The Blinking Brain – A Problem For fMRI?

By Neuroskeptic | March 15, 2012 8:43 pm

Every time we blink, a wave of activity sweeps through our brain – and this could be a serious problem for some fMRI researchers.

French neuroscientists Hupé et al report on A BOLD signature of eyeblinks in the visual cortex. They found that spontaneous blinks are associated with a neural activation pattern over the occipital cortex areas responsible for processing vision.

In many ways this is not surprising – when you blink, everything goes dark, and then lights up again, all within a fraction of second, which means that blinks are a kind of very dramatic visual stimulus, equivalent to a big black object suddenly appearing and then vanishing again. However, it’s long been believed that blink suppression mechanisms in the eye and brain somehow block out the responses that would otherwise happen during a blink.

Don’t be so sure, say Hupé et al. In an elegant experiment, they showed volunteers a standard set of visual stimuli during fMRI scanning, while recording blinks using an eye tracking camera. Then they simply treated the blinks as events, and used standard analysis methods to find neural activation associated with them.

Blinks caused a significant BOLD response over a number of “visual” areas.

Compared to the “real” visual stimuli in the task, the blink signal was less extensive, but no less strong.

So what? The great majority of fMRI experiments don’t use eyetracking to measure blinks, so this study raises the scary possibility that blinks could lie behind some of the “stimulus-related” activations that we all know and love. It would be a problem if subject blinks were correlated with the stimuli or tasks, which they might be, because blink rate may vary with our psychological state.

I don’t think we should be too worried yet. The blink blobs were essentially confined to parts of the visual cortex. So any study that’s not focussed on vision is probably in the clear (although that’s just the average response: in some individual subjects, the activations were a lot wider.)

However, as the authors point out, there is a risk that alterations in blink rate, caused, perhaps, by emotional or cognitive stress, might be wrongly “found” to be causing visual cortex activation, which might call into question claims of “top-down” influences on early visual cortex… oh dear.

ResearchBlogging.orgHupé, J., Bordier, C., and Dojat, M. (2012). A BOLD signature of eyeblinks in the visual cortex NeuroImage DOI: 10.1016/j.neuroimage.2012.03.001

CATEGORIZED UNDER: fMRI, methods, papers, science
  • http://www.blogger.com/profile/15705565128439299346 Bradley Voytek

    Very nice. Another concern is that if your stimuli between conditions are of different luminance, they may elicit more or stronger blinks. Eye movements and blinks have long been known as a source of noise in EEG, because the effects or prominent. It's nice to see fMRI researchers being made aware of these issues as well.

  • http://www.blogger.com/profile/05660407099521700995 petrossa

    “I don't think we should be too worried yet. The blink blobs were essentially confined to parts of the visual cortex.”

    Or otoh we should be very worried because there are other signals getting mixed up in the datastream used for constructing the model (fmri is a model of the brain, not a representation of the brain) making most observations useless.

  • http://www.blogger.com/profile/07387300671699742416 practiCal fMRI

    It's a curious finding in some ways. Basically, their finding is that blinks are a modulation of the visual input, and that modulation is causing pronounced “activation patterns” of its own. Makes sense, even the positive sign of the “activation” to a blanking event, which they explain in the paper (I only skim-read it) as a typical response to sharp changes in luminance. I'll buy it.

    And I can't see obvious ways for motion to have caused the responses, in part because of the ~4 sec lag, although I do wonder about the N/2 ghosts from the eyes, assuming that they ran axial slices. They didn't give the slice prescription. (Not even in the detailed methods, which were in supplementary material for another reference. Sheesh. Since when did the methods get so unimportant…?) But as I say, even if the ghosts from the moving/blinking eyes were increasing the variance in those slices containing eye and eye muscle signal, I'm hard pushed to explain the timing of the response they measured. Still, it could happen, and I'd want that potential excluded to be sure. (I just get suspicious when a lot of the activation is (a) in mid-brain and (b) at the level of the eyes. And now that I think more about it, why slightly less than 4 sec to peak, not closer to 5 sec…?)

