What neural mechanisms underlie “fluid intelligence,” the ability to reason and solve novel problems? This is the question addressed by Gray et al. in Nature Neuroscience. The authors begin by suggesting that fluid intelligence (aka, gF) is related to both attentional control and active maintenance of information in the face of ongoing processing (i.e., working memory). Each of these concepts, in turn, has been associated with the functioning of the lateral prefrontal cortex – a region that has been massively expanded in humans compared to even our closest evolutionary relatives.
To confirm that individual differences in gF are related to prefrontal functioning, Gray et al. measured performance both on a standard gF task (Raven’s matrices) as well as on a standard test of prefrontal function from cognitive neuroscience: the 3-back task. In Raven’s matrices, subjects are required to pick which of several stimuli “fits” as the final item in a matrix of abstract patterns (see an example.) In contrast, the 3-back task provides subjects with a series of stimuli, presented sequentially, and requires that they respond if the current stimulus matches the one presented 3 items previously (i.e., to respond yes to the second “B” in a sequence like “A X B Y X B X A”). This task is performed in an ongoing fashion, such that subjects must constantly displace the third item in memory with the second, and update memory with the current item. (If you can’t tell from my description, this is an extremely difficult task).
Intuitively, one might not expect a strong relationship between these tasks: 3-back relies heavily on memory, whereas all the relevant stimuli are simultaneously present in Raven’s. Conversely, Raven’s requires abstract and somewhat “analogical” reasoning, but 3-back requires only rote memorization. So these tasks seem to require very different computations – an individual’s performance might be expected vary substantially between them.
On the other hand, there’s the concept of the “positive manifold”: performance on any two reliably-measured tasks is positively correlated (indeed, this is part of the basis for the concept of “general intelligence”). Surprisingly, the positive manifold may apply to neuroscience data as well: despite the possibility that different neural regions would underlie performance on these two very different tasks, certain regions in prefrontal cortex reliably mediate the behavioral correlations between these tasks.
To demonstrate this surprising fact, the authors distinguished between 3-back performance on lure trials (where the target item had occurred on perhaps the 2nd or 4th previous trial, but not the 3rd back) and those on non-lure trials (where target items occured on 1 trial ago, or more than 5 trials ago). Lure trials actually seemed more sensitive to performance than target trials (in which an item was actually presented 3 trials ago) insofar as accuracy was just as bad as target trials, but RTs were even longer.
Estimates of gF were positively correlated with accuracy on all trials types, but was most strongly related with lure trial performance: taking into account accuracy on non-lure trials or accuracy on target trials, gF still showed a significant relationship with lure trials. Activity in lateral PFC, anterior cingulate, and lateral cerebellum all predicted accuracy, and activity in these regions during lure trials overlapped with up to 92% of the shared variance between gF and 3-back performance. In contrast, this pattern was much more subtle on both target and non-lure trials.
Interestingly, the magnitude of sustained activation (thought to subserve active maintenance) was correlated with 3-back accuracy but not with gF ability. This finding is somewhat at odds with accounts that put “vanilla” active maintenance at the center of intelligence and executive control – other processes (such as those recall and discrimination processes involved in lure trials) appear to more strongly manifest the variance shared with gF. This would seem to have applications to the notion of “reactive control” and “secondary memory” as discussed recently in the literature – future work will need to clarify the relationships between these constructs.
The authors note that grey matter volume in lateral prefrontal cortex is under “significant” genetic control, suggesting that perhaps gF is itself largely heritable. Word has it that a new (but still under review) publication is showing the heritability of gF as being close to 1. In contrast, the authors here suggest that gF is probably not entirely heritable, and that a better understanding of individual differences in the neural correlates of gF could contribute to future attempts at enhancing fluid intelligence.