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:
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”.
Ouch!
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:
They conclude: 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
7 comments:
Anyone who's ever labelled and traced a nerve fiber tract could have told you this!
Could have told us what? That the grid theory is true, or false?
As an aside, the authors also noted that they have found the neuro-anatomical correlate to the state of being verklempt. It occurs when tension in the pontocerebellar fibers is increased to the point that the pyramidal tract begins to be functionally compressed. @jhewitt123
Told us that it's false. Axon tracts go all over the place, criss crossing at all angles, looping around, fanning out and converging in all sorts of ways.
I will say that the way that the pyramidal tract punctures the caudate-putamen leaving those little orthogonal remnant bridges is a neuro-anatomical phenomenon ripe for theoretical musings.
As I have understood Van Weden, it it not 90 degree angles, but the relation between fibers. Think of if as two (three for the 3d grid) packs of raw spaghetti that you stick into one another perpendicularly. Then cook them, without stirring, and you will have tangled fibers all over, but they are relatively still in a grid-like structure. So the simple argument "I have traced a fiber and it was not straight" is not applicable.
But I do not know who is right.
The problem with the use of technology in neuroscience - in this case diffusion MRI - is that it is too often dressed up as mature and applied to questions that it is just not capable of answering.
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