7/09/2006

What is the Value of fMRI?

Starting with an excellent short article at Seed magazine, a discussion across the blogosphere has focused on the contribution of fMRI technology to our understanding of the brain and cognition. While fascinating, these discussions were wide-ranging both in the topics covered and the blogs on which they were posted, making the thread rather difficult to follow. Here's what I hope is an accurate and chronological summary of the discussion so far:

The original article (Seed Magazine, June 27)

Yale professor Paul Bloom suggests that fMRI has been overhyped, both in the media and within funding agencies, mostly because it produces pretty pictures. According to Bloom's analysis, more mundane techniques, such as reaction time measurements, have contributed far more to the field than fMRI. Bloom argues that this has occurred because of an implicit bias that any theory mentioning the brain is more satisfying than theories which do not, regardless of the underlying validity of the theories. A second cause of this bias is the fact that we are "natural dualists," as Bloom puts it, and are therefore more impressed by any demonstration of how our abstract and conceptual mental experience is connected to anatomy.

A Lot of People in White Coats (Mixing Memory, June 27)

On Chris's excellent (but regrettably anti-cog-neuro) blog, he suggests that in most cases, cognitive neuroscience 'tells us little more than that cognition happens in the brain.'

Commenter Hyperion counters that the distinction between behavioral and imaging work may be a false dichotomy, "with the implicit assumption that they're somehow incompatible."

The Allure of fMRI (Cognitive Daily, June 27)

Here, author Dave Munger cites an experiment mentioned in Bloom's article, in which even completely irrelevant neuroscience terminology was perceived as making scientific explanations more satisfactory, among novices and experts alike. Commenter Matt Weber then poses the interesting question of why EEG, TMS and other quantitative cog neuro techniques don't "attract the same attention"as fMRI. Finally, commenter jbark opines that the quality of imaging studies will always be more dependent on the quality of the behavioral task than the quality of the scanning technologies.

A dissenting view (Small Gray Matters, June 27)

Small & Gray (at the excellent, and new to me Small & Gray Matters blog) attempts a "spirited defense" of fMRI technology. This is where the discussion gets heated; the primary points are:
  • Even if fMRI is overhyped, it may not be detracting from coverage on non-fMRI cognitive science
  • Cognitive neuroscience and cognitive psychology occupy different levels of analysis, and comparisons of "theoretical sophistication" between the two is one between apples and oranges
  • Imaging is a complementary approach to more traditional cognitive approaches
  • The focus on media hype is misplaced; we'd be better to think about how these techniques contribute to our understanding of cognition, rather than the impression they make on lay people
  • Bloom's comparison between RT and fMRI is stacked to favor RT studies, simply because they've been around hundreds of years longer
  • Preference for some techniques, or levels of analysis, above others is merely a question of personal preference

Commenter VTW notes that the lack of sophistication of early imaging studies is an essential part of how science progresses, and that current studies should not only build upon the early work, but replicate it in order to address the power issues inherent to those early studies.

The Dirty Secrets of fMRI (Frontal Cortex, June 28)

Author Jonah Lehrer suggests that a common perception of fMRI research ("pick a sexy question, stuff some people in a magnetic tube, and get technicolor pictures on the cover of Nature") comes from the fact that fMRI has not yet earned it reducitionism credentials. In other words, methodological limitations of the technology itself call into question fMRI's validity, above and beyond the more general question of whether it is overhyped due to the public's biases.

For example, Lehrer cites papers in which increases in blood flow do not always correspond with increases in neural firing, and that even chemically silenced neurons still generate a BOLD signal that appears active. Lehrer also cites a paper showing that BOLD signals emanate only from brain regions with dense vascular networks, which apparently has "little, if any, relationship to our neural activity."

Commenter Steve_HT notes that one of the works cited by Lehrer actually suggests that fMRI "provides a reasonable measure of the net neural input" into a particular brain region, because it is indexing local field potential (LFP), not the spiking rate of individual neurons.

A dissenting response (Small Gray Matters, June 28)

Small & Gray mounts a second defense of fMRI, by responding to Lehrer's post described above. The essentials of the argument are as follows:
  • The timecourse of the BOLD signal directly reflects monotonic changes in neural activity, though at the level of LFP rather than spiking rate
  • Correlation between LFP and BOLD is nearly 67%, reflecting 67% shared variance
  • It is fortunate that BOLD correlates with LFP, and not with spiking rate, since individual neurons rapidly habituate to stimuli, and since BOLD has a well known temporal lag; if this were not the case, fMRI would have insurmountable power issues
  • Because of this, the chemical silencer mentioned by Lehrer affects only individual neurons, and not LFP, thus explaining why the BOLD signal continues after chemical silencing
  • Vascular density likely develops based on the historical neural activity of a brain region; so even if fMRI does detects regions with greater vascular density more easily than others, at least it is indexing more active regions! Furthermore, the study showing these effects is questionable in terms of general applicability, partly because it used chinchillas, a rather unusual choice for this kind of study
  • The temporal lag of fMRI is well known and well understood, and can be computationally corrected for

fMRI Redux (Frontal Cortex, June 29)

Lehrer responds to Small & Gray's critique by reiterating his point that the connection between a BOLD signal and underlying neural activity is highly complex, and may not be commonly appreciated. Lehrer also argues that fMRI researchers would do well to earn their "reductionistic credentials" by focusing on anomalies such as the lack of correlation between spiking rate and LFP, and aptly concludes with the phrase, "the devil is always in the details."

fMRI studies overrated? (Neuromarketing, July 3)

NeuroGuy notes that similar problems exist in the burgeoning neuromarketing literature, in which the "flashy, full-color pictures sometimes obscure the fact that the actual marketing conclusions that can be drawn from the work are tenuous at best." Neuroguy also argues that fMRI may have become so popular because it is a real-time view at the actual anatomy that connects "input" to "output." NeuroGuy ends with a word to the wise: "today’s marketers should avoid being seduced by brain images that suggest it’s now possible to read the minds of customers."

Flickering Lights: One-Shot Wonders versus the Network Model (Smooth Pebbles, July 4)

Author David Dobbs notes that Bloom's article covers much of the same ground as his excellent article in Sci Am Mind, and expresses his agreement that skepticism of fMRI work is justified in the current climate of extreme overhyping by the media.

Dobbs argues that one feature of fMRI research that has been heretofore overlooked is the distinction between two types of imaging study: relatively simple correlational work, versus network analysis of brain regions involved in specific tasks.

As he points out, these more sophisticated inquiries involve a systems-level understanding of how inter-regional brain dynamics contribute to complex phenomena such as depression, attention, and cognitive control.

[My apologies if I left out contributions from blogs not listed here; I did my best to compile the major points into this post. If you feel that your contribution to the discussion was overlooked or mischaracterized, please let me know!]

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