Background: Individuals experience a visual illusion created by shape interaction: when two shapes are presented successively and briefly, the form of the second (test) shape appears distorted due to the form of the first (prime) shape; this shape interaction is called the shape distortion effect. While age-related deterioration in performance is found in various aspects of visual perception, the effect of aging on the shape distortion effect has not been evaluated. Objective: The purpose of this study is to determine the effect of aging on the shape distortion effect. Methods: We measured the perception of briefly presented elementary shapes and of the shape distortion effect in 29 healthy volunteers, with ages ranging from 18 to 83 years. For each shape interaction trial, a prime rectangle was presented (vertical or horizontal) for 45 ms, followed by an interstimulus interval for 135 ms, a test circle for 60 ms and finally a random dot mask for 300 ms. The test circle was presented in each quadrant of the visual field. The prime rectangle was flashed either at the same position as the test circle (0° offset, intrahemifield) or 11° apart, displaced horizontally in the opposite hemifield (11° offset, interhemifield). In the elementary-shape trials, there was no prime, and the test shape was a circle or an ellipse. Using the method of adjustment, the percent elongation [(longer diameter – shorter diameter)/(shorter diameter) ×100] of the reproduced ellipse was computed. Results: The mean percent elongation in response to the elementary shapes did not vary with increasing age. The shape distortion effect decreased significantly with increasing age during both intra- and interhemifield conditions. The mean shape distortion effect was larger for the intrahemifield condition than for the interhemifield one. Conclusion: The shape distortion effect decreases with advancing age while the perception of elementary shapes does not. These results indicate a severer age-related dysfunction of the cerebral processing of shape interaction than that of elementary-shape perception.

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