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Over the past few decades, different theories have been advanced to explain geometric-optical illusions based on various perceptual processes such as assimilation and/or contrast. Consistent with the contradictory effects of assimilation and contrast, Pressey's assimilation theory provided an explanation for the Müller-Lyer illusion, but failed to account for the Titchener (Ebbinghaus) illusion. A model that explains both Müller-Lyer and Titchener illusions according to a common underlying process may outline a unified explanation for a variety of geometric-optical illusions. In order to develop such a model, the concept of empty space is introduced as an area of the illusory figure that is not filled by line drawings. It was predicted that the magnitude of illusion would increase with the area of the empty space around the illusory figures. The effect of empty space on the magnitude of perceptual distortion was measured in Müller-Lyer figures, with outward arrowheads of different length. The results indicated an overestimation of the target stimulus in all of the figures. Nevertheless, consistent with the prediction of the present model, the horizontal line in the Müller-Lyer figure with the longest arrowheads appeared shorter than that with the shortest arrowheads, although the size contrast of these figures was the same. According to the analysis proposed in the present study, the area of empty space not only affects the magnitude of illusion but also serves as a contextual cue for the perceptual system to determine the direction of illusion (orientation). The functional relationships between the size contrast and empty space provide a common explanation for the Müller-Lyer, Titchener, and a variety of other geometric-optical illusions.

Although there are many studies investigating the effects of early cortical injury on brain and behavioral development in laboratory animals, there are virtually no studies examining the effects of cortical injury in adolescence. The purpose of present study was to investigate the effects of unilateral motor cortex lesion received in early and late adolescence periods (Postnatal days 35 and 55 [P35, P55]) on spontaneous neural reorganization and behavioral recovery in adulthood. Rats were given unilateral motor cortex lesions at P35 or P55 and their motor behaviors were compared to sham controls in adulthood. The results of behavioral tests (skilled reaching, postural asymmetry, sunflower seed manipulation, forepaw inhibition in swimming) revealed that rats with P35 lesions had significant functional deficits whereas the rats with P55 lesions showed nearly complete recovery. Golgi-Cox analysis of pyramidal neurons showed bilateral hypertrophy of dendritic fields in the remaining sensorimotor cortex in P55 but not P35 rat brains. Thus, there appears to be an age-related pattern of morphological and behavioral changes in response to cortical injury in the early and late adolescent periods leading to better functional recovery from later injuries, much as is seen in human children.