Earlier this year, Harvard neuroscientist
a Science paper
saying that brain white matter 'wiring' is organized in a grid-like fashion, with sheets of fibres crossing each other.
As Ed Yong put it, that the brain is full of Manhattan-like grids.
However, they were wrong - and that neat grid structure was purely an artefact of the method they used. So say London-based critics Marco Catani and colleagues in a Technical Comment just published.
Catani et al argue that the analysis
Wedeen et al used was unable to distinguish two crossing fibres, unless the angle between them was very large, i.e. close to a right angle. In other words, they only saw right angles - and hence neat parallel sheets - but the other angles were still there.
They present some data of their own, showing the distribution of fibre-crossing angles in 10 healthy brains:
This shows no clear peak at 90 degrees as the grid theory proposes. Some fibres do cross at right angles, but only about 12% of them. The rest cross at a wide range of other angles.
Catani et al finish up by saying that we know, from cutting up brains, that it's just not a grid:
Crossing ... fanning, merging, and kissing are other modalities that are frequently observed in postmortem anatomy and not visible with current diffusion methods. Finally, the grid model does not take into account the presence of thalamic fibers, which project radially in all brain regions. This implies that most white matter voxels have multiple populations of fibers... merging at progressively tangential angles to reach the same cortical areas. Current tractography reconstructions are biased toward solving only crossing. This information is well known to anatomists, and there is a serious risk in proposing the grid model as “a means to validate MRI tractography through consistency with grid structure”.
Wedeen et al respond robustly, however. They defend the accuracy of their technique, holding that it's more sensitive than alternatives, and say that even supposing their method had a bias towards detecting right angles, that wouldn't explain why they saw neat overlapping sheets.
They also point out that some earlier neuroanatomists did spot the kind of abrupt 'corners' and orthogonal tracts predicted by their theory, such as this drawing of a monkey brain:
The thesis that brain pathways adhere to a simple geometric system best accounts for the available evidence—not like London, but Manhattan; not unfathomable, but unlimited.
Marco Catani, Istvan Bodi, and Flavio Dell’Acqua (2012). Comment on “The Geometric Structure of the Brain Fiber Pathways” Science DOI: 10.1126/science.1223425