http://www.pnas.org/content/early/2013/11/27/1316909110 The data in the study concerned are derived from a technique called diffusion tensor imaging (DTI), which estimates how water diffuses along white matter tracts in the brain. The big advantage of DTI is that it can be used in living humans. Its big disadvantage is that even at best it gives only a very low resolution and thus partial picture of possible connections in the brain, and DTI is also rather poorly validated against 'gold standard' anatomical tracing methods that are routinely used in animal experiments. Thus, of the billions of connections made via the white matter of the human brain, DTI detects a tiny fraction of a percent and the method only indicates hypothetical anatomical connections and not any function. As a reality check one should take the example of the nematode worm, C elegans, which is the only animal for which we have a complete map of every connection in its nervous system. It has 302 neurons, compared to 85 billion in the human brain, yet no neuroscientist (or journalist) can tell you what is in the mind of this worm. Our knowledge of the connections in the human brain is poverty-stricken by comparison, yet this has not stopped some neuroscientists from linking their hypothetical structures of human neural networks, derived from techniques like DTI, directly to complex psychological process, as in the paper under discussion. Worse still, the media reinterpret and amplify the scientists conjectures to build their own confection of irresponsible speculation that bears little relation to the original data, as the present case so ably illustrates.
- K. Martin