This study examined developmental changes in concept formation, rule switching, and perseverative behaviors of children in the WCST by altering visual features of the test and using a new test score-the izigzag' error score-which shows the number of shifts made between two incorrect concepts or rules. Instead of the original four 3-dimensional WCST target cards, 12 unidimensional target cards were used in the present study. Consistent with previous research using the original WCST, the results of the current study revealed age-related differences in the unidimensional WCST performance. Six-to seven-year-olds produced more perseverative errors than older children, but as many zigzag errors-shifts between incorrect categories -as the eight-to nine-year-olds did. This suggested that the main difficulty for six-to seven-year-olds was one of the representational inflexibility rather than of switching inflexibility. (c) 2006 Elsevier Inc. All rights reserved.