Perception of visual image size by school students of different ages

In psychophysical experiments, 209 high school students with normal vision, 8 to 16 years old, were examined to study the perception of visual image sizes. Observers assessed the length of linear arrow-like figures (the Müller-Lyer illusion) or the length of single lines without arrow-like ends. Distortion of line size perception by 17–21% was obtained in the Müller-Lyer illusion independent of the age of observers. Distortion of size perception was absent in the case of estimation of the length of single line segments. The size differentiation thresholds gradually decreased in both cases with increasing age of observers and were correlated with the acuity of vision. For single lines, they were, on average, 1.3 times lower than the thresholds in the Müller-Lyer illusion. The probable mechanisms of the Müller-Lyer illusion are discussed. The experimental results demonstrate stability of illusion for observers aged 8–16 years, which may be connected with preferential contribution of the lower levels of the visual system to the appearance of the illusion.