    Still, we know that attention is another good modulator of visual signal, so I wouldn't discount this finding. If someone replicates it I'll be happier. Happier still if the next experiment includes some “simulated blinks,” perhaps some short null events timed to provide a similar period of darkness at a similar frequency to real blinks, and compare the effects of simulated to real blinks in the model. The simulated blinks won't be perfect matches for the real thing, but they would allow robust test of the hypothesis that it's the luminance changes that are the primary response generators in visual areas.

  • http://www.blogger.com/profile/00856481031749235750 Tom Johnstone

    Nice article demonstrating how important it is to consider alternative sources of variation in fMRI BOLD signal. In response to Bradley's comment above, increasingly researchers are using eye tracking to control for eye movements, etc. We have shown, for example that up to 50-70% of the interindividual variance in BOLD contrast in areas during a task involving more complex visual picture stimuli can be accounted for by eye movements. That's either noise or a major confound, depending on the analysis being run. Without measuring gaze, blinking etc., researchers have no real way of being sure their results are not affected.

  • Lili Marleaux

    Somewhat tangentially related, I noticed several years ago that some people blink in synchrony with MRI coil noise, especially during EPI scans.

  • DS

    I find it unacceptable that this paper nor the Hupe paper it references for further elaboration of methods nor the supplemental material about methods mentions anything about the receiver head coil used.

    Does anybody know what the standard head coil is for this Bruker 3T Medspec scanner. For all I know they could have used a surface coil!

    The paper (nor the Hupe paper or the supplemental) does not even mention the estimated motion parameters. Not that I trust these rigid image volume motion correction algorithms – I don't – but if the motion parameters were relatively large that would be something extremely important for the reader (and the reviewers) to know!

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Bradley: Yeah, fMRI is now paying attention to things like eye movement and blinks, that EEG researchers have been all too aware of for some time.

    Actually though… I wonder if the EEG researchers might have something to learn in return.

    This study shows that blinks cause real, strong neural activation.

    EEG researchers tend to just discard blink-contaminated data on the assumption that all the signal is artefactual, but this study suggests that some of the signal could be neural.

    Might be hard to disentangle the two with EEG, but it should be possible.

  • Anonymous

    Intracranial EEG study also showed that eye blinks increased the amplitude of gamma-oscillations.

    Neuroimage. 2011 Oct 15;58(4):1101-9.

    (http://www.ncbi.nlm.nih.gov/pubmed/21816225)

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Anonymous: Oh, that's interesting. I wonder if the same might be true of microsaccades, which are known to cause 'artefactual' gamma EEG activity – maybe some of it is real after all? (but still eye-movement related…) It's a stretch, but if blinks can do it, there's no reason eye movements couldn't.

  • http://www.priceperheadcostarica.com/ pay per head

    Whoa! Amazing! Bad with words 'cause everything is so good!

  • http://www.blogger.com/profile/00890404304081225894 StokesBlog

    Really interesting, if only scanners routinely incorporated eye tracking equipment! I think people have avoided the issue not because they really believed it would have no effect, but just because eye tracking in the scanner is too difficult. Especially when experimenters have limited control over the scanner set up. This is changing, of course, but this paper is a timely reminder to get on with it!
    Also, nice point by Neuroskeptic re: EEG. Very often people think of the eye movement artefact simply as signal corruption, and forget about true brain effects. Signal processing methods to remove the artefact, such as ICA, may not be optimal for dealing with real brain effects.
    As well as gamma mentioned above, it is clear even from the raw data that eye blinks trigger two or three alpha cycles in visual cortex. The amplitude can be really massive in some subjects. And eye blinks almost certainly correlate with many cognitive manipulations, such as task difficulty, duration of trial, etc. Epochs contaminated by eye blinks (or saccades) should really be excluded, just to be safe. It might reduce the number of trials in the analysis, but almost certainly worth it… Might even improve your SNR!

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Neuroskeptic

No brain. No gain.

About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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