Over half a million fMRI and EEG papers have been published to date . What drives the trajectory? What have we learned? Who would actually know?
Of all the techniques to observe the living human brain, fMRI and EEG are the most popular, providing a completely safe and non-invasive functional view with no radioactive tracers or harmful radiations. This post is about the quantitative trajectory of research using these techniques based on an analysis of Pubmed data over the last 80 years. (The two techniques are fundamentally different.One measures electrical activity through sensors placed on the skull and the other measures blood oxygen. You can read more about the techniques and their differences here or do a deep dive into the EEG signal here).
So what has the research community produced over the decades? Here’s what we found out:
Research by the Numbers
EEG was first measured by Hans Berger in 1924 (read about its interesting history here) though he did not publish his results for many years and the technique did not gain traction in the research community until after his death in 1941. After this slow start EEG began to gain popularity in the 1960s. By 1980 however it began to lose luster as perhaps its promise began to fade or as advances in magnetic resonance imaging began to take center stage and interest.
The BOLD (blood oxygen) signal behind fMRI was first discovered and used in the late 1980s with the first paper published in 1990 by a Japanese group led by Seiji Ogawa. It immediately took human research by storm and the number of papers simply took off in an astronomical rise, quickly outpacing EEG.
Beginning in the year 2000 however, EEG began to regain its popularity as advances in computing have allowed more interesting and sophisticated approaches to signal analysis. When the number of papers is normalized to that in the year 2000, you can see that the growth of both techniques are now matched.
To date a total of 581,000 EEG and fMRI papers (maybe some MRI sneaked into the count, it was hard to separate) have been published with the volume growing each year over the past decades. In 2015 alone a total of 29,295 fMRI papers and 5,979 EEG papers were published.
Economics of a Paper
In contrast to EEG however, fMRI is an extraordinarily expensive technique. Compared to the $100,000 cost of a high-end EEG system today, an fMRI system only begins at $300,000 and can go all the way up to $3 million. Part of the reason, perhaps, that fMRI gained such volume relative to EEG is that when Universities invest in an fMRI machine, they must have sufficient utilization of the device to justify the cost and therefore a critical mass of researchers dedicated to the technique. This would likely leave little budget to hire researchers of EEG (someone is looking at ROI yes? Even if the scientists aren’t).
Of course, another reason for the dominance of fMRI over EEG was likely the relative difficulty of interpretation of the EEG signal into easily visualized results. fMRI was able to provide a visualization of brain activity with ‘this part lights up’ pictures that was easier to understand and perpetuate to the public. This no doubt made for easier presentations to non-scientific committees managing the purse strings.
That said, despite almost 5X more fMRI papers, if you look simply at the quantitative output per unit device cost (assuming a mid value for EEG at $50,000 and FMRI at $1M), EEG has consistently done 4X better.
What do they say?
Of course, science is not simply a numbers game. What matters is what these papers say. With over half a million papers out there and over 30k published just in the last year, one thing is abundantly clear: no one can actually know the literature. It is simply too vast. If I am at all representative, I skim perhaps 200-300 papers in a year (about 0.7% of annual output) and thoroughly read about 30-40 (0.1% of annual output). So what has really been discovered about the human brain so far? Who’s keeping track?