A Tale of Two Studies: Voxel-Based Lesion-Symptom Mapping

Brain imaging has contributed greatly to our understanding of the functional neuroataomy of the human brain. A lot these contributions have been blogged about by my bestest buddy Neuroskeptic (why don't you return my phone calls anymore!?). One of the more popular methods used to capture brain function is the functional magnetic resonance (fMRI). However, the results of fMRI studies are correlational and do not represent causation. There is another method, however, that "can identify regions, including white matter tracts, playing a causal role in a particular cognitive domain." This method is known as voxel-based lesion-symptom mapping (VLSM). A voxel is the three-dimensional analog of a pixel, and represents a volume of about 1 cubic millimeter. This method produces pretty images such as this one below.

A team of researchers from various important sounding universities published a study in this month's Proceedings of the National Academy of Sciences (PNAS; 1). In this issue of PNAS (pronounced penis), is an article titled "Distributed Neural System for General Intelligence Revealed by Lesion Mapping." The researchers created 3-D representations of the lesions of 241 subjects who had "single, focal, stable, chronic lesions of the brain." The subjects also had undergone neuropsychological testing, which included either the WAIS-R/WAIS-III.

The researchers were trying to discover where in the brain is general intelligence (often designated as "g"). Specifically,

"we address the question of whether g draws upon specific brain regions, as opposed to being correlated with global brain properties (such as total brain volume). Identifying such brain regions would help shed light on how g contributes to information processing and open the door to further exploration of its biological underpinnings, such as its emergence through evolution and development, and its alteration through psychiatric or neurological disease."

If "g" sounds like a highly abstract to concept to you, that's because it is. It's actually a really controversial concept within the field (2, 3). Below are the "g" loadings from this study.

The closer the color is to red, the closer that particular subtest loaded onto one of three g-related functions (i.e., verbal, spatial, working memory). The statistics of this study are admittedly over my head, since calculating g loadings require factor analysis. Since "g" is an abstraction, no actual number is presented for "g." Only how well a specific test loads onto "g" is provided.

What the researchers discovered should not be surprising to any biped mammal with working frontal lobes,

"One of the main findings that really struck us was that there was a distributed system here. Several brain regions, and the connections between them, were what was most important to general intelligence." (4)

More specifically,

"Statistically significant associations were found between g and damage to a remarkably circumscribed albeit distributed network in frontal and parietal cortex, critically including white matter association tracts and frontopolar cortex. We suggest that general intelligence draws on connections between regions that integrate verbal, visuospatial, working memory, and executive processes." (1)

"Statistically significant associations" is not same as "causal role." It's correlational. Still, nice sleight of hand.

What this group of geniuses is saying is that different brain functions are located in different parts of the brain, and when everything works in harmony, you have general intelligence.

"The researchers say the findings will open the door to further investigations about how the brain, intelligence, and environment all interact."

Open doors? That would mean that this research is original and ground breaking. It's not. In fact, in the March 2009 issue of Neuron (5) appeared this study, "Lesion Mapping of Cognitive Abilities Linked to Intelligence." Here is the press release (6). In this study, there were 241 patients with "single, focal, stable, chronic lesions of the brain," who had their lesions mapped and were also administered either the WAIS-R/WAIS-III. Also, the researchers are the same in both studies.

This study also found that performance on these (same) tests mapped primarily onto the frontal and parietal lobes.

The main difference is that the former study examined the anatomical location of intelligence in general, while the latter examine the anatomical location of general intelligence.

"So, what's the difference smart ass!?"

It depends on who you ask. Some say there is no difference, while others say there is a difference. At this point in the debate, however, we're engaging in mental masturbation (which is equally satisfying, plus people don't stare when you do it on the bus).

What I've been trying to figure out is if this counts as a duplicate publication? Sure, this doesn't have the far reaching consequences of these douche baggers (7, 8). There is a slight theoretical difference, the results are essentially identical. Curiously, in the most recent study, there is no citation to the other study. You'd think that the researchers want other people to read both of their studies.

Perhaps I'm looking too much into this. Or, perhaps I just enjoy mental masturbation...



Gläscher J, Rudrauf D, Colom R, Paul LK, Tranel D, Damasio H, & Adolphs R (2010). Distributed neural system for general intelligence revealed by lesion mapping. Proceedings of the National Academy of Sciences of the United States of America PMID: 20176936

Gläscher J, Tranel D, Paul LK, Rudrauf D, Rorden C, Hornaday A, Grabowski T, Damasio H, & Adolphs R (2009). Lesion mapping of cognitive abilities linked to intelligence. Neuron, 61 (5), 681-91 PMID: 19285465