Nature
PUBLISHED
A few years ago, scientists put a dead salmon inside an fMRI scanner and appeared to find evidence of life after death. Maybe there was a little fishy heaven up there after all. But these results were not all they seemed to be at first glance. The shock finding was actually a sharp reminder that results from even the fanciest high-tech equipment can be extraordinarily misleading if not understood correctly.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.The curious study came to be in 2005 when Craig Bennett, then a first-year graduate student at Dartmouth College, and his lab partner were looking into new ways to carry out and interpret fMRI scans. In an experimental spirit, they set out to image the most “curious objects” they could find.
“I picked up the salmon from our local supermarket early on an early Saturday morning in spring of 2005. The clerk behind the counter was a little shocked to be selling a full-length Atlantic salmon at 6:30 AM, especially when I told her what was about it happen to it,” Bennett wrote in a blog post.
In the study, eventually published in 2009 after Bennett had joined the University of California Santa Barbara, they detailed how they put the dead Atlantic salmon (Salmo salar) in the fMRI while showing emotive photographs of human behavior. The lifeless fish was then asked – yes, seriously – to determine what emotion the person in the photo must have been experiencing.
To their surprise, the scans seemed to show the deceased salmon's brain was flickering with activity.
“I ran the fish data through my SPM processing pipelines and couldn’t believe what I saw,” wrote Bennett.
“Sure, there were some false positives. Just about any volume with 65,000 voxels is going to have some false positives with uncorrected statistics. Rather, it was where the false positives occurred that really floored me. A cluster of three significant voxels was arranged together right along the midline of the salmon’s brain,” he added.
The findings were, of course, not all they appeared to be – and that was the whole point. The researchers and their dead salmon were attempting to make a point about false positive results and the need to be very careful when interpreting fMRI scans. The results demonstrated that fMRI scans cannot simply be taken at face value; they must be subjected to precise statistical scrutiny before any conclusions are drawn.
“The more I think about the affair the more I believe that the fish has the chance to impact the field of neuroimaging in a very positive way. Predefined significance thresholds with a specified cluster extent are a weak control to the problem of false positives in imaging data,” Bennett explained.
“Statisticians and methods researchers have argued about the need for multiple comparisons correction for some time. In just one figure the salmon data illustrates exactly why we need stronger controls for the false positive problem in fMRI,” he continued.
As it turned out, the study made precisely the kind of splash a dead fish deserves. In 2012, their work was honored at the Ig Nobel Prizes, an annual celebration of ridiculous research that first makes you laugh, then makes you think. Years on, the infamous salmon continues to be cited in scientific papers when discussing the difficulties of interpreting fMRI scans.
If nothing else, it's a reminder that decent doses of skepticism and creativity are some of the most important tools in science.
