Observers pointing to a target viewed directly may elevate their fingertip close to the line of sight. However, pointing blindfolded, after viewing the target, they may pivot lower, from the shoulder, aligning the arm with the target as if reaching to the target. Indeed, in Experiment 1 participants elevated their arms more in visually monitored than blindfolded pointing. In Experiment 2, pointing to a visible target they elevated a short pointer more than a long one, raising its tip to the line of sight. In Experiment 3, the Experimenter aligned the participant's arm with the target. Participants judged they were pointing below a visually monitored target. In Experiment 4, participants viewing another person pointing, eyes-open or eyes-closed, judged the target was aligned with the pointing arm. In Experiment 5, participants viewed their arm and the target via a mirror and posed their arm so that it was aligned with the target. Arm elevation was higher in pointing directly.
Three groups of observers pointed to target circles in a path on the ground, in two parallel rows. Participants in one group viewed the circles and then pointed blindfolded. Those in a second group were blindfolded and then touched the circles with a stick while walking past them. Volunteers in the third group were blind adults, a diverse group, who also used a stick to detect the circles. For all three groups, as distance to the circles increased, pointing azimuths shrank and elevations increased. We suggest that directions to targets on major environmental surfaces may be appreciated similarly by the blind and sighted. We challenge the assumption that the principle of convergence to the horizon, available through vision because of the way in which visual angle decreases on the retina, is not available through touch.
Observers viewed pictures of a simulated ground plane and judged the orientation of lines pictured as lying on the ground. We presented three lines at a time and manipulated three factors: (1) the declination of the lines below the horizon (depicting distance to the target angles), (2) their length, and (3) whether or not they converged to a point on the horizon. Only the first factor had a substantial effect on these errors. We conclude that perspective foreshortening in pictures produces errors in perceived 3-D orientation. Our explanation is based on the different rates of change of elevation and azimuth with distance.
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