lunedì 9 settembre 2013

fMRI: NOT A MIND READER (YET?) (II)

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Too-Big Brother

Maybe not just yet. In fairness to their developers, these brain-decoding devices do have a plausible scientific use, namely to help scientists test their theories of how the brain works. Such devices also should be helpful in devising high-tech communications and control systems for people whose disabilities — including “locked-in syndrome”—preclude speech, sign language, and any use of their arms or legs....



It’s also worth bearing in mind that fMRI cannot “read” your private thoughts without a considerable amount of compliance on your part.

A typical fMRI unit weighs tons, costs a fortune, and requires a huge electromagnet that must be kept supercooled by hundreds of gallons of liquid helium. Into this noisy, high-tech leviathan you as the subject must consent to enter, lying flat on your back — after leaving any watch, necklace, earring, or other ferromagnetic item far away.

Smaller, more portable MRI devices with permanent magnets are feasible, but have lower field strengths that make them less useful for functional neuroimaging — as opposed to the ordinary structural imaging of a typical hospital MRI. “Functional MRI probably is never going to be very portable,” says Jack Gallant, Ph.D., whose laboratory at the University of California at Berkeley accomplished the recent movie-decoding feat.

The primary magnetic field inside an fMRI electromagnet is strong enough — tens of thousands of times stronger than the ambient planetary field — to force the realignment of hydrogen nuclei in your brain, as if they were so many compass needles. The device uses a radio-frequency pulse to, in effect, pluck those aligned particles, like a harpist plucking harp strings, and it maps the resulting tones in all their informative variety. The technique is ingenious — the basic MRI concept earned its inventors a Nobel Prize — but it is so sensitive that it can easily be disrupted. Even after consenting to be fMRI-scanned and being positioned within the machine, you could spoil the scan simply by moving your head a bit during the procedure. In principle, you could muddy the data covertly, for example by thinking of the wrong things, by wiggling a toe or clenching a muscle, or merely by failing to follow the operator’s instructions.

Even when subjects are fully compliant, fMRI cannot offer anything close to a fine-grained, neuron-by-neuron picture of the working brain — which is just one of the reasons fMRI applications have been slow to emerge from the lab.

The usual fMRI technique — blood oxygen level-dependent or “BOLD” fMRI — aims to record the tiny surges of oxygen-laden blood that active brain cells draw from surrounding vessels. (The technique takes advantage of the tiny difference in magnetic properties between incoming, oxygen-carrying red blood cells and those that have delivered their O2 loads.) These surges, even when measured precisely, are imperfect proxies for brain activity. And in practice, the raw BOLD signal they yield is also quite “noisy,” making it likely that a typical large fMRI dataset — in the absence of the right statistical techniques—will show spurious correlations between neural activity, somewhere in the brain, and the behavior of interest. A team of researchers jokingly illustrated this potential noise problem last year by using fMRI to find “neural correlates of interspecies perspective taking” — in a dead fish.

The spatial resolution of an fMRI scan is not very high; it typically can resolve individual “voxels” of 3D brain-space that are on the order of one cubic millimeter — a volume that often contains hundreds of thousands of neurons and their support cells. The time resolution of fMRI is even worse; the changes in the BOLD signal occur over seconds, which is to say: thousands of times more slowly than the changes in neuronal activity that drive them. The content of an fMRI mind-scan movie could never reproduce the rat-a-tat pace of the average Hollywood blockbuster. Even much simpler applications are limited by BOLD signals’ slowness. A Chinese report in 2007 on fMRI-based lie-detection noted plaintively that “all images of fMRI are just the final results of brain changes after lying.”

In other words, fMRI provides a big, slow picture of brain activity — a rough sketch, not a microscopically detailed one. “It helps us understand how the brain processes information at a macroscopic level,” says John-Dylan Haynes, Ph.D., who directs the Berlin Center for Advanced Neuroimaging at the Charité Hospital in Berlin.


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