September 21, 2009
Something Fishy With The Technology
While it may sound like one of the wackiest research studies done in recent years, neuroscientist Craig Bennett, from the University of California at Santa Barbara and colleagues in an effort to evaluate fMRI noise and false positives, purchased a whole Atlantic Salmon and sat the fish in an fMRI scanner. To maintain the rigor of a protocol typically used in humans, they "showed" it a series of photographs depicting individuals in social situations and then "asked" it to determine what emotion the individual in the photo must have been experiencing.
If that is all that happened, it would have been reported as a silly experiment. However, to the researchers surprise, something more took place. When they analyzed the voxel (volumetric pixel) data, the voxels representing the area where the salmon’s tiny brain sat showed evidence of activity. In the fMRI scan, it looked like the dead salmon was actually thinking about the pictures it had been shown.
According to Bennett, "by complete, random chance, we found some voxels that were significant that just happened to be in the fish’s brain. And if I were a ridiculous researcher, I’d say, ‘a dead salmon perceiving humans can tell their emotional state."
The result is completely nuts - but that’s exactly the point. Bennett wrote up the work as a warning about the dangers of false positives in fMRI data. They wanted to call attention to ways the field could improve its statistical methods. Each set of brain scans elicit up to 130,000 voxels. The fMRI data has a lot of natural noise and with the amounts of data generated in the work, chance can play some tricks. Bennett compared the fMRI data problems to a particularly strange kind of darts game.
"In fMRI, you have 160,000 darts, and so just by random chance, by the noise that’s inherent in the fMRI data, you’re going to have some of those darts hit a bull’s-eye by accident," he said. Neuroscientists can filter the fMRI data to highlight the signal within the noise, but in so doing, rigorous statistical checks have to be maintained. "We could set our threshold so high that we have no false positives, but we have no legitimate results," Bennett said. "We could also set it so low that we end up getting voxels in the fish’s brain. It’s the fine line that we walk."
Bennett’s point is that a suite of methods known as multiple comparisons correction can allow researchers to maintain most of their statistical power while keeping the danger of false positives at bay.
The work highlights that brain science is highly data-driven and statistical now. Although the visualizations " usually some orangey spots on an otherwise dark brain scan " seem simple, the data collection and interpretation that go into producing them is intense.
This work was presented as a poster at Human Brain Mapping conference earlier this summer. Neuroscience researchers have been forwarding it to each other for weeks.
The authors didn't mention if they enjoyed the salmon.
Conference Poster:
Neural correlates of interspecies perspective taking in the post-mortem Atlantic Salmon: An argument for multiple comparisons correction
Sources/Links:
MindHacks
Wired
Conference Poster
Dr Bennett's Home Page