slant of the palm-board relative to the instructed slant was less. We then used this relationship to predict the data in task-1 from the empirical data in task-2. We found a linear relationship between the predicted and empirical data in task-1 (y=0.91×+1.77, r=0.987), which suggests a lawful relationship between tasks 1 and 2. An independent, but noteworthy finding from task-1 is that the horizontal ground was judged as slanted upward (∼3.8deg).
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