Participants (N = 246; ages, 7–89 years; 45.5% female; 10.7% left-handed) completed a card sorting task (Fig. 2a) modeled after Berg (1948) (link) and described more fully elsewhere (Lyvers & Tobias-Webb, 2010 (link)). The instructions were as follows:

You are about to take part in an experiment in which you need to categorize cards based on the pictures appearing on them. To begin, you will see four piles. Each pile has a different number, color, and shape. You will see a series of cards and need to determine which pile each belongs to…. The correct answer depends upon a rule, but you will not know what the rule is. But, we will tell you on each trial whether or not you were correct. Finally, the rule may change during the task, so when it does, you should figure out what the rule is as quickly as possible and change with it. Press any key to begin.

After each trial, feedback of “correct!” or “incorrect” was displayed for 500 ms. The maximum number of trials was 128 (i.e., two decks of 64 cards) but could be shorter (100) on the basis of optimal category completions. The rule (color, shape, or number) could switch as quickly as every tenth trial. The primary dependent measure was the percentage of the total number of trials with perseverative errors. A perseverative error was defined as an incorrect response to a shifted or new category that would have been correct for the immediately preceding category. Response time was also obtained for correct and incorrect decisions for each participant, although excessively short (<100 ms) or long (>10 s) trial times were excluded prior to calculating the mean for each participant.