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Sunday, September 27, 2009

Dead Fish Thinking

One of the popular techniques in psychology these days is to analyze brain activity in an MRI machine. In a typical experiment, the subject will be shown various images and the machine will maps those parts of the brain that are responding.

In the latest study, a group of psychologists showed a bunch of photographs to a dead salmon and measured the brain activity. The results were announced in a poster: Neural correlates of interspecies perspective taking in the post-mortem Atlantic Salmon: An argument for multiple comparisons correction. Apparently, the salmon was thinking about the photos.

Here's the method ....
Subject. One mature Atlantic Salmon (Salmo salar) participated in the fMRI study. The salmon was approximately 18 inches long, weighed 3.8 lbs, and was not alive at the time of scanning.

Task. The task administered to the salmon involved completing an open-ended mentalizing task. The salmon was shown a series of photographs depicting human individuals in social situations with a specified emotional valence. The salmon was asked to determine what emotion the individual in the photo must have been experiencing.

Design. Stimuli were presented in a block design with each photo presented for 10 seconds followed by 12 seconds of rest. A total of 15 photos were displayed. Total scan time was 5.5 minutes.

Preprocessing. Image processing was completed using SPM2. Preprocessing steps for the functional imaging data included a 6-parameter rigid-body affine realignment of the fMRI timeseries, coregistration of the data to a T1-weighted anatomical image, and 8 mm full-width at half-maximum (FWHM) Gaussian smoothing.

Analysis. Voxelwise statistics on the salmon data were calculated through an ordinary least-squares estimation of the general linear model (GLM). Predictors of the hemodynamic response were modeled by a boxcar function convolved with a canonical hemodynamic response. A temporal high pass filter of 128 seconds was include to account for low frequency drift. No autocorrelation correction was applied.

Voxel Selection. Two methods were used for the correction of multiple comparisons in the fMRI results. The first method controlled the overall false discovery rate (FDR) and was based on a method defined by Benjamini and Hochberg (1995). The second method controlled the overall familywise error rate (FWER) through the use of Gaussian random field theory. This was done using algorithms originally devised by Friston et al. (1994).
The point of the study was to show that you need to be aware of false positives and make the effort to filter the data in order to remove them.

The bigger point, as far as the rest of of us are concerned, is that all aspects of science are complicated and we need to be appropriately skeptical. There's a tendency in the popular press to treat all these brain activity studies as gospel truth because they use a fancy machine.

[Hat Tip: John Hawks]

1 comment :

  1. Thanks - you made my day. My nephew and I just had a conversation about healthy skeptical thought, and now you introduce me to dead salmon thoughts. Great post!