The Limitations of Brain Scanning Technology

It is often said that a picture is worth a thousand words, and nowhere is this truer than in the world of neuroscience where the invention of Functional Magnetic Resonance Imaging (fMRI) appears to have fulfilled the dream of providing images of activity inside the brains of healthy humans.

fMRI technology has its origins in the pioneering work of Linus Pauling and Charles Coryell who, in 1936, discovered that oxygen-rich and oxygen-depleted blood had different magnetic properties. It was quickly recognised that this finding could augment the existing 'magnetic resonance imaging' technology by enabling researchers to analyse changes in the blood oxygenation over time in the brains of living subjects.

To obtain an fMRI brain scan a subject is placed inside a large and hugely expensive machine that contains powerful magnets. The machine is designed to measure the relative concentrations of oxygenated and deoxygenated blood in small areas of brain tissue. This produces what is known as a BOLD (Blood Oxygen Level Dependant) response. Since brain cells get their energy from oxygen, if an fMRI scan reveals an increase in the level of oxygenated blood in a particular region, the assumption is that the region in question is active and requires additional oxygen for fuel.

A computer then compares the scan data from the 'non-stimulated' state with that of the 'stimulated' state in order to identify which regions of the brain become relatively more active than others during the stimulation. This results in images of the brain with the various colours representing the relative strengths of the BOLD data.

The appeal of being able to 'see inside the brain' is so great that today the application of brain scanning has extended far beyond the realms of neuroscientific research. For example, firms such as Google, Motorola, Unilever and Disney have all used 'neuromarketeers' in an attempt to, as the head of the US marketing firm NeuroFocus puts it, unlock the "secrets for selling to the unconscious mind"1.

Neuroimaging is even finding its way into legal proceedings in an area known as 'neurolaw' - a discipline that sits at the intersection of neuroscience, legal theory and moral philosophy. This is particularly prevalent in the United States where the death sentence still exists in many states. In murder trials, defence lawyers are increasingly turning to brain scanning technology in an attempt to demonstrate that the guilt of a defendant is at least partially due to neurological disorders or immaturity. In fact several convicted murderers have already appealed their death sentences on the grounds that their lawyers wrongly denied them brain-scan evaluations2.

As the marketeers and lawyers have realised, not only is the concept of brain-scan imagery highly appealing, it can also be highly persuasive. To prove the point, two US psychologists undertook an experiment in which they asked people to review a series of academic papers, some of which were credible and others less so. Their conclusion was as follows:

'The use of brain images to represent the level of brain activity associated with cognitive processes influenced ratings of the scientific merit of the reported research, compared to identical articles including no image, a bar graph, or a topographical map. This effect occurred for fictional articles that included errors in the scientific reasoning in the articles, and in a real article in which there were no such errors. The present results lend support to the oft mentioned notion that there is something particularly persuasive about brain images with respect to conferring credibility to cognitive neuroscience data'3.

The real concern surrounding brain imaging is that people are affording far greater significance to these appealing images than they strictly deserve. This is because the technology used to produce them is in its infancy and the data the technology supplies can rarely be relied upon to provide definitive answers to even the simplest of questions.

To add some substance to this argument, the following points illustrate some of the more significant limitations, in this case, of fMRI scanning:

Delay: We know that our brains respond to stimulation almost instantly, yet the oxygenation of the blood that an fMRI scanner is measuring only occurs 2-5 seconds later. Therefore, the mental process and the instrument purporting to measure it are out of synch. We also know that some neurons are 'excitatory', ramping up the activity in brain regions, while others are 'inhibitory', damping activity down. Since all these activities will consume oxygen there is no hard evidence to know whether an fMRI scan is measuring stimulation or the suppression of stimulation.

Resolution: During an fMRI scan the computer is analysing the BOLD data from tiny regions known as 'voxels', a term derived from the words 'volume' and 'pixel'. A voxel measures about 3 cubic millimetres with an average brain containing about 50,000 voxels.

A human brain however is estimated to contain around 86 billion neurons (brain cells), with each individual cell having an average of 7-10 thousand connections to other cells in the body. So if we assume that these neurons are evenly distributed throughout the brain (which they are not), each voxel would contain about 2 million neurons, which in turn could account for 30 billion synaptic connections.

While the results of fMRI scans are interesting, to suggest that they provide anything more than evidence of neurological activity is fanciful.

Lies, damn lies and statistics: Computers are involved in fMRI scanning because they are needed to analyse the vast amount of data that is produced. The role of the computer is to identify aspects of the data that are deemed to be 'statistically significant'. Those areas of data are then further analysed to reinforce and corroborate the findings.

Although this is an accepted scientific approach, it is not without its flaws. The fact is that when analysing such vast amounts of data it is highly probable that some of the data will demonstrate 'statistical significance' regardless of whether it is truly significant or not.

To make the point, neuroscientist Craig Bennett scanned the brain of a dead Atlantic salmon that he purchased from a fishmonger. They placed the salmon in a scanning machine and 'showed' it photographs of people in various situations. When they analysed the data, they found that a tiny area of the salmon's brain appeared to spark into life in response to the task. Obviously, this was not possible as the salmon was dead, but the data-crunching identified statistical significance in some areas. Rather satisfyingly the salmon study won a 2012 Ig Nobel Prize for work that 'makes people laugh, and then think'4;.

Cause and effect: While there is a good argument that the areas that 'light up' in fMRI scans correlate with the brain regions associated with the activity being examined, it is not possible to go beyond this to explain cause and effect. For example, a 2011 study into the neurological effects in young people of playing violent video games found evidence of alterations in activity in regions associated with cognitive function and emotional control5. However, this finding does nothing to explain whether some people are more or less prone to these changes and why, whether the violent images increase or decrease their emotional sensitivity to violence in the real world or whether they are more or less likely to commit violent acts themselves as a result. In short, it is difficult to see what the scan results are telling us other than the fact that our brains react to what we see!

It may seem as though I am being wholly negative about brain scanning technologies - that is not my intention. The technology is amazing and is making significant contributions to our understanding of the workings of the brain. But the tremendous enthusiasm with which the technology is being embraced should be tempered with a healthy dose of realism. To put this in perspective, and to make this precise point, during a recent seminar we ran in London my colleague Gill McKay quoted a neuroscientist who estimated that we only know about 3% of what there is to know about the brain. When she said this a gentleman at the back of the room laughed out loud because, as he explained, this was precisely the same figure that had been quoted to him 35 years earlier when he was studying for his degree in psychology. It seems that we still have a lot to learn!

1 Pradeep, A.K. (2010) The Buying Brain, Secrets for Selling to the Unconscious Mind. Hoboken, NJ: John Wiley & Sons.

2 The Admissibility of Brain Scans in Criminal Trials: The Case of Positron Emission Tomography by Susan E Rushing

3 Seeing is believing: The effect of brain images on judgments of scientific reasoning by David P. McCabe, Alan D. Castel (2007)

4 Neural correlates of interspecies perspective taking in the post-mortem Atlantic Salmon:
An argument for multiple comparisons correction. By Craig M. Bennett, Abigail A. Baird, Michael B. Miller, and George L. Wolford

5 Radiological Society of North America: Violent Video Games Alter Brain Function in Young Men

Published February 2018

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