Complex reasoning and abstract thought—hallmarks of human behavior—are thought to originate in the region of the brain known as the prefrontal cortex. Now, a research team from RIKEN in Japan and the University of Oxford in the UK has differentiated the roles of several sub-regions of the prefrontal cortex in the macaque monkey1.
Figure 1: An example of the analog of the Wisconsin Card Sorting Test. Monkeys are rewarded after correctly applying the ‘shape-matching’ (top) or ‘color-matching’ (bottom) rule that will change without notice after several consecutive correct trials.
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Reproduced, with permission, from Ref. 1 © (2009) AAAS
Led by Keiji Tanaka of the RIKEN Brain Science Institute in Wako, the team trained fourteen macaques in an analog of a diagnostic test of frontal lobe function known as the Wisconsin Card Sorting Test. In this test, after viewing an image, a monkey must select a matching image from a choice of three, according to an unspecified rule, either ‘match shape’ or ‘match color’ (Fig. 1). A reward is given only for applying the currently relevant rule, which must be a guess on the first trial.
After maintaining a success rate of at least 85% over several trials, the rule changes without notice. If the monkey makes its next choice using the previously correct rule, it is not rewarded. To receive further rewards, the monkey must switch the rule, remember to apply the new rule under varying conditions in subsequent trials, and be able to update the rule again.
The researchers compared the performance of normal monkeys to those with brain damage to specific areas of the prefrontal cortex. They discovered that damage to the cortex surrounding the principal sulcus impaired the ability to retain the current rule in working memory, whereas damage to other regions did not affect this ability.
Tanaka and colleagues also found a link between lesions to the orbitofrontal cortex and difficulties in rapidly updating the changing rules. These monkeys needed to successfully complete multiple consecutive successful trials before they modified their behavior. Monkeys with damage to the anterior cingulate cortex exhibited neither of these deficits; they made rapid, incorrect choices at a higher rate. According to Tanaka, these monkeys often ignore the rule and act impulsively, based solely on long-term habits.
Similar deficits have been observed previously about the effects of frontal lobe damage in humans, but with no clear localization. Now, the specific links between these specialized brain regions and separable components of cognitive flexibility in using behavioral rules can be used to more thoroughly diagnose patients and to help scientists understand how the brain works to solve complex tasks.
The corresponding author for this highlight is based at the Laboratory for Cognitive Brain Mapping, RIKEN Brain Science Institute
