The task they used is the so-called "conservation of number" task. Present a child younger than seven with two rows of items (buttons, for example) and ask them to say whether the two rows contain an equal number of items or not. You'll probably find that they base their answer on the length of the rows, regardless of the actual number of items in each row. The error reveals the child's use of an inappropriate "length=number" strategy.
When Leroux's team asked nine men to perform this task, they took longer to answer when the longer row wasn't necessarily the row with most items, compared with an easier condition, in which row length and number of items always matched. Moreover, the more difficult condition, involving length-number mismatches, was associated with increased activity in a swathe of brain areas compared with the easier condition. These regions included frontal regions such as the middle frontal gyrus and anterior cingulate cortex, which are known to be involved in inhibitory control.
The researchers said their findings show that the adults were having to inhibit a child-like tendency to follow the erroneous "length=number" strategy. They also noted that these frontal brain areas involved in inhibitory control are known to develop later in childhood than other areas which are associated with more basic functions. "These developmental findings support the idea of a crucial role of executive frontal areas in solving a task like Piaget's," they said, "as they mature precisely during a period when children become able to inhibit the perceptual bias - age 7."
Gaëlle Leroux, Jeanne Spiess, Laure Zago, Sandrine Rossi, Amélie Lubin, Marie-Renée Turbelin, Bernard Mazoyer, Nathalie Tzourio-Mazoyer, Olivier Houdé, Marc Joliot (2009). Adult brains don't fully overcome biases that lead to incorrect performance during cognitive development: an fMRI study in young adults completing a Piaget-like task. Developmental Science, 12 (2), 326-338 DOI: 10.1111/j.1467-7687.2008.00785.x
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