Studies of cortical function in primates have traditionally relied on a combination of electrophysiology in monkeys and functional imaging in humans. But this makes it hard to know how reliably we can extrapolate results from monkeys to humans. Recent technical advances have made it possible to use functional imaging in monkeys, and a new study by Nakahara et al. takes advantage of this to revisit a favourite test of cognitive psychologists — the Wisconsin Card Sorting Test (WCST).

The WCST is used to measure behavioural flexibility — subjects have to sort cards by a single feature (for example, colour) and then, without warning, 'shift set' to sort them by a different feature (perhaps the shapes shown on the cards). The group set out to compare directly the brain activation produced by this task in monkey and human brains. The method used to record activation — functional magnetic resonance imaging (fMRI) — was the same for the two sets of subjects, and the task design was also identical.

We know that 'set shifting' involves the prefrontal cortex, because patients with damage to prefrontal cortex often 'perseverate' — they carry on sorting the cards by a particular feature even when they have been told that it is incorrect. So, Nakahara and colleagues used this task to compare the functional organization of the prefrontal cortex in monkeys and humans.

They found that the parts of the prefrontal cortex that were activated by set shifting — which can therefore be considered to be functionally homologous — were in the same cytoarchitectonic areas in monkeys and humans. This might not seem all that surprising, but other functionally homologous areas, such as the frontal eye fields (which are involved in the control of eye movements), are located in different cytoarchitectonic areas in the two species. So, this kind of direct comparison across species will be very important for the study of the organization and evolution of the prefrontal cortex.