A new approach to fMRI scanning offers a three-dimensional look at brain activation.
fMRI is already a 3D technique, of course, but in the case of the cerebral cortex - which is what the great majority of neuroscientists are most interested in - the 3D data are effectively just 2D images folded up in space.
The cortex can be thought of a big sheet crumpled up into the shape of a brain, and it's possible to use software to 'unfold' the cortex into a 2D map for the purposes of fMRI data visualization. It's more informative because it shows you which areas are closest to each other.
But the cortex isn't really a sheet. It's more like six sheets stacked up - the cortex is formed of six layers, each with distinct cell types, connections, and functions. The difference between Layer III and Layer V of a particular cortical area is, in some ways, as important as the difference between two adjacent areas, but fMRI can't distinguish them because they're too close together.
Until now. In a new paper, Minnesota neuroscientists Olman et al say that they've given fMRI a third dimension -
Layer-specific FMRI reflects different neuronal computations at different depths in human v1.
They used a powerful 7 Tesla MRI scanner and a T2-weighted 3D GRASE pulse sequence that provides extremely high spatial resolution (0.7 mm - whereas 3 mm is the fMRI standard). The trade-off was that they were only able to scan a small chunk of the brain, namely the primary visual cortex. However, this is a good place to start, because it has a very well-understood layering system.
Does it work?
Probably, although the data they present are a little messy. By showing volunteers various kinds of pictures, they tried to find evidence of layer-specific visual cortex activation. However, most of the stimuli they used activated all layers equally. In my view the best evidence for layer-specific results was this, from two people -
Showing that the upper layers of the cortex were more activated by colourful stimuli that activate "P cells" compared to rapidly changing stimuli that act on "M cells".
We'll need more data to be sure that this technique works, but if it does, it promises some awesome science in the future. Still, it's not all good news for us neuroscientists. We'll have to relearn all the facts about cortical layers that most of us studied in Neuroscience 101 and then promptly forgot about.
Someone remind me, is Layer I or VI the top one...?
Olman CA, Harel N, Feinberg DA, He S, Zhang P, Ugurbil K, and Yacoub E (2012). Layer-specific FMRI reflects different neuronal computations at different depths in human v1. PloS one, 7 (3) PMID: 22448223
