One of the ongoing challenges in neuroscience is to do equal justice to both functional localization and functional integration in the brain. We know the brain is not equipotential, not every part can do everything, but we also know that individual brain regions rarely do much of interest on their own, the popularity of stories touting the discovery of “the brain region for X” notwithstanding. And yet the field tends too often to break into warring camps, with each side rallying around one or the other of these poles, each hoping to show that localization (no! integration!) is “the” fundamental fact of the brain.
Neuropsychology in general, and the neuropsychological assessment of brain injury patients in particular, has long been associated with support for the localization side of this spurious debate, because when focal (i.e. highly localized) brain injuries appear to result in very specific cognitive deficits, it can be hard to resist the conclusion that the patient has damaged (and the scientist has thereby discovered) “the” brain region for the impaired aspect of cognition. Of course, one should resist, if for no other reason (and there are so many other reasons!) than the injury might merely have cut off communication between the brain regions actually responsible for the impaired ability.
Thankfully, however, the field (although not always the media) has largely put aside such over-simplified inferences, as is beautifully illustrated by an article in this month’s issue of Brain: A Journal of Neurology (also available from the author’s website here). In this study of 182 patients with focal brain injuries, Dr. Aron Barbey and colleagues use a simple but elegant method known as voxel-based lesion-symptom mapping to identify the many regions of the brain that are causally related to general intelligence. The idea is straightforward: for each few millimeters of brain tissue, compare the test scores of patients with an injury there to the scores of patients without an injury there. If there is a significant difference in the average scores, that looks like pretty good evidence that those millimeters of brain tissue play some role supporting the abilities measured by the test.
Although the amount of information gained at each brain location is thus somewhat limited, the method nevertheless allows researchers to thoroughly map the causal network implicated in the cognitive functions of interest. In the current case, the authors report finding an extensive, largely left-lateralized network of regions implicated in supporting general intelligence. The regions include those involved in supporting verbal reasoning, working memory, cognitive flexibility, and executive control, among other things. Their results specifically highlight importance of the white-matter communication pathways between these regions, suggesting that the communication between and the integration of information from these regions is at least as important to intelligence as the activities of these regions.
Of course, that shouldn’t be at all surprising, and it’s true that the results are not necessarily earth-shattering by themselves. But the study is nevertheless important, because it represents a serious and successful attempt to do simultaneous justice to both functional specialization and functional integration in the brain. We need more such work.