A 1969 study by Parrish, Lundy, and Leibowitz which investigated the effect of hypnotic age regression on the magnitude of the Ponzo and Poggendorff illusions was partially replicated. In the present study, the Ponzo illusion was presented to eight college students under three conditions (waking, regressed to age 9, and regressed to age 5). Results indicated no significant difference in the magnitude of the illusion across the three conditions. These findings were in contradiction to those cited by Parrish et al., whose age-regressed 5s responded to the illusion in a manner typical of young children. It was concluded that hypnotic age regression does not affect the magnitude of the Ponzo illusion, and it was suggested that age regression is a questionable tool for investigating developmental aspects of perception.

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